Current FastRPC message context uses a 12-bit mask where the upper
8 bits are context ID from idr_alloc_cyclic and the lower 4 bits
represent PD type. This design works for normal FastRPC calls but
doesn't work as expected for polling mode. To enable polling mode
support from DSP(DSP writes to poll memory), DSP expects a 16-bit
context where the upper 8 bits are context ID, the lower 4 bits are
PD type and the 5th bit from the end denotes async mode(not yet
upstreamed). If this bit is set, DSP disables polling. With the
current design, odd context IDs set this bit, causing DSP to skip
poll memory updates. Update the context mask to ensure a hole
which won't get populated, ensuring polling mode works as expected.
This is not a bug and the change is added to support polling mode.

Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
---
 drivers/misc/fastrpc.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 679cd8997a00..ccba3b6dfdfa 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -37,7 +37,8 @@
 #define FASTRPC_CTX_MAX (256)
 #define FASTRPC_INIT_HANDLE	1
 #define FASTRPC_DSP_UTILITIES_HANDLE	2
-#define FASTRPC_CTXID_MASK (0xFF0)
+#define FASTRPC_CTXID_MASK (0xFF00)
+#define FASTRPC_CTXID_POS (8)
 #define INIT_FILELEN_MAX (2 * 1024 * 1024)
 #define INIT_FILE_NAMELEN_MAX (128)
 #define FASTRPC_DEVICE_NAME	"fastrpc"
@@ -489,7 +490,7 @@ static void fastrpc_context_free(struct kref *ref)
 		fastrpc_buf_free(ctx->buf);
 
 	spin_lock_irqsave(&cctx->lock, flags);
-	idr_remove(&cctx->ctx_idr, ctx->ctxid >> 4);
+	idr_remove(&cctx->ctx_idr, ctx->ctxid >> FASTRPC_CTXID_POS);
 	spin_unlock_irqrestore(&cctx->lock, flags);
 
 	kfree(ctx->maps);
@@ -625,7 +626,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
 		spin_unlock_irqrestore(&cctx->lock, flags);
 		goto err_idr;
 	}
-	ctx->ctxid = ret << 4;
+	ctx->ctxid = ret << FASTRPC_CTXID_POS;
 	spin_unlock_irqrestore(&cctx->lock, flags);
 
 	kref_init(&ctx->refcount);
@@ -2451,7 +2452,7 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
 	if (len < sizeof(*rsp))
 		return -EINVAL;
 
-	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
+	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> FASTRPC_CTXID_POS);
 
 	spin_lock_irqsave(&cctx->lock, flags);
 	ctx = idr_find(&cctx->ctx_idr, ctxid);
-- 
2.34.1

On Thu, Nov 13, 2025 at 12:11:10PM +0530, Ekansh Gupta wrote:
> Current FastRPC message context uses a 12-bit mask where the upper
> 8 bits are context ID from idr_alloc_cyclic and the lower 4 bits
> represent PD type. This design works for normal FastRPC calls but
> doesn't work as expected for polling mode.

Doesn't work why / how?

> To enable polling mode
> support from DSP(DSP writes to poll memory), DSP expects a 16-bit
> context where the upper 8 bits are context ID, the lower 4 bits are
> PD type and the 5th bit from the end denotes async mode(not yet

What's in the middle between bits 5 and 8?

> upstreamed). If this bit is set, DSP disables polling. With the
> current design, odd context IDs set this bit, causing DSP to skip
> poll memory updates. Update the context mask to ensure a hole
> which won't get populated, ensuring polling mode works as expected.
> This is not a bug and the change is added to support polling mode.
> 
> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
> ---
>  drivers/misc/fastrpc.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 679cd8997a00..ccba3b6dfdfa 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -37,7 +37,8 @@
>  #define FASTRPC_CTX_MAX (256)
>  #define FASTRPC_INIT_HANDLE	1
>  #define FASTRPC_DSP_UTILITIES_HANDLE	2
> -#define FASTRPC_CTXID_MASK (0xFF0)
> +#define FASTRPC_CTXID_MASK (0xFF00)
> +#define FASTRPC_CTXID_POS (8)

Use FIELD_PREP instead.

