[PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Sayantan Nandy]

The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
sub-system, an extra 4 byte tag is added to each frame. To allow users
to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
to 9220 bytes (9216+4).

Signed-off-by: Sayantan Nandy <sayantann11@gmail.com>
---
 drivers/net/ethernet/airoha/airoha_eth.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/airoha/airoha_eth.h b/drivers/net/ethernet/airoha/airoha_eth.h
index fbbc58133364..20e602d61e61 100644
--- a/drivers/net/ethernet/airoha/airoha_eth.h
+++ b/drivers/net/ethernet/airoha/airoha_eth.h
@@ -21,7 +21,7 @@
 #define AIROHA_MAX_NUM_IRQ_BANKS	4
 #define AIROHA_MAX_DSA_PORTS		7
 #define AIROHA_MAX_NUM_RSTS		3
-#define AIROHA_MAX_MTU			9216
+#define AIROHA_MAX_MTU			9220
 #define AIROHA_MAX_PACKET_SIZE		2048
 #define AIROHA_NUM_QOS_CHANNELS		4
 #define AIROHA_NUM_QOS_QUEUES		8
-- 
2.43.0

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Lorenzo Bianconi]

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> The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
> sub-system, an extra 4 byte tag is added to each frame. To allow users
> to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
> to 9220 bytes (9216+4).
> 
> Signed-off-by: Sayantan Nandy <sayantann11@gmail.com>

I think the patch is fine, but here you are missing to specify this is v2
and this patch targets net-next. Moreover, please wait 24h before reposting
a new version of the same patch.

Regards,
Lorenzo

> ---
>  drivers/net/ethernet/airoha/airoha_eth.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/airoha/airoha_eth.h b/drivers/net/ethernet/airoha/airoha_eth.h
> index fbbc58133364..20e602d61e61 100644
> --- a/drivers/net/ethernet/airoha/airoha_eth.h
> +++ b/drivers/net/ethernet/airoha/airoha_eth.h
> @@ -21,7 +21,7 @@
>  #define AIROHA_MAX_NUM_IRQ_BANKS	4
>  #define AIROHA_MAX_DSA_PORTS		7
>  #define AIROHA_MAX_NUM_RSTS		3
> -#define AIROHA_MAX_MTU			9216
> +#define AIROHA_MAX_MTU			9220
>  #define AIROHA_MAX_PACKET_SIZE		2048
>  #define AIROHA_NUM_QOS_CHANNELS		4
>  #define AIROHA_NUM_QOS_QUEUES		8
> -- 
> 2.43.0
> 

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Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Jakub Kicinski]

On Thu, 15 Jan 2026 14:40:03 +0100 Lorenzo Bianconi wrote:
> > The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
> > sub-system, an extra 4 byte tag is added to each frame. To allow users
> > to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
> > to 9220 bytes (9216+4).
> > 
> > Signed-off-by: Sayantan Nandy <sayantann11@gmail.com>  
> 
> I think the patch is fine, but here you are missing to specify this is v2
> and this patch targets net-next. Moreover, please wait 24h before reposting
> a new version of the same patch.

FWIW this is a good reminder for Sayantan for the future but it doesn't
merit a repost in itself. net-next is our default tree, and pw bot
guessed correctly.

That said I think Andrews request warrants some extra testing here so:
-- 
pw-bot: cr

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Andrew Lunn]

On Thu, Jan 15, 2026 at 02:18:37PM +0530, Sayantan Nandy wrote:
> The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
> sub-system, an extra 4 byte tag is added to each frame. To allow users
> to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
> to 9220 bytes (9216+4).

What does the hardware actually support? Is 9220 the real limit? 10K?
16K?

	Andrew

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Benjamin Larsson]

On 15/01/2026 18:41, Andrew Lunn wrote:
> On Thu, Jan 15, 2026 at 02:18:37PM +0530, Sayantan Nandy wrote:
>> The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
>> sub-system, an extra 4 byte tag is added to each frame. To allow users
>> to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
>> to 9220 bytes (9216+4).
> What does the hardware actually support? Is 9220 the real limit? 10K?
> 16K?
>
> 	Andrew
>
Hi, datasheets say 16k and I have observed packet sizes close to that on 
the previous SoC generation EN7523 on the tx path.

MvH

Benjamin Larsson

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Andrew Lunn]

On Thu, Jan 15, 2026 at 08:10:20PM +0100, Benjamin Larsson wrote:
> On 15/01/2026 18:41, Andrew Lunn wrote:
> > On Thu, Jan 15, 2026 at 02:18:37PM +0530, Sayantan Nandy wrote:
> > > The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
> > > sub-system, an extra 4 byte tag is added to each frame. To allow users
> > > to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
> > > to 9220 bytes (9216+4).
> > What does the hardware actually support? Is 9220 the real limit? 10K?
> > 16K?
> > 
> > 	Andrew
> > 
> Hi, datasheets say 16k and I have observed packet sizes close to that on the
> previous SoC generation EN7523 on the tx path.

