linux-kernel - RE: [PATCH] net: fec: allow disable coalescing

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Message-ID:
 <PAXPR04MB8510411D7846F1028CDE9C2D887AA@PAXPR04MB8510.eurprd04.prod.outlook.com>
Date: Thu, 26 Jun 2025 08:40:36 +0000
From: Wei Fang <wei.fang@....com>
To: Jonas Rebmann <jre@...gutronix.de>, Andrew Lunn <andrew@...n.ch>
CC: "imx@...ts.linux.dev" <imx@...ts.linux.dev>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "kernel@...gutronix.de"
	<kernel@...gutronix.de>, Shenwei Wang <shenwei.wang@....com>, Clark Wang
	<xiaoning.wang@....com>, Andrew Lunn <andrew+netdev@...n.ch>, "David S.
 Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub
 Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: RE: [PATCH] net: fec: allow disable coalescing

> On 2025-06-26 10:12, Andrew Lunn wrote:
> > On Thu, Jun 26, 2025 at 02:36:37AM +0000, Wei Fang wrote:
> >>>
> >>> -       /* Must be greater than zero to avoid unpredictable behavior */
> >>> -       if (!fep->rx_time_itr || !fep->rx_pkts_itr ||
> >>> -           !fep->tx_time_itr || !fep->tx_pkts_itr)
> >>> -               return;
> >
> > Hi Wei
> >
> > When i see a comment like this being removed, i wounder if there is
> > any danger of side effects? Do you know what is being done here is
> > actually safe, for all the different versions of the FEC which support
> > coalescence?
> 
> For reference, this comment is taken in plain from section 11.6.4.1.16.3
> in the i.MX 8M Plus Applications Processor Reference Manual (and is the
> same for the 6UL).
> 
> I was also worried about this so I made sure that in any case where
> either of those is zero, the coalescing enable bit (FEC_ITR_EN) is
> explicitly disabled.
> 
> fec_enet_itr_coal_set is only ever called if FEC_QUIRK_HAS_COALESCE is
> set and for those models, we expect disabling coalescing via FEC_ITR_EN
> -- and consequently also setting the parameters to zero -- to be
> unproblematic. This is also the reset default.
> 

Yes, ICFT and ICTT cannot be set to zero when FEC_ITR_EN is set. But it is
safe to 0 when FEC_ITR_EN is not set.