From: Adam Young <admiyo@os.amperecomputing.com>

Define a new, optional, callback that allows the driver to
specify how the return data buffer is allocated.  If that callback
is set,  mailbox/pcc.c is now responsible for reading from and
writing to the PCC shared buffer.

This also allows for proper checks of the Commnand complete flag
between the PCC sender and receiver.

For Type 4 channels, initialize the command complete flag prior
to accepting messages.

Since the mailbox does not know what memory allocation scheme
to use for response messages, the client now has an optional
callback that allows it to allocate the buffer for a response
message.

When an outbound message is written to the buffer, the mailbox
checks for the flag indicating the client wants an tx complete
notification via IRQ.  Upon receipt of the interrupt It will
pair it with the outgoing message. The expected use is to
free the kernel memory buffer for the previous outgoing message.

Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
---
 drivers/mailbox/pcc.c | 102 ++++++++++++++++++++++++++++++++++++++++--
 include/acpi/pcc.h    |  29 ++++++++++++
 2 files changed, 127 insertions(+), 4 deletions(-)

diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
index f6714c233f5a..0a00719b2482 100644
--- a/drivers/mailbox/pcc.c
+++ b/drivers/mailbox/pcc.c
@@ -306,6 +306,22 @@ static void pcc_chan_acknowledge(struct pcc_chan_info *pchan)
 		pcc_chan_reg_read_modify_write(&pchan->db);
 }
 
+static void *write_response(struct pcc_chan_info *pchan)
+{
+	struct pcc_header pcc_header;
+	void *buffer;
+	int data_len;
+
+	memcpy_fromio(&pcc_header, pchan->chan.shmem,
+		      sizeof(pcc_header));
+	data_len = pcc_header.length - sizeof(u32) + sizeof(struct pcc_header);
+
+	buffer = pchan->chan.rx_alloc(pchan->chan.mchan->cl, data_len);
+	if (buffer != NULL)
+		memcpy_fromio(buffer, pchan->chan.shmem, data_len);
+	return buffer;
+}
+
 /**
  * pcc_mbox_irq - PCC mailbox interrupt handler
  * @irq:	interrupt number
@@ -317,6 +333,8 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
 {
 	struct pcc_chan_info *pchan;
 	struct mbox_chan *chan = p;
+	struct pcc_header *pcc_header = chan->active_req;
+	void *handle = NULL;
 
 	pchan = chan->con_priv;
 
@@ -340,7 +358,17 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
 	 * required to avoid any possible race in updatation of this flag.
 	 */
 	pchan->chan_in_use = false;
-	mbox_chan_received_data(chan, NULL);
+
+	if (pchan->chan.rx_alloc)
+		handle = write_response(pchan);
+
+	if (chan->active_req) {
+		pcc_header = chan->active_req;
+		if (pcc_header->flags & PCC_CMD_COMPLETION_NOTIFY)
+			mbox_chan_txdone(chan, 0);
+	}
+
+	mbox_chan_received_data(chan, handle);
 
 	pcc_chan_acknowledge(pchan);
 
@@ -384,9 +412,24 @@ pcc_mbox_request_channel(struct mbox_client *cl, int subspace_id)
 	pcc_mchan = &pchan->chan;
 	pcc_mchan->shmem = acpi_os_ioremap(pcc_mchan->shmem_base_addr,
 					   pcc_mchan->shmem_size);
-	if (pcc_mchan->shmem)
-		return pcc_mchan;
+	if (!pcc_mchan->shmem)
+		goto err;
+
+	pcc_mchan->manage_writes = false;
+
+	/* This indicates that the channel is ready to accept messages.
+	 * This needs to happen after the channel has registered
+	 * its callback. There is no access point to do that in
+	 * the mailbox API. That implies that the mailbox client must
+	 * have set the allocate callback function prior to
+	 * sending any messages.
+	 */
+	if (pchan->type == ACPI_PCCT_TYPE_EXT_PCC_SLAVE_SUBSPACE)
+		pcc_chan_reg_read_modify_write(&pchan->cmd_update);
+
+	return pcc_mchan;
 
+err:
 	mbox_free_channel(chan);
 	return ERR_PTR(-ENXIO);
 }
@@ -417,8 +460,38 @@ void pcc_mbox_free_channel(struct pcc_mbox_chan *pchan)
 }
 EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
 
+static int pcc_write_to_buffer(struct mbox_chan *chan, void *data)
+{
+	struct pcc_chan_info *pchan = chan->con_priv;
+	struct pcc_mbox_chan *pcc_mbox_chan = &pchan->chan;
+	struct pcc_header *pcc_header = data;
+
+	if (!pchan->chan.manage_writes)
+		return 0;
+
+	/* The PCC header length includes the command field
+	 * but not the other values from the header.
+	 */
+	int len = pcc_header->length - sizeof(u32) + sizeof(struct pcc_header);
+	u64 val;
+
+	pcc_chan_reg_read(&pchan->cmd_complete, &val);
+	if (!val) {
+		pr_info("%s pchan->cmd_complete not set", __func__);
+		return -1;
+	}
+	memcpy_toio(pcc_mbox_chan->shmem,  data, len);
+	return 0;
+}
+
+
 /**
- * pcc_send_data - Called from Mailbox Controller code. Used
+ * pcc_send_data - Called from Mailbox Controller code. If
+ *		pchan->chan.rx_alloc is set, then the command complete
+ *		flag is checked and the data is written to the shared
+ *		buffer io memory.
+ *
+ *		If pchan->chan.rx_alloc is not set, then it is used
  *		here only to ring the channel doorbell. The PCC client
  *		specific read/write is done in the client driver in
  *		order to maintain atomicity over PCC channel once
@@ -434,17 +507,37 @@ static int pcc_send_data(struct mbox_chan *chan, void *data)
 	int ret;
 	struct pcc_chan_info *pchan = chan->con_priv;
 
+	ret = pcc_write_to_buffer(chan, data);
+	if (ret)
+		return ret;
+
 	ret = pcc_chan_reg_read_modify_write(&pchan->cmd_update);
 	if (ret)
 		return ret;
 
 	ret = pcc_chan_reg_read_modify_write(&pchan->db);
+
 	if (!ret && pchan->plat_irq > 0)
 		pchan->chan_in_use = true;
 
 	return ret;
 }
 
+
+static bool pcc_last_tx_done(struct mbox_chan *chan)
+{
+	struct pcc_chan_info *pchan = chan->con_priv;
+	u64 val;
+
+	pcc_chan_reg_read(&pchan->cmd_complete, &val);
+	if (!val)
+		return false;
+	else
+		return true;
+}
+
+
+
 /**
  * pcc_startup - Called from Mailbox Controller code. Used here
  *		to request the interrupt.
@@ -490,6 +583,7 @@ static const struct mbox_chan_ops pcc_chan_ops = {
 	.send_data = pcc_send_data,
 	.startup = pcc_startup,
 	.shutdown = pcc_shutdown,
+	.last_tx_done = pcc_last_tx_done,
 };
 
 /**
diff --git a/include/acpi/pcc.h b/include/acpi/pcc.h
index 840bfc95bae3..9af3b502f839 100644
--- a/include/acpi/pcc.h
+++ b/include/acpi/pcc.h
@@ -17,6 +17,35 @@ struct pcc_mbox_chan {
 	u32 latency;
 	u32 max_access_rate;
 	u16 min_turnaround_time;
+
+	/* Set to true to indicate that the mailbox should manage
+	 * writing the dat to the shared buffer. This differs from
+	 * the case where the drivesr are writing to the buffer and
+	 * using send_data only to  ring the doorbell.  If this flag
+	 * is set, then the void * data parameter of send_data must
+	 * point to a kernel-memory buffer formatted in accordance with
+	 * the PCC specification.
+	 *
+	 * The active buffer management will include reading the
+	 * notify_on_completion flag, and will then
+	 * call mbox_chan_txdone when the acknowledgment interrupt is
+	 * received.
+	 */
+	bool manage_writes;
+
+	/* Optional callback that allows the driver
+	 * to allocate the memory used for receiving
+	 * messages.  The return value is the location
+	 * inside the buffer where the mailbox should write the data.
+	 */
+	void *(*rx_alloc)(struct mbox_client *cl,  int size);
+};
+
+struct pcc_header {
+	u32 signature;
+	u32 flags;
+	u32 length;
+	u32 command;
 };
 
 /* Generic Communications Channel Shared Memory Region */
-- 
2.43.0

On Mon, Jul 14, 2025 at 08:10:07PM -0400, admiyo@os.amperecomputing.com wrote:
> From: Adam Young <admiyo@os.amperecomputing.com>
> 
> Define a new, optional, callback that allows the driver to
> specify how the return data buffer is allocated.  If that callback
> is set,  mailbox/pcc.c is now responsible for reading from and
> writing to the PCC shared buffer.
> 
> This also allows for proper checks of the Commnand complete flag
> between the PCC sender and receiver.
> 
> For Type 4 channels, initialize the command complete flag prior
> to accepting messages.
> 
> Since the mailbox does not know what memory allocation scheme
> to use for response messages, the client now has an optional
> callback that allows it to allocate the buffer for a response
> message.
> 
> When an outbound message is written to the buffer, the mailbox
> checks for the flag indicating the client wants an tx complete
> notification via IRQ.  Upon receipt of the interrupt It will
> pair it with the outgoing message. The expected use is to
> free the kernel memory buffer for the previous outgoing message.
>

I know this is merged. Based on the discussions here, I may send a revert
to this as I don't think it is correct.

> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
> ---
>  drivers/mailbox/pcc.c | 102 ++++++++++++++++++++++++++++++++++++++++--
>  include/acpi/pcc.h    |  29 ++++++++++++
>  2 files changed, 127 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
> index f6714c233f5a..0a00719b2482 100644
> --- a/drivers/mailbox/pcc.c
> +++ b/drivers/mailbox/pcc.c
> @@ -306,6 +306,22 @@ static void pcc_chan_acknowledge(struct pcc_chan_info *pchan)
>  		pcc_chan_reg_read_modify_write(&pchan->db);
>  }
>  
> +static void *write_response(struct pcc_chan_info *pchan)
> +{
> +	struct pcc_header pcc_header;
> +	void *buffer;
> +	int data_len;
> +
> +	memcpy_fromio(&pcc_header, pchan->chan.shmem,
> +		      sizeof(pcc_header));
> +	data_len = pcc_header.length - sizeof(u32) + sizeof(struct pcc_header);
> +
> +	buffer = pchan->chan.rx_alloc(pchan->chan.mchan->cl, data_len);
> +	if (buffer != NULL)
> +		memcpy_fromio(buffer, pchan->chan.shmem, data_len);
> +	return buffer;
> +}
> +
>  /**
>   * pcc_mbox_irq - PCC mailbox interrupt handler
>   * @irq:	interrupt number
> @@ -317,6 +333,8 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>  {
>  	struct pcc_chan_info *pchan;
>  	struct mbox_chan *chan = p;
> +	struct pcc_header *pcc_header = chan->active_req;
> +	void *handle = NULL;
>  
>  	pchan = chan->con_priv;
>  
> @@ -340,7 +358,17 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>  	 * required to avoid any possible race in updatation of this flag.
>  	 */
>  	pchan->chan_in_use = false;
> -	mbox_chan_received_data(chan, NULL);
> +
> +	if (pchan->chan.rx_alloc)
> +		handle = write_response(pchan);
> +
> +	if (chan->active_req) {
> +		pcc_header = chan->active_req;
> +		if (pcc_header->flags & PCC_CMD_COMPLETION_NOTIFY)
> +			mbox_chan_txdone(chan, 0);
> +	}
> +
> +	mbox_chan_received_data(chan, handle);
>  
>  	pcc_chan_acknowledge(pchan);
>  
> @@ -384,9 +412,24 @@ pcc_mbox_request_channel(struct mbox_client *cl, int subspace_id)
>  	pcc_mchan = &pchan->chan;
>  	pcc_mchan->shmem = acpi_os_ioremap(pcc_mchan->shmem_base_addr,
>  					   pcc_mchan->shmem_size);
> -	if (pcc_mchan->shmem)
> -		return pcc_mchan;
> +	if (!pcc_mchan->shmem)
> +		goto err;
> +
> +	pcc_mchan->manage_writes = false;
> +

Who will change this value as it is fixed to false always.
That makes the whole pcc_write_to_buffer() reduntant. It must go away.
Also why can't you use tx_prepare callback here. I don't like these changes
at all as I find these redundant. Sorry for not reviewing it in time.
I was totally confused with your versioning and didn't spot the mailbox/pcc
changes in between and assumed it is just MCTP net driver changes. My mistake.

> +	/* This indicates that the channel is ready to accept messages.
> +	 * This needs to happen after the channel has registered
> +	 * its callback. There is no access point to do that in
> +	 * the mailbox API. That implies that the mailbox client must
> +	 * have set the allocate callback function prior to
> +	 * sending any messages.
> +	 */
> +	if (pchan->type == ACPI_PCCT_TYPE_EXT_PCC_SLAVE_SUBSPACE)
> +		pcc_chan_reg_read_modify_write(&pchan->cmd_update);
> +
> +	return pcc_mchan;
>  
> +err:
>  	mbox_free_channel(chan);
>  	return ERR_PTR(-ENXIO);
>  }
> @@ -417,8 +460,38 @@ void pcc_mbox_free_channel(struct pcc_mbox_chan *pchan)
>  }
>  EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
>  
> +static int pcc_write_to_buffer(struct mbox_chan *chan, void *data)
> +{
> +	struct pcc_chan_info *pchan = chan->con_priv;
> +	struct pcc_mbox_chan *pcc_mbox_chan = &pchan->chan;
> +	struct pcc_header *pcc_header = data;
> +
> +	if (!pchan->chan.manage_writes)
> +		return 0;
> +
> +	/* The PCC header length includes the command field
> +	 * but not the other values from the header.
> +	 */
> +	int len = pcc_header->length - sizeof(u32) + sizeof(struct pcc_header);
> +	u64 val;
> +
> +	pcc_chan_reg_read(&pchan->cmd_complete, &val);
> +	if (!val) {
> +		pr_info("%s pchan->cmd_complete not set", __func__);
> +		return -1;
> +	}
> +	memcpy_toio(pcc_mbox_chan->shmem,  data, len);
> +	return 0;
> +}
> +
> +
>  /**
> - * pcc_send_data - Called from Mailbox Controller code. Used
> + * pcc_send_data - Called from Mailbox Controller code. If
> + *		pchan->chan.rx_alloc is set, then the command complete
> + *		flag is checked and the data is written to the shared
> + *		buffer io memory.
> + *
> + *		If pchan->chan.rx_alloc is not set, then it is used
>   *		here only to ring the channel doorbell. The PCC client
>   *		specific read/write is done in the client driver in
>   *		order to maintain atomicity over PCC channel once
> @@ -434,17 +507,37 @@ static int pcc_send_data(struct mbox_chan *chan, void *data)
>  	int ret;
>  	struct pcc_chan_info *pchan = chan->con_priv;
>  
> +	ret = pcc_write_to_buffer(chan, data);
> +	if (ret)
> +		return ret;
> +

Completely null as manages_write is false always.

>  	ret = pcc_chan_reg_read_modify_write(&pchan->cmd_update);
>  	if (ret)
>  		return ret;
>  
>  	ret = pcc_chan_reg_read_modify_write(&pchan->db);
> +
>  	if (!ret && pchan->plat_irq > 0)
>  		pchan->chan_in_use = true;
>  
>  	return ret;
>  }
>  
> +
> +static bool pcc_last_tx_done(struct mbox_chan *chan)
> +{
> +	struct pcc_chan_info *pchan = chan->con_priv;
> +	u64 val;
> +
> +	pcc_chan_reg_read(&pchan->cmd_complete, &val);

Not checking return from pcc_chan_reg_read(). Be consistent with the
other code in the file.

> +	if (!val)
> +		return false;
> +	else
> +		return true;
> +}
> +
> +
> +
>  /**
>   * pcc_startup - Called from Mailbox Controller code. Used here
>   *		to request the interrupt.
> @@ -490,6 +583,7 @@ static const struct mbox_chan_ops pcc_chan_ops = {
>  	.send_data = pcc_send_data,
>  	.startup = pcc_startup,
>  	.shutdown = pcc_shutdown,
> +	.last_tx_done = pcc_last_tx_done,
>  };
>  
>  /**
> diff --git a/include/acpi/pcc.h b/include/acpi/pcc.h
> index 840bfc95bae3..9af3b502f839 100644
> --- a/include/acpi/pcc.h
> +++ b/include/acpi/pcc.h
> @@ -17,6 +17,35 @@ struct pcc_mbox_chan {
>  	u32 latency;
>  	u32 max_access_rate;
>  	u16 min_turnaround_time;
> +
> +	/* Set to true to indicate that the mailbox should manage
> +	 * writing the dat to the shared buffer. This differs from
> +	 * the case where the drivesr are writing to the buffer and
> +	 * using send_data only to  ring the doorbell.  If this flag
> +	 * is set, then the void * data parameter of send_data must
> +	 * point to a kernel-memory buffer formatted in accordance with
> +	 * the PCC specification.
> +	 *
> +	 * The active buffer management will include reading the
> +	 * notify_on_completion flag, and will then
> +	 * call mbox_chan_txdone when the acknowledgment interrupt is
> +	 * received.
> +	 */
> +	bool manage_writes;
> +
> +	/* Optional callback that allows the driver
> +	 * to allocate the memory used for receiving
> +	 * messages.  The return value is the location
> +	 * inside the buffer where the mailbox should write the data.
> +	 */
> +	void *(*rx_alloc)(struct mbox_client *cl,  int size);

Why this can't be in rx_callback ?


