Re: [PATCH net v4] net: axienet: cap the TX poll return value at the NAPI budget

["Gupta, Suraj" <suraj.gupta2@amd.com>]

On 9/30/2026 12:45 PM, Sagi Maimon wrote:
> axienet_tx_poll() reclaims every completed TX descriptor in one pass:
> it passes lp->tx_bd_num to axienet_free_tx_chain() as the descriptor
> limit, and @budget is only used as the napi_consume_skb() bulk-free
> hint.  It then returns the number of packets reclaimed, which is bounded
> by the ring size rather than by the budget, so the poll can report more
> work than it was given.  This was seen after TX completion interrupts
> had not been taken for a while and a full ring was reclaimed at once:
> 
>    eth0: NAPI poll function axienet_tx_poll+0x0/0x180 [xilinx_emac]
>          returned 96, exceeding its budget of 64.
> 
> netpoll is affected as well.  poll_one_napi() polls with a budget of 0
> to reclaim the TX path only, and warns once if any work is reported.
> 
> Keep reclaiming the whole ring and cap only the value returned.
> Documentation/networking/napi.rst allows a poll to process any number
> of TX completions; it is the reported work that must stay within the
> budget.  Stopping the reclaim at the budget instead would also leave
> completed descriptors for a later poll, which does not come when
> napi_disable() is pending.  When more than @budget packets were
> reclaimed, returning @budget keeps the poll scheduled, and the next poll
> completes NAPI and re-enables the interrupt as before.  A budget of 0
> now yields 0.
> 
> Suggested-by: Paolo Abeni <pabeni@redhat.com>
> Fixes: 5a6caa2cfabb ("net: xilinx: axienet: Fix packet counting")
> Assisted-by: LLM sparse
> Signed-off-by: Sagi Maimon <maimon.sagi@gmail.com>
> ---
> 

LGTM

Reviewed-by: Suraj Gupta <suraj.gupta2@amd.com>

Thanks,
Suraj

> Notes:
>      Changes in v4:
>      - Different approach, as Paolo Abeni and the review of v3 suggested:
>        keep reclaiming the whole ring and cap only the value returned,
>        instead of stopping the reclaim at the budget.  napi.rst allows any
>        number of TX completions per poll, and stopping early could leave
>        completed descriptors behind while napi_disable() is pending.
>      - Subject changed to match; v1-v3 were "net: axienet: bound TX
>        completion cleanup by the NAPI budget".
>      - Fixes: now names 5a6caa2cfabb, which made the return value unbounded
>        (review of v3).
>      - Only axienet_tx_poll() and its kernel-doc change now.
>      - Suggested-by: Paolo Abeni.
>      - Build-tested only so far; v3 was tested on hardware, v4 not yet.
>      - v3: https://lore.kernel.org/netdev/20260924135052.185129-1-maimon.sagi@gmail.com/
>      
>      Changes in v3:
>      - Report no work for a budget of 0, so netpoll cannot trip the
>        WARN_ONCE() in poll_one_napi().
>      - Add the Assisted-by: tag that v1 and v2 omitted.
>      - v2: https://lore.kernel.org/netdev/20260917115657.20697-1-maimon.sagi@gmail.com/
>      
>      Changes in v2:
>      - Treat a budget of 0 as no limit, so that netpoll still drains the TX
>        ring.
>      - v1: https://lore.kernel.org/netdev/20260914114821.55503-1-maimon.sagi@gmail.com/
> 
>   drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index 1722b7038f34..243b07fd5be8 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -984,9 +984,9 @@ axienet_start_xmit_dmaengine(struct sk_buff *skb, struct net_device *ndev)
>    * axienet_tx_poll - Invoked once a transmit is completed by the
>    * Axi DMA Tx channel.
>    * @napi:	Pointer to NAPI structure.
> - * @budget:	Max number of TX packets to process.
> + * @budget:	NAPI budget, or 0 when polled by netpoll.
>    *
> - * Return: Number of TX packets processed.
> + * Return: Number of TX packets processed, capped at @budget.
>    *
>    * This function is invoked from the NAPI processing to notify the completion
>    * of transmit operation. It clears fields in the corresponding Tx BDs and
> @@ -1027,7 +1027,12 @@ static int axienet_tx_poll(struct napi_struct *napi, int budget)
>   		axienet_dma_out32(lp, XAXIDMA_TX_CR_OFFSET, lp->tx_dma_cr);
>   		spin_unlock_irq(&lp->tx_cr_lock);
>   	}
> -	return packets;
> +
> +	/* The whole ring was reclaimed above, which may be more than the
> +	 * budget, but a poll must not report more work than it was given.
> +	 * netpoll polls with a budget of 0 and expects no work reported.
> +	 */
> +	return min(packets, budget);
>   }
>   
>   /**
> 
> base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7