Re: [PATCH net] net: mana: Ring doorbell at 4 CQ wraparounds

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 25/02/2026 18:49, Long Li wrote:
> MANA hardware requires at least one doorbell ring every 8 wraparounds
> of the CQ. The driver rings the doorbell as a form of flow control to
> inform hardware that CQEs have been consumed.
> 
> The NAPI poll functions mana_poll_tx_cq() and mana_poll_rx_cq() can
> poll up to CQE_POLLING_BUFFER (512) completions per call. If the CQ
> has fewer than 512 entries, a single poll call can process more than
> 4 wraparounds without ringing the doorbell. The doorbell threshold
> check also uses ">" instead of ">=", delaying the ring by one extra
> CQE beyond 4 wraparounds. Combined, these issues can cause the driver
> to exceed the 8-wraparound hardware limit, leading to missed
> completions and stalled queues.
> 
> Fix this by capping the number of CQEs polled per call to 4 wraparounds
> of the CQ in both TX and RX paths. Also change the doorbell threshold
> from ">" to ">=" so the doorbell is rung as soon as 4 wraparounds are
> reached.
> 
> Cc: stable@vger.kernel.org
> Fixes: 58a63729c957 ("net: mana: Fix doorbell out of order violation and avoid unnecessary doorbell rings")
> Signed-off-by: Long Li <longli@microsoft.com>
> ---
>   drivers/net/ethernet/microsoft/mana/mana_en.c | 23 +++++++++++++++----
>   1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
> index 9919183ad39e..fe667e0d930d 100644
> --- a/drivers/net/ethernet/microsoft/mana/mana_en.c
> +++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
> @@ -1770,8 +1770,14 @@ static void mana_poll_tx_cq(struct mana_cq *cq)
>   	ndev = txq->ndev;
>   	apc = netdev_priv(ndev);
>   
> +	/* Limit CQEs polled to 4 wraparounds of the CQ to ensure the
> +	 * doorbell can be rung in time for the hardware's requirement
> +	 * of at least one doorbell ring every 8 wraparounds.
> +	 */
>   	comp_read = mana_gd_poll_cq(cq->gdma_cq, completions,
> -				    CQE_POLLING_BUFFER);
> +				    min_t(u32, (cq->gdma_cq->queue_size /

no need for min_t, simple min() can be used, queue_size is already u32

> +					   COMP_ENTRY_SIZE) * 4,
> +					  CQE_POLLING_BUFFER));
>   
>   	if (comp_read < 1)
>   		return;
> @@ -2156,7 +2162,14 @@ static void mana_poll_rx_cq(struct mana_cq *cq)
>   	struct mana_rxq *rxq = cq->rxq;
>   	int comp_read, i;
>   
> -	comp_read = mana_gd_poll_cq(cq->gdma_cq, comp, CQE_POLLING_BUFFER);
> +	/* Limit CQEs polled to 4 wraparounds of the CQ to ensure the
> +	 * doorbell can be rung in time for the hardware's requirement
> +	 * of at least one doorbell ring every 8 wraparounds.
> +	 */
> +	comp_read = mana_gd_poll_cq(cq->gdma_cq, comp,
> +				    min_t(u32, (cq->gdma_cq->queue_size /

same here

> +					   COMP_ENTRY_SIZE) * 4,
> +					  CQE_POLLING_BUFFER));
>   	WARN_ON_ONCE(comp_read > CQE_POLLING_BUFFER);
>   
>   	rxq->xdp_flush = false;