Re: [PATCH/RFC net-next] net: fec: allow "mini jumbo" frames

[Vivien Didelot <vivien.didelot@savoirfairelinux.com>]

Hi Nikita,

Nikita Yushchenko <nikita.yoush@cogentembedded.com> writes:

> This adds support for MTU slightly larger than default, on modern
> FEC flavours.
>
> Currently FEC driver uses single hardware Rx buffer per frame. On most
> FEC flavours, size of single buffer is limited by 11-bit field, and
> has to be multiple of 64 (in the worst case). Thus maximum usable Rx
> buffer size is 1984 bytes.
>
> Of those:
> - 2 bytes are used for IP header alignment,
> - 14 bytes are used by ethhdr,
> - up to 8 bytes are needed for VLAN and/or DSA tags,
> - 4 bytes are needed for CRC.
>
> Thus maximum MTU possible within current RX architecture is 1956.
>
> This patch allows exactly that. For further increase, Rx architecture
> change is needed.
>
> Use of MTU=1956 gives about 1.5% throughput improvement between two Vybrid
> boards, compared to default MTU=1500.
>
> Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>

For what it's worth, I have tested your patch on my ZII Rev B boards
(see vf610-zii-dev-rev-b.dts) which have a FEC as the master net device
of their DSA trees. They still work as expected.

Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>

Thanks,

        Vivien