Can you test 16K?

Does it make any difference to the memory allocation? Some drivers
allocate receive buffers based on the MAX MTU, not the current MTU, so
can eat up a lot of memory which is unlikely to be used. We should try
to avoid that.

Thanks
	Andrew

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Benjamin Larsson]

Hi.

On 16/01/2026 02:08, Andrew Lunn wrote:
> On Thu, Jan 15, 2026 at 08:10:20PM +0100, Benjamin Larsson wrote:
>> On 15/01/2026 18:41, Andrew Lunn wrote:
>>> On Thu, Jan 15, 2026 at 02:18:37PM +0530, Sayantan Nandy wrote:
>>>> The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
>>>> sub-system, an extra 4 byte tag is added to each frame. To allow users
>>>> to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
>>>> to 9220 bytes (9216+4).
>>> What does the hardware actually support? Is 9220 the real limit? 10K?
>>> 16K?
>>>
>>> 	Andrew
>>>
>> Hi, datasheets say 16k and I have observed packet sizes close to that on the
>> previous SoC generation EN7523 on the tx path.
> Can you test 16K?

I probably can but it would take some time (weeks) as I dont have any 
current setup with AN7581.

>
> Does it make any difference to the memory allocation? Some drivers
> allocate receive buffers based on the MAX MTU, not the current MTU, so
> can eat up a lot of memory which is unlikely to be used. We should try
> to avoid that.
>
> Thanks
> 	Andrew

Larger packets will consume more dma descriptors (a larger packet will 
be split into several dma descriptors). So you dont allocate more memory 
to be able to send jumbo frames.

MvH

Benjamin Larsson

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Andrew Lunn]

On Fri, Jan 16, 2026 at 11:48:30AM +0100, Benjamin Larsson wrote:
> Hi.
> 
> On 16/01/2026 02:08, Andrew Lunn wrote:
> > On Thu, Jan 15, 2026 at 08:10:20PM +0100, Benjamin Larsson wrote:
> > > On 15/01/2026 18:41, Andrew Lunn wrote:
> > > > On Thu, Jan 15, 2026 at 02:18:37PM +0530, Sayantan Nandy wrote:
> > > > > The Industry standard for jumbo frame MTU is 9216 bytes. When using DSA
> > > > > sub-system, an extra 4 byte tag is added to each frame. To allow users
> > > > > to set the standard 9216-byte MTU via ifconfig,increase AIROHA_MAX_MTU
> > > > > to 9220 bytes (9216+4).
> > > > What does the hardware actually support? Is 9220 the real limit? 10K?
> > > > 16K?
> > > > 
> > > > 	Andrew
> > > > 
> > > Hi, datasheets say 16k and I have observed packet sizes close to that on the
> > > previous SoC generation EN7523 on the tx path.
> > Can you test 16K?
> 
> I probably can but it would take some time (weeks) as I dont have any
> current setup with AN7581.
> 
> > 
> > Does it make any difference to the memory allocation? Some drivers
> > allocate receive buffers based on the MAX MTU, not the current MTU, so
> > can eat up a lot of memory which is unlikely to be used. We should try
> > to avoid that.
> > 
> > Thanks
> > 	Andrew
> 
> Larger packets will consume more dma descriptors (a larger packet will be
> split into several dma descriptors). So you dont allocate more memory to be
> able to send jumbo frames.

So it 'costs' nothing to declare the actual maximum.

For the moment, 9220 seems acceptable, but please include in the
commit message that the real maximum is higher, probably somewhere
around 16K.

	Andrew

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Andrew Lunn]

On Fri, Jan 16, 2026 at 08:09:51PM +0530, sayantan nandy wrote:
> Hi all,
> 
> Thanks for the review and comments.
> 
> I checked the AN7581 HW and it does support MTU sizes up to 16K. However, as
> mentioned by Benjamin, larger packets consume more DMA descriptors, and while
> no extra buffers are allocated, this can put pressure on the descriptor rings.

Does the hardware consume DMA descriptors for the full 16K, not just
the number of descriptors needed for the actual received packet size?
That seems like a bad design.

      Andrew

Re: [PATCH] net: airoha_eth: increase max mtu to 9220 for DSA jumbo frames

[Benjamin Larsson]

On 16/01/2026 15:56, Andrew Lunn wrote:
> On Fri, Jan 16, 2026 at 08:09:51PM +0530, sayantan nandy wrote:
>> Hi all,
>>
>> Thanks for the review and comments.
>>
>> I checked the AN7581 HW and it does support MTU sizes up to 16K. However, as
>> mentioned by Benjamin, larger packets consume more DMA descriptors, and while
>> no extra buffers are allocated, this can put pressure on the descriptor rings.
> Does the hardware consume DMA descriptors for the full 16K, not just
> the number of descriptors needed for the actual received packet size?
> That seems like a bad design.
>
>        Andrew

The descriptor buffer size is configurable. The amount of used 
descriptors will be the received packet size % descriptor buffer size + 1.

MvH

Benjamin Larsson