I am convinced to send a revert, please respond so that I can understand
the requirements better.

-- 
Regards,
Sudeep

On 9/4/25 07:00, Sudeep Holla wrote:
> On Mon, Jul 14, 2025 at 08:10:07PM -0400,admiyo@os.amperecomputing.com wrote:
>> From: Adam Young<admiyo@os.amperecomputing.com>
>>
>> Define a new, optional, callback that allows the driver to
>> specify how the return data buffer is allocated.  If that callback
>> is set,  mailbox/pcc.c is now responsible for reading from and
>> writing to the PCC shared buffer.
>>
>> This also allows for proper checks of the Commnand complete flag
>> between the PCC sender and receiver.
>>
>> For Type 4 channels, initialize the command complete flag prior
>> to accepting messages.
>>
>> Since the mailbox does not know what memory allocation scheme
>> to use for response messages, the client now has an optional
>> callback that allows it to allocate the buffer for a response
>> message.
>>
>> When an outbound message is written to the buffer, the mailbox
>> checks for the flag indicating the client wants an tx complete
>> notification via IRQ.  Upon receipt of the interrupt It will
>> pair it with the outgoing message. The expected use is to
>> free the kernel memory buffer for the previous outgoing message.
>>
> I know this is merged. Based on the discussions here, I may send a revert
> to this as I don't think it is correct.

Have you decided what to do?  The MCTP over PCC driver depends on the 
behavior in this patch. If you do revert, I will need a path forward.

Based on other code review feed back, I need to make an additional 
change:  the rx_alloc callback function needs to be atomically set, and 
thus needs to move to the mailbox API.  There it will pair with the 
prepare transaction function.  It is a small change, but I expect some 
feedback from the mailbox maintainers.

I know all of the other drivers that use the PCC mailbox currently do 
direct management of the shared buffer.  I suspect that is the biggest 
change that is causing you concern.  Are you OK with maintaining a 
mailbox-managed path to buffer management as well?  I think it will be 
beneficial to other drivers in the long run.

On Mon, Sep 08, 2025 at 10:58:55AM -0400, Adam Young wrote:
> 
> On 9/4/25 07:00, Sudeep Holla wrote:
> > On Mon, Jul 14, 2025 at 08:10:07PM -0400,admiyo@os.amperecomputing.com wrote:
> > > From: Adam Young<admiyo@os.amperecomputing.com>
> > > 
> > > Define a new, optional, callback that allows the driver to
> > > specify how the return data buffer is allocated.  If that callback
> > > is set,  mailbox/pcc.c is now responsible for reading from and
> > > writing to the PCC shared buffer.
> > > 
> > > This also allows for proper checks of the Commnand complete flag
> > > between the PCC sender and receiver.
> > > 
> > > For Type 4 channels, initialize the command complete flag prior
> > > to accepting messages.
> > > 
> > > Since the mailbox does not know what memory allocation scheme
> > > to use for response messages, the client now has an optional
> > > callback that allows it to allocate the buffer for a response
> > > message.
> > > 
> > > When an outbound message is written to the buffer, the mailbox
> > > checks for the flag indicating the client wants an tx complete
> > > notification via IRQ.  Upon receipt of the interrupt It will
> > > pair it with the outgoing message. The expected use is to
> > > free the kernel memory buffer for the previous outgoing message.
> > > 
> > I know this is merged. Based on the discussions here, I may send a revert
> > to this as I don't think it is correct.
> 
> Have you decided what to do?  The MCTP over PCC driver depends on the
> behavior in this patch. If you do revert, I will need a path forward.
> 

Sorry not yet. I still need to understand and analyse your last reply.

> Based on other code review feed back, I need to make an additional change: 
> the rx_alloc callback function needs to be atomically set, and thus needs to
> move to the mailbox API.  There it will pair with the prepare transaction
> function.  It is a small change, but I expect some feedback from the mailbox
> maintainers.
> 
> I know all of the other drivers that use the PCC mailbox currently do direct
> management of the shared buffer.  I suspect that is the biggest change that
> is causing you concern.  Are you OK with maintaining a mailbox-managed path
> to buffer management as well?  I think it will be beneficial to other
> drivers in the long run.
> 

If you are really interesting to consolidating, then move all the buffer
management into the core. Just don't introduce things that just work on
your platform and for your use case. You need move all the drivers to this
new model of accessing the buffers. Otherwise I see no point as it is just
another churn but in core mailbox PCC driver instead of a client driver
using PCC. So, I am not OK with the change as is and needs to be reworked
or reverted. I need sometime to understand your email and requirements.

-- 
Regards,
Sudeep

On 9/8/25 11:20, Sudeep Holla wrote:
> If you are really interesting to consolidating, then move all the buffer
> management into the core. Just don't introduce things that just work on
> your platform and for your use case. You need move all the drivers to this
> new model of accessing the buffers. Otherwise I see no point as it is just
> another churn but in core mailbox PCC driver instead of a client driver
> using PCC. So, I am not OK with the change as is and needs to be reworked
> or reverted. I need sometime to understand your email and requirements.

This is kindof a large request.  I can start working on getting the 
other drivers to use the common mechanisms, but I do not have a way to 
test most of them, and there are numerous drivers.  I don't like making 
changes that I cannot test myself, but if we can get the other driver 
maintainers to test and review, I am happy to participate.

I know we use the CPPC driver, and I can show how that one would be 
consolidated.

Here is a potential path forward:

1. Revert the current patch, specifically to remove the callback function.

2.  I will post a minimal patch for the change to the mailbox api

3.  I will post patches that modify the other drivers to pass NULL in to 
the send_message path.  These will need to be reviewed and tested by the 
other driver maintainers

4. I will post a revised patch that only performs the buffer management 
for the send path.  This is essential for the MCTP over PCC driver, and 
for anything that wants to follow the PCC spec correctly.  This will 
remove pcc_mchan->manage_writes = false; This path will be triggered by 
passing a non-NULL pointer into the send data path. This is roughly half 
to the current patch.

5.  I will post a revised patch that makes use of the mailbox API 
callback defined in step 2 to allocate the memory used for the read stage.

6.I will repost my MCTP over PCC driver  that makes  use  of the updated 
patches.

Any point after step 3, we can start migrating the drivers to use the 
send mechanism.  After step 5 we can migrate the drivers to use the 
receive mechanism.


A shorter path forward would be:

1.   I will post a minimal patch for the change to the mailbox api

2. I will post a revised PCC Mailbox patch that makes use of the 
callback function, as well as an updated MCTP-PCC driver that makes use 
of that callback.  We deprecate the existing rx_alloc callback in the 
PCC Mailbox driver and ignore it in the PCC Mailbox.

3. Convert the other drivers to use the managed send/receive mechanism.

4.  Remove the flag pcc_mchan->manage_writes

I will stay engaged through the entire process, to include providing 
patches for the updated drivers, testing whatever I have access to, and 
reviewing all code that comes along these paths. I obviously prefer the 
shorter path, as it will allow me to get the MCTP-PCC driver merged. My 
team is going to be writing another, similar Mailbox implementation to 
the PCC mailbox. The more common behavior between the two 
implementations, the less code we will have to vary between drivers that 
make use of the mailboxes.

Answers inline.