>  #define INIT_FILELEN_MAX (2 * 1024 * 1024)
>  #define INIT_FILE_NAMELEN_MAX (128)
>  #define FASTRPC_DEVICE_NAME	"fastrpc"
> @@ -489,7 +490,7 @@ static void fastrpc_context_free(struct kref *ref)
>  		fastrpc_buf_free(ctx->buf);
>  
>  	spin_lock_irqsave(&cctx->lock, flags);
> -	idr_remove(&cctx->ctx_idr, ctx->ctxid >> 4);
> +	idr_remove(&cctx->ctx_idr, ctx->ctxid >> FASTRPC_CTXID_POS);
>  	spin_unlock_irqrestore(&cctx->lock, flags);
>  
>  	kfree(ctx->maps);
> @@ -625,7 +626,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
>  		spin_unlock_irqrestore(&cctx->lock, flags);
>  		goto err_idr;
>  	}
> -	ctx->ctxid = ret << 4;
> +	ctx->ctxid = ret << FASTRPC_CTXID_POS;
>  	spin_unlock_irqrestore(&cctx->lock, flags);
>  
>  	kref_init(&ctx->refcount);
> @@ -2451,7 +2452,7 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
>  	if (len < sizeof(*rsp))
>  		return -EINVAL;
>  
> -	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
> +	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> FASTRPC_CTXID_POS);
>  
>  	spin_lock_irqsave(&cctx->lock, flags);
>  	ctx = idr_find(&cctx->ctx_idr, ctxid);
> -- 
> 2.34.1
> 

-- 
With best wishes
Dmitry

On 11/19/2025 1:59 PM, Dmitry Baryshkov wrote:
> On Thu, Nov 13, 2025 at 12:11:10PM +0530, Ekansh Gupta wrote:
>> Current FastRPC message context uses a 12-bit mask where the upper
>> 8 bits are context ID from idr_alloc_cyclic and the lower 4 bits
>> represent PD type. This design works for normal FastRPC calls but
>> doesn't work as expected for polling mode.
> Doesn't work why / how?
Explained this in the next statements, I'll rephrase this.
>
>> To enable polling mode
>> support from DSP(DSP writes to poll memory), DSP expects a 16-bit
>> context where the upper 8 bits are context ID, the lower 4 bits are
>> PD type and the 5th bit from the end denotes async mode(not yet
> What's in the middle between bits 5 and 8?
It's unused as of now.
>
>> upstreamed). If this bit is set, DSP disables polling. With the
>> current design, odd context IDs set this bit, causing DSP to skip
>> poll memory updates. Update the context mask to ensure a hole
>> which won't get populated, ensuring polling mode works as expected.
>> This is not a bug and the change is added to support polling mode.
>>
>> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
>> ---
>>  drivers/misc/fastrpc.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
>> index 679cd8997a00..ccba3b6dfdfa 100644
>> --- a/drivers/misc/fastrpc.c
>> +++ b/drivers/misc/fastrpc.c
>> @@ -37,7 +37,8 @@
>>  #define FASTRPC_CTX_MAX (256)
>>  #define FASTRPC_INIT_HANDLE	1
>>  #define FASTRPC_DSP_UTILITIES_HANDLE	2
>> -#define FASTRPC_CTXID_MASK (0xFF0)
>> +#define FASTRPC_CTXID_MASK (0xFF00)
>> +#define FASTRPC_CTXID_POS (8)
> Use FIELD_PREP instead.
Checking this, thanks
>
>>  #define INIT_FILELEN_MAX (2 * 1024 * 1024)
>>  #define INIT_FILE_NAMELEN_MAX (128)
>>  #define FASTRPC_DEVICE_NAME	"fastrpc"
>> @@ -489,7 +490,7 @@ static void fastrpc_context_free(struct kref *ref)
>>  		fastrpc_buf_free(ctx->buf);
>>  
>>  	spin_lock_irqsave(&cctx->lock, flags);
>> -	idr_remove(&cctx->ctx_idr, ctx->ctxid >> 4);
>> +	idr_remove(&cctx->ctx_idr, ctx->ctxid >> FASTRPC_CTXID_POS);
>>  	spin_unlock_irqrestore(&cctx->lock, flags);
>>  
>>  	kfree(ctx->maps);
>> @@ -625,7 +626,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
>>  		spin_unlock_irqrestore(&cctx->lock, flags);
>>  		goto err_idr;
>>  	}
>> -	ctx->ctxid = ret << 4;
>> +	ctx->ctxid = ret << FASTRPC_CTXID_POS;
>>  	spin_unlock_irqrestore(&cctx->lock, flags);
>>  
>>  	kref_init(&ctx->refcount);
>> @@ -2451,7 +2452,7 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
>>  	if (len < sizeof(*rsp))
>>  		return -EINVAL;
>>  
>> -	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
>> +	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> FASTRPC_CTXID_POS);
>>  
>>  	spin_lock_irqsave(&cctx->lock, flags);
>>  	ctx = idr_find(&cctx->ctx_idr, ctxid);
>> -- 
>> 2.34.1
>>