netdev - Re: [PATCH net-next 1/1] net: stmmac: Remove redundant checking for rx_coalesce

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Date: Tue, 23 May 2023 14:47:33 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Gan Yi Fang <yi.fang.gan@...el.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@...com>,
	Alexandre Torgue <alexandre.torgue@...com>,
	Jose Abreu <joabreu@...opsys.com>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Looi Hong Aun <hong.aun.looi@...el.com>,
	Michael Sit Wei Hong <michael.wei.hong.sit@...el.com>
Subject: Re: [PATCH net-next 1/1] net: stmmac: Remove redundant checking for
 rx_coalesce_usecs

On Tue, May 23, 2023 at 02:19:52AM -0400, Gan Yi Fang wrote:
> The datatype of rx_coalesce_usecs is u32, always larger or equal to zero.
> Previous checking does not include value 0, this patch removes the
> checking to handle the value 0.
> 
> Signed-off-by: Gan Yi Fang <yi.fang.gan@...el.com>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
> index 35c8dd92d369..6ed0e683b5e0 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ethtool.c
> @@ -917,7 +917,7 @@ static int __stmmac_set_coalesce(struct net_device *dev,
>  	else if (queue >= max_cnt)
>  		return -EINVAL;
>  
> -	if (priv->use_riwt && (ec->rx_coalesce_usecs > 0)) {
> +	if (priv->use_riwt) {
>  		rx_riwt = stmmac_usec2riwt(ec->rx_coalesce_usecs, priv);
>  
>  		if ((rx_riwt > MAX_DMA_RIWT) || (rx_riwt < MIN_DMA_RIWT))

This appears to be a user visible ABI change. For the current code, a
value of zero here is ignored, and 0 is returned. With this change, 0
will result in rx_riwt being calculated as 0, which is less than
MIN_DMA_RIWT, so you get -EINVAL returned.

I don't know this uAPI too well. What values are passed to this
function for:

ethtool -C eth24 tx-usecs 42

where you only want to change transmit coalesce? Is rx_usecs 0?

At minimum you need to explain in the commit message: "This change in
behaviour making the value of 0 cause an error is not a problem
because...."

    Andrew

---
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