On 9/4/25 07:00, Sudeep Holla wrote:
> On Mon, Jul 14, 2025 at 08:10:07PM -0400, admiyo@os.amperecomputing.com wrote:
>> From: Adam Young <admiyo@os.amperecomputing.com>
>>
>> Define a new, optional, callback that allows the driver to
>> specify how the return data buffer is allocated.  If that callback
>> is set,  mailbox/pcc.c is now responsible for reading from and
>> writing to the PCC shared buffer.
>>
>> This also allows for proper checks of the Commnand complete flag
>> between the PCC sender and receiver.
>>
>> For Type 4 channels, initialize the command complete flag prior
>> to accepting messages.
>>
>> Since the mailbox does not know what memory allocation scheme
>> to use for response messages, the client now has an optional
>> callback that allows it to allocate the buffer for a response
>> message.
>>
>> When an outbound message is written to the buffer, the mailbox
>> checks for the flag indicating the client wants an tx complete
>> notification via IRQ.  Upon receipt of the interrupt It will
>> pair it with the outgoing message. The expected use is to
>> free the kernel memory buffer for the previous outgoing message.
>>
> I know this is merged. Based on the discussions here, I may send a revert
> to this as I don't think it is correct.
>
>> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
>> ---
>>   drivers/mailbox/pcc.c | 102 ++++++++++++++++++++++++++++++++++++++++--
>>   include/acpi/pcc.h    |  29 ++++++++++++
>>   2 files changed, 127 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
>> index f6714c233f5a..0a00719b2482 100644
>> --- a/drivers/mailbox/pcc.c
>> +++ b/drivers/mailbox/pcc.c
>> @@ -306,6 +306,22 @@ static void pcc_chan_acknowledge(struct pcc_chan_info *pchan)
>>   		pcc_chan_reg_read_modify_write(&pchan->db);
>>   }
>>   
>> +static void *write_response(struct pcc_chan_info *pchan)
>> +{
>> +	struct pcc_header pcc_header;
>> +	void *buffer;
>> +	int data_len;
>> +
>> +	memcpy_fromio(&pcc_header, pchan->chan.shmem,
>> +		      sizeof(pcc_header));
>> +	data_len = pcc_header.length - sizeof(u32) + sizeof(struct pcc_header);
>> +
>> +	buffer = pchan->chan.rx_alloc(pchan->chan.mchan->cl, data_len);
>> +	if (buffer != NULL)
>> +		memcpy_fromio(buffer, pchan->chan.shmem, data_len);
>> +	return buffer;
>> +}
>> +
>>   /**
>>    * pcc_mbox_irq - PCC mailbox interrupt handler
>>    * @irq:	interrupt number
>> @@ -317,6 +333,8 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>>   {
>>   	struct pcc_chan_info *pchan;
>>   	struct mbox_chan *chan = p;
>> +	struct pcc_header *pcc_header = chan->active_req;
>> +	void *handle = NULL;
>>   
>>   	pchan = chan->con_priv;
>>   
>> @@ -340,7 +358,17 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>>   	 * required to avoid any possible race in updatation of this flag.
>>   	 */
>>   	pchan->chan_in_use = false;
>> -	mbox_chan_received_data(chan, NULL);
>> +
>> +	if (pchan->chan.rx_alloc)
>> +		handle = write_response(pchan);
>> +
>> +	if (chan->active_req) {
>> +		pcc_header = chan->active_req;
>> +		if (pcc_header->flags & PCC_CMD_COMPLETION_NOTIFY)
>> +			mbox_chan_txdone(chan, 0);
>> +	}
>> +
>> +	mbox_chan_received_data(chan, handle);
>>   
>>   	pcc_chan_acknowledge(pchan);
>>   
>> @@ -384,9 +412,24 @@ pcc_mbox_request_channel(struct mbox_client *cl, int subspace_id)
>>   	pcc_mchan = &pchan->chan;
>>   	pcc_mchan->shmem = acpi_os_ioremap(pcc_mchan->shmem_base_addr,
>>   					   pcc_mchan->shmem_size);
>> -	if (pcc_mchan->shmem)
>> -		return pcc_mchan;
>> +	if (!pcc_mchan->shmem)
>> +		goto err;
>> +
>> +	pcc_mchan->manage_writes = false;
>> +
> Who will change this value as it is fixed to false always.
> That makes the whole pcc_write_to_buffer() reduntant. It must go away.
> Also why can't you use tx_prepare callback here. I don't like these changes
> at all as I find these redundant. Sorry for not reviewing it in time.
> I was totally confused with your versioning and didn't spot the mailbox/pcc
> changes in between and assumed it is just MCTP net driver changes. My mistake.

This was a case of leaving the default as is to not-break the existing 
mailbox clients.

The maibox client can over ride it in its driver setup.



>
>> +	/* This indicates that the channel is ready to accept messages.
>> +	 * This needs to happen after the channel has registered
>> +	 * its callback. There is no access point to do that in
>> +	 * the mailbox API. That implies that the mailbox client must
>> +	 * have set the allocate callback function prior to
>> +	 * sending any messages.
>> +	 */
>> +	if (pchan->type == ACPI_PCCT_TYPE_EXT_PCC_SLAVE_SUBSPACE)
>> +		pcc_chan_reg_read_modify_write(&pchan->cmd_update);
>> +
>> +	return pcc_mchan;
>>   
>> +err:
>>   	mbox_free_channel(chan);
>>   	return ERR_PTR(-ENXIO);
>>   }
>> @@ -417,8 +460,38 @@ void pcc_mbox_free_channel(struct pcc_mbox_chan *pchan)
>>   }
>>   EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
>>   
>> +static int pcc_write_to_buffer(struct mbox_chan *chan, void *data)
>> +{
>> +	struct pcc_chan_info *pchan = chan->con_priv;
>> +	struct pcc_mbox_chan *pcc_mbox_chan = &pchan->chan;
>> +	struct pcc_header *pcc_header = data;
>> +
>> +	if (!pchan->chan.manage_writes)
>> +		return 0;
>> +
>> +	/* The PCC header length includes the command field
>> +	 * but not the other values from the header.
>> +	 */
>> +	int len = pcc_header->length - sizeof(u32) + sizeof(struct pcc_header);
>> +	u64 val;
>> +
>> +	pcc_chan_reg_read(&pchan->cmd_complete, &val);
>> +	if (!val) {
>> +		pr_info("%s pchan->cmd_complete not set", __func__);
>> +		return -1;
>> +	}
>> +	memcpy_toio(pcc_mbox_chan->shmem,  data, len);
>> +	return 0;
>> +}
>> +
>> +
>>   /**
>> - * pcc_send_data - Called from Mailbox Controller code. Used
>> + * pcc_send_data - Called from Mailbox Controller code. If
>> + *		pchan->chan.rx_alloc is set, then the command complete
>> + *		flag is checked and the data is written to the shared
>> + *		buffer io memory.
>> + *
>> + *		If pchan->chan.rx_alloc is not set, then it is used
>>    *		here only to ring the channel doorbell. The PCC client
>>    *		specific read/write is done in the client driver in
>>    *		order to maintain atomicity over PCC channel once
>> @@ -434,17 +507,37 @@ static int pcc_send_data(struct mbox_chan *chan, void *data)
>>   	int ret;
>>   	struct pcc_chan_info *pchan = chan->con_priv;
>>   
>> +	ret = pcc_write_to_buffer(chan, data);
>> +	if (ret)
>> +		return ret;
>> +
> Completely null as manages_write is false always.
Not if re-set by the client.
>
>>   	ret = pcc_chan_reg_read_modify_write(&pchan->cmd_update);
>>   	if (ret)
>>   		return ret;
>>   
>>   	ret = pcc_chan_reg_read_modify_write(&pchan->db);
>> +
>>   	if (!ret && pchan->plat_irq > 0)
>>   		pchan->chan_in_use = true;
>>   
>>   	return ret;
>>   }
>>   
>> +
>> +static bool pcc_last_tx_done(struct mbox_chan *chan)
>> +{
>> +	struct pcc_chan_info *pchan = chan->con_priv;
>> +	u64 val;
>> +
>> +	pcc_chan_reg_read(&pchan->cmd_complete, &val);
> Not checking return from pcc_chan_reg_read(). Be consistent with the
> other code in the file.
OK, this is legit.
>
>> +	if (!val)
>> +		return false;
>> +	else
>> +		return true;
>> +}
>> +
>> +
>> +
>>   /**
>>    * pcc_startup - Called from Mailbox Controller code. Used here
>>    *		to request the interrupt.
>> @@ -490,6 +583,7 @@ static const struct mbox_chan_ops pcc_chan_ops = {
>>   	.send_data = pcc_send_data,
>>   	.startup = pcc_startup,
>>   	.shutdown = pcc_shutdown,
>> +	.last_tx_done = pcc_last_tx_done,
>>   };
>>   
>>   /**
>> diff --git a/include/acpi/pcc.h b/include/acpi/pcc.h
>> index 840bfc95bae3..9af3b502f839 100644
>> --- a/include/acpi/pcc.h
>> +++ b/include/acpi/pcc.h
>> @@ -17,6 +17,35 @@ struct pcc_mbox_chan {
>>   	u32 latency;
>>   	u32 max_access_rate;
>>   	u16 min_turnaround_time;
>> +
>> +	/* Set to true to indicate that the mailbox should manage
>> +	 * writing the dat to the shared buffer. This differs from
>> +	 * the case where the drivesr are writing to the buffer and
>> +	 * using send_data only to  ring the doorbell.  If this flag
>> +	 * is set, then the void * data parameter of send_data must
>> +	 * point to a kernel-memory buffer formatted in accordance with
>> +	 * the PCC specification.
>> +	 *
>> +	 * The active buffer management will include reading the
>> +	 * notify_on_completion flag, and will then
>> +	 * call mbox_chan_txdone when the acknowledgment interrupt is
>> +	 * received.
>> +	 */
>> +	bool manage_writes;
>> +
>> +	/* Optional callback that allows the driver
>> +	 * to allocate the memory used for receiving
>> +	 * messages.  The return value is the location
>> +	 * inside the buffer where the mailbox should write the data.
>> +	 */
>> +	void *(*rx_alloc)(struct mbox_client *cl,  int size);
> Why this can't be in rx_callback ?

