[v3] Initial support for PIC64-HPSC/HX Ethernet endpoint

[PATCH net-next v3 2/3] net: macb: add safeguards for jumbo frame larger than 10240

Posted by Charles Perry 3 weeks, 3 days ago

The RX buffers for GEM can have a maximum size of 16320 bytes
(0xff in the RXBS field of the DMACFG register means 255*64 =
16320 bytes).

The GEM IP has configurable maximum jumbo frame length that can go up to
16383. The actual value for this limit can be found in the
       "jumbo_max_length" field (bits 0..13) of the DCFG2 register.
Currently, the macb driver doesn't use the DCFG2 register when
determining the max MTU, instead an hardcoded value (jumbo_max_len in
struct macb_config) is used for each platform. Right now the maximum
value for jumbo_max_len is 10240 (0x2800).

GEM uses one buffer per packet which means that one buffer must allow
room for the max MTU plus L2 encapsulation and alignment. This is a
limitation of the driver.

This commit adds a limit to max_mtu and rx_buffer_size so that the RXBS
field can never overflow when a large MTU is used.

With this commit, it is now possible to add new platforms with a
jumbo_max_len of 16383 so that the hardware properties of each IP can be
properly captured in struct macb_config.

Signed-off-by: Charles Perry <charles.perry@microchip.com>
Reviewed-by: Simon Horman <horms@kernel.org>
---
 drivers/net/ethernet/cadence/macb_main.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index 32f9463edff0..c653ea6cadef 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -52,6 +52,7 @@ struct sifive_fu540_macb_mgmt {
 
 #define MACB_RX_BUFFER_SIZE	128
 #define RX_BUFFER_MULTIPLE	64  /* bytes */
+#define RX_BUFFER_MAX		(0xFF * RX_BUFFER_MULTIPLE) /* 16320 bytes */
 
 #define DEFAULT_RX_RING_SIZE	512 /* must be power of 2 */
 #define MIN_RX_RING_SIZE	64
@@ -2603,7 +2604,7 @@ static void macb_init_rx_buffer_size(struct macb *bp, size_t size)
 	if (!macb_is_gem(bp)) {
 		bp->rx_buffer_size = MACB_RX_BUFFER_SIZE;
 	} else {
-		bp->rx_buffer_size = size;
+		bp->rx_buffer_size = MIN(size, RX_BUFFER_MAX);
 
 		if (bp->rx_buffer_size % RX_BUFFER_MULTIPLE) {
 			netdev_dbg(bp->dev,
@@ -5794,7 +5795,8 @@ static int macb_probe(struct platform_device *pdev)
 	/* MTU range: 68 - 1518 or 10240 */
 	dev->min_mtu = GEM_MTU_MIN_SIZE;
 	if ((bp->caps & MACB_CAPS_JUMBO) && bp->jumbo_max_len)
-		dev->max_mtu = bp->jumbo_max_len - ETH_HLEN - ETH_FCS_LEN;
+		dev->max_mtu = MIN(bp->jumbo_max_len, RX_BUFFER_MAX) -
+				ETH_HLEN - ETH_FCS_LEN;
 	else
 		dev->max_mtu = 1536 - ETH_HLEN - ETH_FCS_LEN;
 
-- 
2.47.3

Re: [PATCH net-next v3 2/3] net: macb: add safeguards for jumbo frame larger than 10240

Posted by Théo Lebrun 3 weeks ago

Hello Charles,

On Fri Mar 13, 2026 at 3:06 PM CET, Charles Perry wrote:
> The RX buffers for GEM can have a maximum size of 16320 bytes
> (0xff in the RXBS field of the DMACFG register means 255*64 =
> 16320 bytes).
>
> The GEM IP has configurable maximum jumbo frame length that can go up to
> 16383. The actual value for this limit can be found in the
>        "jumbo_max_length" field (bits 0..13) of the DCFG2 register.
> Currently, the macb driver doesn't use the DCFG2 register when
> determining the max MTU, instead an hardcoded value (jumbo_max_len in
> struct macb_config) is used for each platform. Right now the maximum
> value for jumbo_max_len is 10240 (0x2800).

If DCFG2 contains the value then we can runtime detect it. With that, we
could make the macb_config->jumbo_max_len attribute optional. Then
start dropping it from platforms where we know we can trust the DCFG2
value.

An alternative would be to validate macb_config->jumbo_max_len against
the DCFG2 value, but that is less useful.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Re: [PATCH net-next v3 2/3] net: macb: add safeguards for jumbo frame larger than 10240

Posted by Charles Perry 3 weeks ago

On Mon, Mar 16, 2026 at 06:21:38PM +0100, Théo Lebrun wrote:
> Hello Charles,
> 
> On Fri Mar 13, 2026 at 3:06 PM CET, Charles Perry wrote:
> > The RX buffers for GEM can have a maximum size of 16320 bytes
> > (0xff in the RXBS field of the DMACFG register means 255*64 =
> > 16320 bytes).
> >
> > The GEM IP has configurable maximum jumbo frame length that can go up to
> > 16383. The actual value for this limit can be found in the
> >        "jumbo_max_length" field (bits 0..13) of the DCFG2 register.
> > Currently, the macb driver doesn't use the DCFG2 register when
> > determining the max MTU, instead an hardcoded value (jumbo_max_len in
> > struct macb_config) is used for each platform. Right now the maximum
> > value for jumbo_max_len is 10240 (0x2800).
> 
> If DCFG2 contains the value then we can runtime detect it. With that, we
> could make the macb_config->jumbo_max_len attribute optional. Then
> start dropping it from platforms where we know we can trust the DCFG2
> value.
> 

Hello Théo,

That would be a good idea. We could use "jumbo_max_len == 0" as a way to
signal that the DCFG2 register should be used for determining the max MTU.

