Re: [PATCH net-next v4 7/8] net: bcmgenet: allow the MTU to be changed

[Florian Fainelli]

On 10/12/26 02:00, Nicolai Buchwitz wrote:
> The driver never sets dev->max_mtu, so the MTU is stuck at ETH_DATA_LEN.
>
> One descriptor reaches as far as the packet ready threshold, so derive the
> maximum from it. The threshold registers are 8 bit in units of 16 bytes and
> want a multiple of the 256 byte burst size. A descriptor is one page and
> also holds skb_shared_info behind the frame. On 4K pages the page is the
> tighter limit and leaves 3564 bytes. That includes room for a VLAN tag so a
> VLAN interface can run at the parent MTU. The MAC accepts a little more
> than one descriptor holds, so a double tagged frame at that maximum is
> dropped as fragmented until a later patch reassembles those.
>
> Resize the buffers and rewrite the registers in place. The PHY keeps
> running and the link stays up. Two MTUs at or below ETH_DATA_LEN derive the
> same values, so a change between them skips the restart.
>
> A failed allocation retries at the previous size. If that fails too, take
> the interface down. Running on rings that were never allocated is worse.
>
> Suggested-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
> Link: https://github.com/raspberrypi/linux/issues/5561
> Tested-by: Pierre-Marin Leclercq <pierremarinleclercq88@xxxxxxxxx>
> Signed-off-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>

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