Because that is too late.

The problem is that the client needs  to allocate the memory that the 
message comes in in order to hand it off.

In the case of a network device, the rx_alloc code is going to return 
the memory are of a struct sk_buff. The Mailbox does not know how to 
allocate this. If the driver just kmallocs memory for the return 
message, we would have a re-copy of the message.

This is really a mailbox-api level issue, but I was trying to limit the 
scope of my changes as much as possible.

The PCC mailbox code really does not match the abstractions of the 
mailbox in general.  The idea that copying into and out of the buffer is 
done by each individual driver leads to a lot of duplicated code.  With 
this change, most of the other drivers could now be re-written to let 
the mailbox manage the copying, while letting the mailbox client specify 
only how to allocate the message buffers.


Much of this change  was driven by the fact that the PCC mailbox does 
not properly check the flags before allowing writes to the rx channel, 
and that code is not exposed to the driver.  Thus, it was impossible to 
write everything in the rx callback regardless. This work was based on 
Huisong's comments on version 21 of the patch series.

>
>
> I am convinced to send a revert, please respond so that I can understand
> the requirements better.

On Thu, Sep 04, 2025 at 01:06:09PM -0400, Adam Young wrote:
> Answers inline.
> 
> On 9/4/25 07:00, Sudeep Holla wrote:

[...]

> > Who will change this value as it is fixed to false always.
> > That makes the whole pcc_write_to_buffer() reduntant. It must go away.
> > Also why can't you use tx_prepare callback here. I don't like these changes
> > at all as I find these redundant. Sorry for not reviewing it in time.
> > I was totally confused with your versioning and didn't spot the mailbox/pcc
> > changes in between and assumed it is just MCTP net driver changes. My mistake.
> 
> This was a case of leaving the default as is to not-break the existing
> mailbox clients.
> 
> The maibox client can over ride it in its driver setup.
> 

What if driver changes in the middle of an ongoing transaction ? That
doesn't sound like a good idea to me.

You didn't respond as why tx_prepare callback can be used to do exactly
same thing ?

> > > +	void *(*rx_alloc)(struct mbox_client *cl,  int size);
> > Why this can't be in rx_callback ?
> 
> Because that is too late.
> 
> The problem is that the client needs  to allocate the memory that the
> message comes in in order to hand it off.
> 
> In the case of a network device, the rx_alloc code is going to return the
> memory are of a struct sk_buff. The Mailbox does not know how to allocate
> this. If the driver just kmallocs memory for the return message, we would
> have a re-copy of the message.
>

I still don't understand the requirement. The PCC users has access to shmem
and can do what they want in rx_callback, so I don't see any reason for
this API.

> This is really a mailbox-api level issue, but I was trying to limit the
> scope of my changes as much as possible.
> 

Please explain the issue. Sorry if I have missed, pointer are enough if
already present in some mail thread.

> The PCC mailbox code really does not match the abstractions of the mailbox
> in general.  The idea that copying into and out of the buffer is done by
> each individual driver leads to a lot of duplicated code.  With this change,
> most of the other drivers could now be re-written to let the mailbox manage
> the copying, while letting the mailbox client specify only how to allocate
> the message buffers.
> 

Yes that's because each user have their own requirement. You can do what
you want in rx_callback.

> Much of this change  was driven by the fact that the PCC mailbox does not
> properly check the flags before allowing writes to the rx channel, and that
> code is not exposed to the driver.  Thus, it was impossible to write
> everything in the rx callback regardless. This work was based on Huisong's
> comments on version 21 of the patch series.
> 

Pointers please, sorry again. But I really don't like the merged code and
looking for ways to clean it up as well as address the requirement if it
is not available esp. if we have to revert this change.

-- 
Regards,
Sudeep

On 9/5/25 10:37, Sudeep Holla wrote:
> On Thu, Sep 04, 2025 at 01:06:09PM -0400, Adam Young wrote:
>> Answers inline.
>>
>> On 9/4/25 07:00, Sudeep Holla wrote:
> [...]
>
>>> Who will change this value as it is fixed to false always.
>>> That makes the whole pcc_write_to_buffer() reduntant. It must go away.
>>> Also why can't you use tx_prepare callback here. I don't like these changes
>>> at all as I find these redundant. Sorry for not reviewing it in time.
>>> I was totally confused with your versioning and didn't spot the mailbox/pcc
>>> changes in between and assumed it is just MCTP net driver changes. My mistake.
>> This was a case of leaving the default as is to not-break the existing
>> mailbox clients.
>>
>> The maibox client can over ride it in its driver setup.
>>
> What if driver changes in the middle of an ongoing transaction ? That
> doesn't sound like a good idea to me.
It would not be a good idea.  This should be setup only.  Is there a 
cleaner way to pass an initialization value like this in the mailbox API?
>
> You didn't respond as why tx_prepare callback can be used to do exactly
> same thing ?

Note that write_to_buffer checks pcc_chan_reg_read(&pchan->cmd_complete, &val);  This flag comes from struct pcc_chan_info which is defined in the pcc.c, not in a header.  Thus it is not accessible outside of the mailbox driver.  This needs to be done before data is written into the buffer, or there is a chance the far side is still reading it.  Checking it before send_data leads to a race condition.

>
>>>> +	void *(*rx_alloc)(struct mbox_client *cl,  int size);
>>> Why this can't be in rx_callback ?
>> Because that is too late.
>>
>> The problem is that the client needs  to allocate the memory that the
>> message comes in in order to hand it off.
>>
>> In the case of a network device, the rx_alloc code is going to return the
>> memory are of a struct sk_buff. The Mailbox does not know how to allocate
>> this. If the driver just kmallocs memory for the return message, we would
>> have a re-copy of the message.
>>
> I still don't understand the requirement. The PCC users has access to shmem
> and can do what they want in rx_callback, so I don't see any reason for
> this API.

I was not around for the discussion where it was decided to diverge from 
the mailbox abstraction, and provide direct access to the buffer.  I 
assume it was due to the desire not to copy into temporary-allocated 
memory.  However, the pcc mailbox driver should be handling the access 
into and out of the buffer, and not each individual driver.  Looking at 
most of them, there is very little type 4 usage where you are getting 
significant amounts of data back in the buffer, most of them are using 
rx for notification only, not transfer of data into the buffer.  Every 
single driver that will need to copy data from the shmem into a kernel 
memory buffer will have to do the exact same logic, and you can see it 
is non-trivial and worth getting right in the mailbox driver.


>
>> This is really a mailbox-api level issue, but I was trying to limit the
>> scope of my changes as much as possible.
>>
> Please explain the issue. Sorry if I have missed, pointer are enough if
> already present in some mail thread.

I don't think it is in any mail-thread: I expected the conversation to 
happen once I submitted this patch.  I did attempt to clarify my 
reasoning as much as possible in the commit message.  But the patch got 
merged with no discussion, and without a maintainer ACK.

The issue is that the mailbox driver is generic, and the actual use of 
it (MCTP-PCC in my case) is bound to other kernel subsystems, that need 
to manage memory in various ways.  The mailbox driver should handle the 
protocol (PCC) but not the memory management.

Thus, the rights solution would be to provide this callback function at 
the mailbox API level, much like the tx_prepare functions.  However, 
doing that would only change where he pcc mailbox driver finds the 
function, and not the logic.  Thus, I limited the change to only the PCC 
subsystem:  I did not want to have to sell this to the entire mailbox 
community without some prior art to point to.


