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

[Théo Lebrun]

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