linux-kernel - Re: [PATCH 1/2] net: ravb: Always process TX descriptor ring

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Date: Tue, 26 Mar 2024 10:38:43 +0100
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>,
	netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] net: ravb: Always process TX descriptor ring

Hi Paul,

Thanks for your work.

On 2024-03-26 08:37:39 +0000, Paul Barker wrote:
> The TX queue should be serviced each time the poll function is called,
> even if the full RX work budget has been consumed. This prevents
> starvation of the TX queue when RX bandwidth usage is high.

Is this not a design decision? That the driver should prioritize Rx over 
Tx if there is contention. I have no opinion on if this design is good 
or bad, I let Sergey judge that.

> 
> Fixes: a0d2f20650e8 ("Renesas Ethernet AVB PTP clock driver")

However, I do not think it is a bug and should not have a fixes tag.  
Also this fixes tag is incorrect, this points to the commit where ravb.c 
was renamed ravb_main.c. ravb_poll() is not touched by this commit.

> Signed-off-by: Paul Barker <paul.barker.ct@...renesas.com>
> ---
>  drivers/net/ethernet/renesas/ravb_main.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> index d1be030c8848..4f98e4e2badb 100644
> --- a/drivers/net/ethernet/renesas/ravb_main.c
> +++ b/drivers/net/ethernet/renesas/ravb_main.c
> @@ -1324,12 +1324,12 @@ static int ravb_poll(struct napi_struct *napi, int budget)
>  	int q = napi - priv->napi;
>  	int mask = BIT(q);
>  	int quota = budget;
> +	bool rearm = true;
>  
>  	/* Processing RX Descriptor Ring */
>  	/* Clear RX interrupt */
>  	ravb_write(ndev, ~(mask | RIS0_RESERVED), RIS0);
> -	if (ravb_rx(ndev, &quota, q))
> -		goto out;
> +	rearm = !ravb_rx(ndev, &quota, q);
>  
>  	/* Processing TX Descriptor Ring */
>  	spin_lock_irqsave(&priv->lock, flags);
> @@ -1339,6 +1339,9 @@ static int ravb_poll(struct napi_struct *napi, int budget)
>  	netif_wake_subqueue(ndev, q);
>  	spin_unlock_irqrestore(&priv->lock, flags);
>  
> +	if (!rearm)
> +		goto out;
> +
>  	napi_complete(napi);
>  
>  	/* Re-enable RX/TX interrupts */
> 
> base-commit: 4cece764965020c22cff7665b18a012006359095
> -- 
> 2.44.0
> 

-- 
Kind Regards,
Niklas Söderlund