>
>> The PCC mailbox code really does not match the abstractions of the mailbox
>> in general.  The idea that copying into and out of the buffer is done by
>> each individual driver leads to a lot of duplicated code.  With this change,
>> most of the other drivers could now be re-written to let the mailbox manage
>> the copying, while letting the mailbox client specify only how to allocate
>> the message buffers.
>>
> Yes that's because each user have their own requirement. You can do what
> you want in rx_callback.

Each driver needs to do the same thing: reading the header from 
iomemory, checking the length, allocating a buffer, and then reading the 
whole message from iomemory and setting the flag that says the buffer 
read is complete.  This is a way to get that logic right once, and  
reuse for any PCC driver that receives data in the buffer.


>
>> Much of this change  was driven by the fact that the PCC mailbox does not
>> properly check the flags before allowing writes to the rx channel, and that
>> code is not exposed to the driver.  Thus, it was impossible to write
>> everything in the rx callback regardless. This work was based on Huisong's
>> comments on version 21 of the patch series.
>>
> Pointers please, sorry again. But I really don't like the merged code and
> looking for ways to clean it up as well as address the requirement if it
> is not available esp. if we have to revert this change.

In retrospect, this patch made two changes, one which was completely 
required (access to the flag before writing to the buffer) and one which 
is a don't-repeat-yourself implementation.

If needs be, I can work around the changes to the RX  path. But I cannot 
work around the changes  in the TX path: it will lead to a race condition.

Making the RX change makes both paths equivalent, and will lead to 
cleaner PCC clients in the future.

And I can imagine your shock in seeing this patch post-merge.  I did 
send email out to you directly, but I realize now it must have gotten 
lost in the noise.  I really wish we had this discussion prior to 
merge.  However, I hope you will consider not reverting it.  I wrote it 
to be backwards compatible with existing mailbox clients, and to only 
take the new path upon explicit enabling.  I am more than happy to 
consider better ways to enable the features.

On 9/5/25 12:57, Adam Young wrote:
>>>> Who will change this value as it is fixed to false always.
>>>> That makes the whole pcc_write_to_buffer() reduntant. It must go away.
>>>> Also why can't you use tx_prepare callback here. I don't like these 
>>>> changes
>>>> at all as I find these redundant. Sorry for not reviewing it in time.
>>>> I was totally confused with your versioning and didn't spot the 
>>>> mailbox/pcc
>>>> changes in between and assumed it is just MCTP net driver changes. 
>>>> My mistake.
>>> This was a case of leaving the default as is to not-break the existing
>>> mailbox clients.
>>>
>>> The maibox client can over ride it in its driver setup.
>>>
>> What if driver changes in the middle of an ongoing transaction ? That
>> doesn't sound like a good idea to me.
> It would not be a good idea.  This should be setup only.  Is there a 
> cleaner way to pass an initialization value like this in the mailbox API? 

My initial idea was that we should use the mssg pointer to dictate 
whether or not the mailbox should attempt the write.  If the client 
passes in a NULL pointer (and they all should, with the exception of new 
ones) then there is nothing to try to write.

But at lease one of the clients seem to set the message, and I don't 
think there is any really good reason for it.

These are  the drivers that explicitly call pcc_mbox_request_channel.  
There might be other drivers that use the mailbox and request the 
channel through the mailbox API, but lets start with these

drivers/acpi/cppc_acpi.c

drivers/hwmon/xgene-hwmon.c

drivers/i2c/busses/i2c-xgene-slimpro.c

drivers/soc/hisilicon/kunpeng_hccs.c

drivers/devfreq/hisi_uncore_freq.c


For example, the last driver calls:          rc = 
mbox_send_message(pchan->mchan, &cmd);

There is actually no reason to assume that the doorbell will be rung at 
that point:  the cmd object is not null, and thus gets added to the 
ring_buffer.  They then have to poll.  The poll may timeout, but the cmd 
pointer may still be in the ring buffer, and get sent on the next 
message.  But there is no benefit to sending the cmd object here, as the 
ring buffer pointer is not read.

slimpro_i2c_send_msg same thing.  The message is a 32bit value. None of 
these calls actually make use of  the PCC buffer: there is no PCC header 
written etc.  You could argue they don't actually meet the protocol 
definition.

Here is drivers/acpi/cppc_acpi.c

         /* Flip CMD COMPLETE bit */
         writew_relaxed(0, &generic_comm_base->status);

         pcc_ss_data->platform_owns_pcc = true;

         /* Ring doorbell */
         ret = mbox_send_message(pcc_ss_data->pcc_channel->mchan, &cmd);

If, instead, these drivers did

         ret = mbox_send_message(pcc_ss_data->pcc_channel->mchan,  NULL);

You would have proper functioning, and the void * mssg parameter could 
be used for the drivers that actually need it.

All of these drivers would have a simpler code path if they called the 
function pcc_send_data directly, without putting values on the ring 
buffer.  That was how I originally wrote my driver, but I actually want 
to make use of  the mailbox abstraction, and can use the ring buffer as 
rate limiter etc.

So, instead of going an changing the other drivers, I provided a default 
that left the existing behavior alone, and only performs the 
mailbox-assisted-write if the flag is set.

On 7/14/25 20:10, admiyo@os.amperecomputing.com wrote:
> From: Adam Young <admiyo@os.amperecomputing.com>
>
> Define a new, optional, callback that allows the driver to
> specify how the return data buffer is allocated.  If that callback
> is set,  mailbox/pcc.c is now responsible for reading from and
> writing to the PCC shared buffer.
>
> This also allows for proper checks of the Commnand complete flag
> between the PCC sender and receiver.
>
> For Type 4 channels, initialize the command complete flag prior
> to accepting messages.
>
> Since the mailbox does not know what memory allocation scheme
> to use for response messages, the client now has an optional
> callback that allows it to allocate the buffer for a response
> message.
>
> When an outbound message is written to the buffer, the mailbox
> checks for the flag indicating the client wants an tx complete
> notification via IRQ.  Upon receipt of the interrupt It will
> pair it with the outgoing message. The expected use is to
> free the kernel memory buffer for the previous outgoing message.
>
> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
> ---
>   drivers/mailbox/pcc.c | 102 ++++++++++++++++++++++++++++++++++++++++--
>   include/acpi/pcc.h    |  29 ++++++++++++
>   2 files changed, 127 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
> index f6714c233f5a..0a00719b2482 100644
> --- a/drivers/mailbox/pcc.c
> +++ b/drivers/mailbox/pcc.c
> @@ -306,6 +306,22 @@ static void pcc_chan_acknowledge(struct pcc_chan_info *pchan)
>   		pcc_chan_reg_read_modify_write(&pchan->db);
>   }
>   
> +static void *write_response(struct pcc_chan_info *pchan)
> +{
> +	struct pcc_header pcc_header;
> +	void *buffer;
> +	int data_len;
> +
> +	memcpy_fromio(&pcc_header, pchan->chan.shmem,
> +		      sizeof(pcc_header));
> +	data_len = pcc_header.length - sizeof(u32) + sizeof(struct pcc_header);
> +
> +	buffer = pchan->chan.rx_alloc(pchan->chan.mchan->cl, data_len);
> +	if (buffer != NULL)
> +		memcpy_fromio(buffer, pchan->chan.shmem, data_len);
> +	return buffer;
> +}
> +
>   /**
>    * pcc_mbox_irq - PCC mailbox interrupt handler
>    * @irq:	interrupt number
> @@ -317,6 +333,8 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>   {
>   	struct pcc_chan_info *pchan;
>   	struct mbox_chan *chan = p;
> +	struct pcc_header *pcc_header = chan->active_req;

OK, so it looks a little strange to re-initialize this later. Would it 
be better to not have it initialized?


> +	void *handle = NULL;
>   
>   	pchan = chan->con_priv;
>   
> @@ -340,7 +358,17 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>   	 * required to avoid any possible race in updatation of this flag.
>   	 */
>   	pchan->chan_in_use = false;
> -	mbox_chan_received_data(chan, NULL);
> +
> +	if (pchan->chan.rx_alloc)
> +		handle = write_response(pchan);
> +
> +	if (chan->active_req) {
> +		pcc_header = chan->active_req;
> +		if (pcc_header->flags & PCC_CMD_COMPLETION_NOTIFY)

Note that this is the counterpoint to my earlier patch that only 
notifies the platform if the platform requests it.  This is part of the 
specification.

