linux-kernel - Re: [PATCH net-next 0/3] net: stmmac: Add support for coarse timestamping

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Message-ID: <20251015145519.280b6263@kmaincent-XPS-13-7390>
Date: Wed, 15 Oct 2025 14:55:19 +0200
From: Kory Maincent <kory.maincent@...tlin.com>
To: Maxime Chevallier <maxime.chevallier@...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 0/3] net: stmmac: Add support for coarse
 timestamping

On Wed, 15 Oct 2025 12:27:20 +0200
Maxime Chevallier <maxime.chevallier@...tlin.com> wrote:

> Hello everyone,
> 
> This is another attempt to support the fine vs coarse timestamping modes
> in stmmac.
> 
> This mode allows trading off PTP clock frequency adjustment precision
> versus timestamping precision.
> 
> In coarse mode, we lose the ability to fine-tune the PTP clock
> frequency, but get better timestamping precision instead. This is
> especially useful when acting as a PTP Grand Master, where the PTP clock
> in sync'd to a high-precision GPS clock through PPS inputs.
> 
> This has been submitted before as a dedicated ioctl() back in 2020 [1].
> Since then, we now have a better representation of timestamp providers
> with a dedicated qualifier (approx vs precise).
> 
> This series attempts to map these new qualifiers to stmmac's
> timestamping modes, see patch 2 for details.
> 
> The main drawback IMO is that the qualifiers don't map very well to our
> timestamping modes, as the "approx" qualifier actually maps to stmmac's
> "coars" mode, but we actually gain in timestamping precision (while
> losing frequency precision).

https://elixir.bootlin.com/linux/v6.17.1/source/include/uapi/linux/net_tstamp.h#L16
"approx" was initially added for DMA timestamp point.
Maybe we should add a new enum value here with a more suitable name.

Regards,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com