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

[Charles Perry <charles.perry@microchip.com>]

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
>