> +			mbox_chan_txdone(chan, 0);
> +	}
> +
> +	mbox_chan_received_data(chan, handle);
>   
>   	pcc_chan_acknowledge(pchan);
>   
> @@ -384,9 +412,24 @@ pcc_mbox_request_channel(struct mbox_client *cl, int subspace_id)
>   	pcc_mchan = &pchan->chan;
>   	pcc_mchan->shmem = acpi_os_ioremap(pcc_mchan->shmem_base_addr,
>   					   pcc_mchan->shmem_size);
> -	if (pcc_mchan->shmem)
> -		return pcc_mchan;
> +	if (!pcc_mchan->shmem)
> +		goto err;
> +
> +	pcc_mchan->manage_writes = false;
> +
> +	/* This indicates that the channel is ready to accept messages.
> +	 * This needs to happen after the channel has registered
> +	 * its callback. There is no access point to do that in
> +	 * the mailbox API. That implies that the mailbox client must
> +	 * have set the allocate callback function prior to
> +	 * sending any messages.
> +	 */
> +	if (pchan->type == ACPI_PCCT_TYPE_EXT_PCC_SLAVE_SUBSPACE)
> +		pcc_chan_reg_read_modify_write(&pchan->cmd_update);
Is there a better  way to do this?  The flag is not accessable from the 
driver.
> +
> +	return pcc_mchan;
>   
> +err:
>   	mbox_free_channel(chan);
>   	return ERR_PTR(-ENXIO);
>   }
> @@ -417,8 +460,38 @@ void pcc_mbox_free_channel(struct pcc_mbox_chan *pchan)
>   }
>   EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
>   
> +static int pcc_write_to_buffer(struct mbox_chan *chan, void *data)
> +{
> +	struct pcc_chan_info *pchan = chan->con_priv;
> +	struct pcc_mbox_chan *pcc_mbox_chan = &pchan->chan;
> +	struct pcc_header *pcc_header = data;
> +
> +	if (!pchan->chan.manage_writes)
> +		return 0;
> +
> +	/* The PCC header length includes the command field
> +	 * but not the other values from the header.
> +	 */
> +	int len = pcc_header->length - sizeof(u32) + sizeof(struct pcc_header);
> +	u64 val;
> +
> +	pcc_chan_reg_read(&pchan->cmd_complete, &val);
> +	if (!val) {
> +		pr_info("%s pchan->cmd_complete not set", __func__);
> +		return -1;
> +	}
> +	memcpy_toio(pcc_mbox_chan->shmem,  data, len);
> +	return 0;
> +}
> +

I think this is the pattern that we  want all of the PCC mailbox clients 
to migrate to.  Is there any reason it was not implmented this way 
originally?-

> +
>   /**
> - * pcc_send_data - Called from Mailbox Controller code. Used
> + * pcc_send_data - Called from Mailbox Controller code. If
> + *		pchan->chan.rx_alloc is set, then the command complete
> + *		flag is checked and the data is written to the shared
> + *		buffer io memory.
> + *
> + *		If pchan->chan.rx_alloc is not set, then it is used
>    *		here only to ring the channel doorbell. The PCC client
>    *		specific read/write is done in the client driver in
>    *		order to maintain atomicity over PCC channel once
> @@ -434,17 +507,37 @@ static int pcc_send_data(struct mbox_chan *chan, void *data)
>   	int ret;
>   	struct pcc_chan_info *pchan = chan->con_priv;
>   
> +	ret = pcc_write_to_buffer(chan, data);
> +	if (ret)
> +		return ret;
> +
>   	ret = pcc_chan_reg_read_modify_write(&pchan->cmd_update);
>   	if (ret)
>   		return ret;
>   
>   	ret = pcc_chan_reg_read_modify_write(&pchan->db);
> +
>   	if (!ret && pchan->plat_irq > 0)
>   		pchan->chan_in_use = true;
>   
>   	return ret;
>   }
>   
> +
> +static bool pcc_last_tx_done(struct mbox_chan *chan)
> +{
> +	struct pcc_chan_info *pchan = chan->con_priv;
> +	u64 val;
> +
> +	pcc_chan_reg_read(&pchan->cmd_complete, &val);
> +	if (!val)
> +		return false;
> +	else
> +		return true;
> +}
> +
> +
> +
>   /**
>    * pcc_startup - Called from Mailbox Controller code. Used here
>    *		to request the interrupt.
> @@ -490,6 +583,7 @@ static const struct mbox_chan_ops pcc_chan_ops = {
>   	.send_data = pcc_send_data,
>   	.startup = pcc_startup,
>   	.shutdown = pcc_shutdown,
> +	.last_tx_done = pcc_last_tx_done,
>   };
>   
>   /**
> diff --git a/include/acpi/pcc.h b/include/acpi/pcc.h
> index 840bfc95bae3..9af3b502f839 100644
> --- a/include/acpi/pcc.h
> +++ b/include/acpi/pcc.h
> @@ -17,6 +17,35 @@ struct pcc_mbox_chan {
>   	u32 latency;
>   	u32 max_access_rate;
>   	u16 min_turnaround_time;
> +
> +	/* Set to true to indicate that the mailbox should manage
> +	 * writing the dat to the shared buffer. This differs from
> +	 * the case where the drivesr are writing to the buffer and
> +	 * using send_data only to  ring the doorbell.  If this flag
> +	 * is set, then the void * data parameter of send_data must
> +	 * point to a kernel-memory buffer formatted in accordance with
> +	 * the PCC specification.
> +	 *
> +	 * The active buffer management will include reading the
> +	 * notify_on_completion flag, and will then
> +	 * call mbox_chan_txdone when the acknowledgment interrupt is
> +	 * received.
> +	 */
> +	bool manage_writes;
> +
> +	/* Optional callback that allows the driver
> +	 * to allocate the memory used for receiving
> +	 * messages.  The return value is the location
> +	 * inside the buffer where the mailbox should write the data.
> +	 */
> +	void *(*rx_alloc)(struct mbox_client *cl,  int size);
> +};
> +
> +struct pcc_header {
> +	u32 signature;
> +	u32 flags;
> +	u32 length;
> +	u32 command;
>   };

Fairly certain these should not be explicitly little endian IAW the 
spec. It has been a source of discussion in the past.


>   
>   /* Generic Communications Channel Shared Memory Region */

Is there some reason that this patch is not showing up in the 
maintainers queues for review?

