linux-kernel - Re: [PATCH net 1/4] net: ravb: Count packets instead of descriptors in R-Car RX path

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Date: Sun, 14 Apr 2024 14:08:43 +0200
From: Niklas Söderlund <niklas.soderlund+renesas@...natech.se>
To: Paul Barker <paul.barker.ct@...renesas.com>
Cc: Sergey Shtylyov <s.shtylyov@....ru>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/4] net: ravb: Count packets instead of descriptors
 in R-Car RX path

Hi Paul,

Thanks for your patch.

On 2024-04-11 12:44:30 +0100, Paul Barker wrote:
> The units of "work done" in the RX path should be packets instead of
> descriptors.
> 
> Descriptors which are used by the hardware to record error conditions or
> are empty in the case of a DMA mapping error should not count towards
> our RX work budget.
> 
> Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
> Signed-off-by: Paul Barker <paul.barker.ct@...renesas.com>
> ---
>  drivers/net/ethernet/renesas/ravb_main.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> index ba01c8cc3c90..70f2900648d4 100644
> --- a/drivers/net/ethernet/renesas/ravb_main.c
> +++ b/drivers/net/ethernet/renesas/ravb_main.c
> @@ -892,29 +892,25 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
>  	struct ravb_private *priv = netdev_priv(ndev);
>  	const struct ravb_hw_info *info = priv->info;
>  	int entry = priv->cur_rx[q] % priv->num_rx_ring[q];
> -	int boguscnt = (priv->dirty_rx[q] + priv->num_rx_ring[q]) -
> -			priv->cur_rx[q];
>  	struct net_device_stats *stats = &priv->stats[q];
>  	struct ravb_ex_rx_desc *desc;
>  	struct sk_buff *skb;
>  	dma_addr_t dma_addr;
>  	struct timespec64 ts;
> +	int rx_packets = 0;
>  	u8  desc_status;
>  	u16 pkt_len;
>  	int limit;
> +	int i;

The loop variable can never be negative, use unsigned int.

>  
> -	boguscnt = min(boguscnt, *quota);
> -	limit = boguscnt;
> +	limit = priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q];
>  	desc = &priv->rx_ring[q].ex_desc[entry];
> -	while (desc->die_dt != DT_FEMPTY) {
> +	for (i = 0; i < limit && rx_packets < *quota && desc->die_dt != DT_FEMPTY; i++) {
>  		/* Descriptor type must be checked before all other reads */
>  		dma_rmb();
>  		desc_status = desc->msc;
>  		pkt_len = le16_to_cpu(desc->ds_cc) & RX_DS;
>  
> -		if (--boguscnt < 0)
> -			break;
> -

nit: It's a matter of taste, but I like this break condition in the code 
instead of modifying the loop as it's much clearer what's going on. But 
feel free to keep it as is as Sergey likes it.

>  		/* We use 0-byte descriptors to mark the DMA mapping errors */
>  		if (!pkt_len)
>  			continue;
> @@ -960,7 +956,7 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
>  			if (ndev->features & NETIF_F_RXCSUM)
>  				ravb_rx_csum(skb);
>  			napi_gro_receive(&priv->napi[q], skb);
> -			stats->rx_packets++;
> +			rx_packets++;

Why do you add this intermediary variable? Is it not confusing to treat 
rx_packets and rx_bytes differently? Why not instead decrement *quota 
here?

>  			stats->rx_bytes += pkt_len;
>  		}
>  
> @@ -995,9 +991,9 @@ static bool ravb_rx_rcar(struct net_device *ndev, int *quota, int q)
>  		desc->die_dt = DT_FEMPTY;
>  	}
>  
> -	*quota -= limit - (++boguscnt);
> -
> -	return boguscnt <= 0;
> +	stats->rx_packets += rx_packets;
> +	*quota -= rx_packets;
> +	return *quota == 0;
>  }
>  
>  /* Packet receive function for Ethernet AVB */
> -- 
> 2.39.2
> 

-- 
Kind Regards,
Niklas Söderlund