Re: [PATCH net] bnxt_en: Prevent queue stop with deferred completions

[Eric Dumazet]

On Fri, Aug 28, 2026 at 1:09 AM Joe Damato <joe@xxxxxxx> wrote:
>
> When the driver receives a burst of packets, it can mark a BD with the
> NO_CMPL bit to defer completions. The expectation is that the last
> packet in the ring will have this bit unset and the completion generated
> by that packet will cleanup that packet and the ones preceding it. This
> helps to reduce the number of completions fired.
>
> The suppressed completions are controlled by the driver and the number
> of packets with suppressed completions scales with the size of the ring.
> SW USO packets, on the other hand, have an upper bound on the maximum
> number of BDs which can be consumed which does not scale with the ring
> size.
>
> So, for small rings it is possible that: a burst of packets is handed to
> the driver, the driver defers completions for all of the packets because
> the number of free descriptors stays above the threshold in the driver.
> Then, a USO packet arrives, but the number of BDs available is not
> enough and the USO code exits early with NETDEV_TX_BUSY.
>
> In this case, you end up in a state where the ring is full of packets
> with their completions suppressed, which can cause the queue to stop and
> never be restarted.
>
> Assuming default CONFIG_MAX_SKB_FRAGS, this is only possible for small
> rings (<= 457 descriptors, below the driver default value) when
> a burst of packets fills the ring, followed by a large USO packet that
> can't fit. For larger rings, the delta between the completion
> suppression threshold and the BDs required for SW USO is large enough
> that completions will fire and this case is unreachable.
>
> This issue was pointed out by Sashiko and while it seems fairly unlikely
> given that the queue size must be small to trigger this, it is indeed
> possible.
>
> Fix this by tracking the last BD which deferred completions. If SW USO
> exits early and there is a doorbell pending, enable completions for the
> last packet in the ring with disabled completions. This ensures that a
> completion will be generated and avoids stopping the queue with no way
> to start it again.
>
> Fixes: cc5d90667db8 ("net: bnxt: Implement software USO")
> Cc: <stable@xxxxxxxxxxxxxxx> # v7.1+: 4e15e89faac9: net: bnxt: ring the doorbell when SW USO exits early
> Signed-off-by: Joe Damato <joe@xxxxxxx>
> ---

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>