linux-kernel - Re: [PATCH v2 net-next] amd-xgbe: Configure and retrieve 'tx-usecs' for Tx coalescing

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Message-ID: <20250813170012.7436b6e6@kernel.org>
Date: Wed, 13 Aug 2025 17:00:12 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Vishal Badole <Vishal.Badole@....com>
Cc: <Shyam-sundar.S-k@....com>, <andrew+netdev@...n.ch>,
 <davem@...emloft.net>, <edumazet@...gle.com>, <pabeni@...hat.com>,
 <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] amd-xgbe: Configure and retrieve 'tx-usecs'
 for Tx coalescing

On Tue, 12 Aug 2025 10:20:35 +0530 Vishal Badole wrote:
> Ethtool has advanced with additional configurable options, but the
> current driver does not support tx-usecs configuration.

Not sure what you mean by this, perhaps:

  current driver does not even support tx-usecs configuration.

? tx-usecs is a very old tunable.

> Add support to configure and retrieve 'tx-usecs' using ethtool, which
> specifies the wait time before servicing an interrupt for Tx coalescing.
> 

> +	/* Check if both tx_usecs and tx_frames are set to 0 simultaneously */
> +	if (!tx_usecs && !tx_frames) {
> +		NL_SET_ERR_MSG_FMT_MOD(extack,
> +				       "tx_usecs and tx_frames must not be 0 together");
> +		return -EINVAL;
> +	}
> +
>  	/* Check the bounds of values for Tx */
> +	if (tx_usecs > XGMAC_MAX_COAL_TX_TICK) {
> +		NL_SET_ERR_MSG_FMT_MOD(extack, "tx-usecs is limited to %d usec",
> +				       XGMAC_MAX_COAL_TX_TICK);
> +		return -EINVAL;
> +	}

Normal configuration granularity for this parameter is in 10s of usecs.
You seem to be using a timer, so I think you should either round the
value up / down to what the jiffy resolution will give you or
reject configuration that's not expressible in jiffies (not a multiple
of jiffies_to_usecs(1)). Otherwise users may waste time turning this
knob by 100usec which will have zero effect.
-- 
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