Re: [PATCH net-next v4 8/8] net: bcmgenet: reassemble jumbo frames from status block fragments

[Florian Fainelli]

On 10/12/26 02:00, Nicolai Buchwitz wrote:
> The hardware does not truncate a frame longer than the packet ready
> threshold. It splits the frame across descriptors and writes a status block
> at the start of each one. The first fragment then arrives with SOP and no
> EOP and is dropped as fragmented. This caps the MTU.
>
> Strip the status blocks and reassemble the fragments. Only the last block
> holds the checksum of the whole frame. Broadcom confirmed from the RTL that
> every GENET revision splits long frames this way, not just the v5 this was
> tested on.
>
> On transmit the MAC only checksums a frame it holds in full, so anything
> longer than the threshold falls back to software. At jumbo sizes the larger
> frame saves more per packet overhead than the checksum costs.
>
> UMAC_MAX_FRAME_LEN is 14 bit and counts the FCS. That puts the maximum MTU
> at 16347.
>
> The TX descriptor length field is wider than DMA_BUFLENGTH_MASK suggests,
> that mask only extracts the length from the RX status word. A frame of
> 16361 bytes, the largest that MTU allows, came back byte exact 200 times
> over UMAC loopback.
>
> Suggested-by: Justin Chen <justin.chen@xxxxxxxxxxxx>
> Tested-by: Pierre-Marin Leclercq <pierremarinleclercq88@xxxxxxxxx>
> Signed-off-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>

Reviewed-by: Florian Fainelli <florian.fainelli@xxxxxxxxxxxx>
--
Florian