However, that's a new feature and it doesn't belong in this patch. All I
want to do in this patchset is put the real value of jumbo_max_length in
the PIC64-HPSC macb_config and make sure the driver doesn't overflow when
that's used.

Thanks,
Charles

> An alternative would be to validate macb_config->jumbo_max_len against
> the DCFG2 value, but that is less useful.
> 
> Thanks,
> 
> --
> Théo Lebrun, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com
>

Re: [PATCH net-next v3 2/3] net: macb: add safeguards for jumbo frame larger than 10240

Posted by Paolo Abeni 2 weeks, 6 days ago

On 3/16/26 7:26 PM, Charles Perry wrote:
> On Mon, Mar 16, 2026 at 06:21:38PM +0100, Théo Lebrun wrote:
>> Hello Charles,
>>
>> On Fri Mar 13, 2026 at 3:06 PM CET, Charles Perry wrote:
>>> The RX buffers for GEM can have a maximum size of 16320 bytes
>>> (0xff in the RXBS field of the DMACFG register means 255*64 =
>>> 16320 bytes).
>>>
>>> The GEM IP has configurable maximum jumbo frame length that can go up to
>>> 16383. The actual value for this limit can be found in the
>>>        "jumbo_max_length" field (bits 0..13) of the DCFG2 register.
>>> Currently, the macb driver doesn't use the DCFG2 register when
>>> determining the max MTU, instead an hardcoded value (jumbo_max_len in
>>> struct macb_config) is used for each platform. Right now the maximum
>>> value for jumbo_max_len is 10240 (0x2800).
>>
>> If DCFG2 contains the value then we can runtime detect it. With that, we
>> could make the macb_config->jumbo_max_len attribute optional. Then
>> start dropping it from platforms where we know we can trust the DCFG2
>> value.
>>
> 
> Hello Théo,
> 
> That would be a good idea. We could use "jumbo_max_len == 0" as a way to
> signal that the DCFG2 register should be used for determining the max MTU.
> 
> However, that's a new feature and it doesn't belong in this patch. All I
> want to do in this patchset is put the real value of jumbo_max_length in
> the PIC64-HPSC macb_config and make sure the driver doesn't overflow when
> that's used.

FWIW, I agree that is better suited for a follow-up than for the initial
bring-up.

/P

Re: [PATCH net-next v3 2/3] net: macb: add safeguards for jumbo frame larger than 10240

Posted by Théo Lebrun 2 weeks, 6 days ago

On Tue Mar 17, 2026 at 1:29 PM CET, Paolo Abeni wrote:
> On 3/16/26 7:26 PM, Charles Perry wrote:
>> On Mon, Mar 16, 2026 at 06:21:38PM +0100, Théo Lebrun wrote:
>>> Hello Charles,
>>>
>>> On Fri Mar 13, 2026 at 3:06 PM CET, Charles Perry wrote:
>>>> The RX buffers for GEM can have a maximum size of 16320 bytes
>>>> (0xff in the RXBS field of the DMACFG register means 255*64 =
>>>> 16320 bytes).
>>>>
>>>> The GEM IP has configurable maximum jumbo frame length that can go up to
>>>> 16383. The actual value for this limit can be found in the
>>>>        "jumbo_max_length" field (bits 0..13) of the DCFG2 register.
>>>> Currently, the macb driver doesn't use the DCFG2 register when
>>>> determining the max MTU, instead an hardcoded value (jumbo_max_len in
>>>> struct macb_config) is used for each platform. Right now the maximum
>>>> value for jumbo_max_len is 10240 (0x2800).
>>>
>>> If DCFG2 contains the value then we can runtime detect it. With that, we
>>> could make the macb_config->jumbo_max_len attribute optional. Then
>>> start dropping it from platforms where we know we can trust the DCFG2
>>> value.
>>>
>> 
>> Hello Théo,
>> 
>> That would be a good idea. We could use "jumbo_max_len == 0" as a way to
>> signal that the DCFG2 register should be used for determining the max MTU.
>> 
>> However, that's a new feature and it doesn't belong in this patch. All I
>> want to do in this patchset is put the real value of jumbo_max_length in
>> the PIC64-HPSC macb_config and make sure the driver doesn't overflow when
>> that's used.
>
> FWIW, I agree that is better suited for a follow-up than for the initial
> bring-up.

then:

Reviewed-by: Théo Lebrun <theo.lebrun@bootlin.com>

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com