linux-kernel - Re: [PATCH net-next 3/3] net: ethtool: tsconfig: Re-configure hwtstamp upon provider change

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Message-ID: <731e8fa7-465b-4470-9036-c59fea602c07@bootlin.com>
Date: Thu, 16 Oct 2025 10:01:53 +0200
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: Kory Maincent <kory.maincent@...tlin.com>
Cc: Alexandre Torgue <alexandre.torgue@...s.st.com>,
 Jose Abreu <joabreu@...opsys.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 davem@...emloft.net, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Maxime Coquelin <mcoquelin.stm32@...il.com>,
 Richard Cochran <richardcochran@...il.com>,
 Russell King <linux@...linux.org.uk>,
 Alexis Lothoré <alexis.lothore@...tlin.com>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] net: ethtool: tsconfig: Re-configure
 hwtstamp upon provider change

Hi Köry,

On 15/10/2025 14:45, Kory Maincent wrote:
> On Wed, 15 Oct 2025 12:27:23 +0200
> Maxime Chevallier <maxime.chevallier@...tlin.com> wrote:
> 
>> When a hwprov timestamping source is changed, but without updating the
>> timestamping parameters, we may want to reconfigure the timestamping
>> source to enable the new provider.
>>
>> This is especially important if the same HW unit implements 2 providers,
>> a precise and an approx one. In this case, we need to make sure we call
>> the hwtstamp_set operation for the newly selected provider.
> 
> This is a design choice.
> Do we want to preserve the hwtstamp config if only the hwtstamp source is
> changed from ethtool?
> If we want to configure the new source to the old source config we will also
> need to remove this condition:
> https://elixir.bootlin.com/linux/v6.17.1/source/net/ethtool/tsconfig.c#L339

What I get from the ethtool output is that the ts config is per-source.
Re-applying the old config to the new source may not work if the new one
doesn't have the same capabilities.

> 
> I do not really have a strong opinion on this, let's discuss which behavior we
> prefer.

Well if we want to support different timestamp providers provided by the same
HW block (same MAC or even same PHY), then we need a way to notify the provider
when the timestamp provider gets selected and unselected.

Otherwise there's no way for the provider to know it has been re-enabled, unless
we perform a config change at the same time.

Maxime