On 7/22/25 13:10, Adam Young wrote:
>
> On 7/14/25 20:10, admiyo@os.amperecomputing.com wrote:
>> From: Adam Young <admiyo@os.amperecomputing.com>
>>
>> Define a new, optional, callback that allows the driver to
>> specify how the return data buffer is allocated.  If that callback
>> is set,  mailbox/pcc.c is now responsible for reading from and
>> writing to the PCC shared buffer.
>>
>> This also allows for proper checks of the Commnand complete flag
>> between the PCC sender and receiver.
>>
>> For Type 4 channels, initialize the command complete flag prior
>> to accepting messages.
>>
>> Since the mailbox does not know what memory allocation scheme
>> to use for response messages, the client now has an optional
>> callback that allows it to allocate the buffer for a response
>> message.
>>
>> When an outbound message is written to the buffer, the mailbox
>> checks for the flag indicating the client wants an tx complete
>> notification via IRQ.  Upon receipt of the interrupt It will
>> pair it with the outgoing message. The expected use is to
>> free the kernel memory buffer for the previous outgoing message.
>>
>> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>
>> ---
>>   drivers/mailbox/pcc.c | 102 ++++++++++++++++++++++++++++++++++++++++--
>>   include/acpi/pcc.h    |  29 ++++++++++++
>>   2 files changed, 127 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
>> index f6714c233f5a..0a00719b2482 100644
>> --- a/drivers/mailbox/pcc.c
>> +++ b/drivers/mailbox/pcc.c
>> @@ -306,6 +306,22 @@ static void pcc_chan_acknowledge(struct 
>> pcc_chan_info *pchan)
>>           pcc_chan_reg_read_modify_write(&pchan->db);
>>   }
>>   +static void *write_response(struct pcc_chan_info *pchan)
>> +{
>> +    struct pcc_header pcc_header;
>> +    void *buffer;
>> +    int data_len;
>> +
>> +    memcpy_fromio(&pcc_header, pchan->chan.shmem,
>> +              sizeof(pcc_header));
>> +    data_len = pcc_header.length - sizeof(u32) + sizeof(struct 
>> pcc_header);
>> +
>> +    buffer = pchan->chan.rx_alloc(pchan->chan.mchan->cl, data_len);
>> +    if (buffer != NULL)
>> +        memcpy_fromio(buffer, pchan->chan.shmem, data_len);
>> +    return buffer;
>> +}
>> +
>>   /**
>>    * pcc_mbox_irq - PCC mailbox interrupt handler
>>    * @irq:    interrupt number
>> @@ -317,6 +333,8 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>>   {
>>       struct pcc_chan_info *pchan;
>>       struct mbox_chan *chan = p;
>> +    struct pcc_header *pcc_header = chan->active_req;
>
> OK, so it looks a little strange to re-initialize this later. Would it 
> be better to not have it initialized?
>
>
>> +    void *handle = NULL;
>>         pchan = chan->con_priv;
>>   @@ -340,7 +358,17 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>>        * required to avoid any possible race in updatation of this flag.
>>        */
>>       pchan->chan_in_use = false;
>> -    mbox_chan_received_data(chan, NULL);
>> +
>> +    if (pchan->chan.rx_alloc)
>> +        handle = write_response(pchan);
>> +
>> +    if (chan->active_req) {
>> +        pcc_header = chan->active_req;
>> +        if (pcc_header->flags & PCC_CMD_COMPLETION_NOTIFY)
>
> Note that this is the counterpoint to my earlier patch that only 
> notifies the platform if the platform requests it.  This is part of 
> the specification.
>
>> +            mbox_chan_txdone(chan, 0);
>> +    }
>> +
>> +    mbox_chan_received_data(chan, handle);
>>         pcc_chan_acknowledge(pchan);
>>   @@ -384,9 +412,24 @@ pcc_mbox_request_channel(struct mbox_client 
>> *cl, int subspace_id)
>>       pcc_mchan = &pchan->chan;
>>       pcc_mchan->shmem = acpi_os_ioremap(pcc_mchan->shmem_base_addr,
>>                          pcc_mchan->shmem_size);
>> -    if (pcc_mchan->shmem)
>> -        return pcc_mchan;
>> +    if (!pcc_mchan->shmem)
>> +        goto err;
>> +
>> +    pcc_mchan->manage_writes = false;
>> +
>> +    /* This indicates that the channel is ready to accept messages.
>> +     * This needs to happen after the channel has registered
>> +     * its callback. There is no access point to do that in
>> +     * the mailbox API. That implies that the mailbox client must
>> +     * have set the allocate callback function prior to
>> +     * sending any messages.
>> +     */
>> +    if (pchan->type == ACPI_PCCT_TYPE_EXT_PCC_SLAVE_SUBSPACE)
>> + pcc_chan_reg_read_modify_write(&pchan->cmd_update);
> Is there a better  way to do this?  The flag is not accessable from 
> the driver.
>> +
>> +    return pcc_mchan;
>>   +err:
>>       mbox_free_channel(chan);
>>       return ERR_PTR(-ENXIO);
>>   }
>> @@ -417,8 +460,38 @@ void pcc_mbox_free_channel(struct pcc_mbox_chan 
>> *pchan)
>>   }
>>   EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
>>   +static int pcc_write_to_buffer(struct mbox_chan *chan, void *data)
>> +{
>> +    struct pcc_chan_info *pchan = chan->con_priv;
>> +    struct pcc_mbox_chan *pcc_mbox_chan = &pchan->chan;
>> +    struct pcc_header *pcc_header = data;
>> +
>> +    if (!pchan->chan.manage_writes)
>> +        return 0;
>> +
>> +    /* The PCC header length includes the command field
>> +     * but not the other values from the header.
>> +     */
>> +    int len = pcc_header->length - sizeof(u32) + sizeof(struct 
>> pcc_header);
>> +    u64 val;
>> +
>> +    pcc_chan_reg_read(&pchan->cmd_complete, &val);
>> +    if (!val) {
>> +        pr_info("%s pchan->cmd_complete not set", __func__);
>> +        return -1;
>> +    }
>> +    memcpy_toio(pcc_mbox_chan->shmem,  data, len);
>> +    return 0;
>> +}
>> +
>
> I think this is the pattern that we  want all of the PCC mailbox 
> clients to migrate to.  Is there any reason it was not implmented this 
> way originally?-
>
>> +
>>   /**
>> - * pcc_send_data - Called from Mailbox Controller code. Used
>> + * pcc_send_data - Called from Mailbox Controller code. If
>> + *        pchan->chan.rx_alloc is set, then the command complete
>> + *        flag is checked and the data is written to the shared
>> + *        buffer io memory.
>> + *
>> + *        If pchan->chan.rx_alloc is not set, then it is used
>>    *        here only to ring the channel doorbell. The PCC client
>>    *        specific read/write is done in the client driver in
>>    *        order to maintain atomicity over PCC channel once
>> @@ -434,17 +507,37 @@ static int pcc_send_data(struct mbox_chan 
>> *chan, void *data)
>>       int ret;
>>       struct pcc_chan_info *pchan = chan->con_priv;
>>   +    ret = pcc_write_to_buffer(chan, data);
>> +    if (ret)
>> +        return ret;
>> +
>>       ret = pcc_chan_reg_read_modify_write(&pchan->cmd_update);
>>       if (ret)
>>           return ret;
>>         ret = pcc_chan_reg_read_modify_write(&pchan->db);
>> +
>>       if (!ret && pchan->plat_irq > 0)
>>           pchan->chan_in_use = true;
>>         return ret;
>>   }
>>   +
>> +static bool pcc_last_tx_done(struct mbox_chan *chan)
>> +{
>> +    struct pcc_chan_info *pchan = chan->con_priv;
>> +    u64 val;
>> +
>> +    pcc_chan_reg_read(&pchan->cmd_complete, &val);
>> +    if (!val)
>> +        return false;
>> +    else
>> +        return true;
>> +}
>> +
>> +
>> +
>>   /**
>>    * pcc_startup - Called from Mailbox Controller code. Used here
>>    *        to request the interrupt.
>> @@ -490,6 +583,7 @@ static const struct mbox_chan_ops pcc_chan_ops = {
>>       .send_data = pcc_send_data,
>>       .startup = pcc_startup,
>>       .shutdown = pcc_shutdown,
>> +    .last_tx_done = pcc_last_tx_done,
>>   };
>>     /**
>> diff --git a/include/acpi/pcc.h b/include/acpi/pcc.h
>> index 840bfc95bae3..9af3b502f839 100644
>> --- a/include/acpi/pcc.h
>> +++ b/include/acpi/pcc.h
>> @@ -17,6 +17,35 @@ struct pcc_mbox_chan {
>>       u32 latency;
>>       u32 max_access_rate;
>>       u16 min_turnaround_time;
>> +
>> +    /* Set to true to indicate that the mailbox should manage
>> +     * writing the dat to the shared buffer. This differs from
>> +     * the case where the drivesr are writing to the buffer and
>> +     * using send_data only to  ring the doorbell.  If this flag
>> +     * is set, then the void * data parameter of send_data must
>> +     * point to a kernel-memory buffer formatted in accordance with
>> +     * the PCC specification.
>> +     *
>> +     * The active buffer management will include reading the
>> +     * notify_on_completion flag, and will then
>> +     * call mbox_chan_txdone when the acknowledgment interrupt is
>> +     * received.
>> +     */
>> +    bool manage_writes;
>> +
>> +    /* Optional callback that allows the driver
>> +     * to allocate the memory used for receiving
>> +     * messages.  The return value is the location
>> +     * inside the buffer where the mailbox should write the data.
>> +     */
>> +    void *(*rx_alloc)(struct mbox_client *cl,  int size);
>> +};
>> +
>> +struct pcc_header {
>> +    u32 signature;
>> +    u32 flags;
>> +    u32 length;
>> +    u32 command;
>>   };
>
> Fairly certain these should not be explicitly little endian IAW the 
> spec. It has been a source of discussion in the past.
>
>
>>     /* Generic Communications Channel Shared Memory Region */

On Mon, Jul 14, 2025 at 08:10:07PM -0400, admiyo@os.amperecomputing.com wrote:
> From: Adam Young <admiyo@os.amperecomputing.com>
> 
> Define a new, optional, callback that allows the driver to
> specify how the return data buffer is allocated.  If that callback
> is set,  mailbox/pcc.c is now responsible for reading from and
> writing to the PCC shared buffer.
> 
> This also allows for proper checks of the Commnand complete flag
> between the PCC sender and receiver.

Command

> 
> For Type 4 channels, initialize the command complete flag prior
> to accepting messages.
> 
> Since the mailbox does not know what memory allocation scheme
> to use for response messages, the client now has an optional
> callback that allows it to allocate the buffer for a response
> message.
> 
> When an outbound message is written to the buffer, the mailbox
> checks for the flag indicating the client wants an tx complete
> notification via IRQ.  Upon receipt of the interrupt It will
> pair it with the outgoing message. The expected use is to
> free the kernel memory buffer for the previous outgoing message.
> 
> Signed-off-by: Adam Young <admiyo@os.amperecomputing.com>

...