net: macb: add safeguards for jumbo frame larger than 10240

net: macb: add safeguards for jumbo frame larger than 10240

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>
Link: https://patch.msgid.link/20260313140610.3681752-3-charles.perry@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>