Subject: Re: [PATCH net v2] net: xilinx: axienet: Fix packet counting

From: Jakub Kicinski <[email-masked]>
Date: Mon, 9 Sep 2024 18:00:13 -0700

On Fri,  6 Sep 2024 12:42:27 -0400 Sean Anderson wrote:
> axienet_free_tx_chain returns the number of DMA descriptors it's
> handled. However, axienet_tx_poll treats the return as the number of
> packets. When scatter-gather SKBs are enabled, a single packet may use
> multiple DMA descriptors, which causes incorrect packet counts. Fix this
> by explicitly keepting track of the number of packets processed as
> separate from the DMA descriptors.
> 
> Fixes: 8a3b7a252dca ("drivers/net/ethernet/xilinx: added Xilinx AXI Ethernet driver")
> Signed-off-by: Sean Anderson <[email-masked]>

> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index 9aeb7b9f3ae4..556033849d55 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -670,21 +670,21 @@ static int axienet_device_reset(struct net_device *ndev)
>   * @force:	Whether to clean descriptors even if not complete
>   * @sizep:	Pointer to a u32 filled with the total sum of all bytes
>   *		in all cleaned-up descriptors. Ignored if NULL.
> - * @budget:	NAPI budget (use 0 when not called from NAPI poll)
> + * @budget:	NAPI budget (use INT_MAX when not called from NAPI poll)

use INT_MAX and force=true when ... ?
To make sure the dependency is clear.
But actually...

>   *
>   * Would either be called after a successful transmit operation, or after
>   * there was an error when setting up the chain.
> - * Returns the number of descriptors handled.
> + * Returns the number of packets handled.
>   */
>  static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>  				 int nr_bds, bool force, u32 *sizep, int budget)
>  {
>  	struct axidma_bd *cur_p;
>  	unsigned int status;
> +	int i, packets = 0;
>  	dma_addr_t phys;
> -	int i;
>  
> -	for (i = 0; i < nr_bds; i++) {
> +	for (i = 0; i < nr_bds && packets < budget; i++) {

why are you doing this? To make sure drivers doesn't complete more 
than "budget" Tx skbs? The budget is really for Rx, for Tx you can
use a reasonable fixed value, independent of what budget core
passes in, e.g. 128. See:
https://www.kernel.org/doc/html/next/networking/napi.html#datapath-api

>  		cur_p = &lp->tx_bd_v[(first_bd + i) % lp->tx_bd_num];
>  		status = cur_p->status;
>  
> @@ -701,8 +701,10 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>  				 (cur_p->cntrl & XAXIDMA_BD_CTRL_LENGTH_MASK),
>  				 DMA_TO_DEVICE);
>  
> -		if (cur_p->skb && (status & XAXIDMA_BD_STS_COMPLETE_MASK))
> -			napi_consume_skb(cur_p->skb, budget);
> +		if (cur_p->skb && (status & XAXIDMA_BD_STS_COMPLETE_MASK)) {
> +			napi_consume_skb(cur_p->skb, force ? 0 : budget);
> +			packets++;
> +		}
>  
>  		cur_p->app0 = 0;
>  		cur_p->app1 = 0;
> @@ -718,7 +720,13 @@ static int axienet_free_tx_chain(struct axienet_local *lp, u32 first_bd,
>  			*sizep += status & XAXIDMA_BD_STS_ACTUAL_LEN_MASK;
>  	}
>  
> -	return i;
> +	if (!force) {
> +		lp->tx_bd_ci += i;
> +		if (lp->tx_bd_ci >= lp->tx_bd_num)
> +			lp->tx_bd_ci %= lp->tx_bd_num;
> +	}

Moving this chunk into axienet_free_tx_chain() is a noop, right?
Please avoid code cleanups in fixes.

> +	return packets;
>  }
>  /**
-- 
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