Re: [PATCH net] eth: fbnic: ring the doorbell if a burst ends in a drop

[Simon Horman <horms@kernel.org>]

On Mon, Sep 14, 2026 at 07:23:27PM -0700, Jakub Kicinski wrote:
> fbnic_tx_map() skips the doorbell write, and the completion request,
> for every packet handed to it with xmit_more set, counting on the
> packet which ends the burst to publish them all. When that packet is
> dropped instead - skb_put_padto(), skb_cow_head() or a DMA mapping
> failure - nothing rings. The descriptors of the preceding packets stay
> invisible to the HW until the next transmit on that queue, which for a
> burst-then-idle workload may never come.
> 
> Remember the meta descriptor of the last packet left without a doorbell
> and flush it from the error paths. The completion request has to be set
> on that descriptor rather than simply writing the tail, otherwise the HW
> would transmit the packets but never report a head, and the ring would
> fill up and stall for good.
> 
> This is very similar to Joe's recent series of fixes for bnxt.
> Not seen in real life, reproduced under QEMU with failure injection.
> 
> Fixes: 9a57bacd574b ("eth: fbnic: Add basic Tx handling")
> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
> Reproduced under the QEMU fbnic model by failing the DMA mapping of the
> last segment of a software-GSO burst: 21 descriptors (7 packets) stayed
> in the ring with head stuck at 24 and tail at 45, 10094 bytes stuck in
> BQL, and QEMU saw none of the 7 frames.

Reviewed-by: Simon Horman <horms@kernel.org>