Message-ID: <20240219142936.62112d34@kmaincent-XPS-13-7390>
Date: Mon, 19 Feb 2024 14:29:36 +0100
From: Köry Maincent <kory.maincent@...tlin.com>
To: Rahul Rameshbabu <rrameshbabu@...dia.com>
Cc: Florian Fainelli <florian.fainelli@...adcom.com>, Broadcom internal
 kernel review list <bcm-kernel-feedback-list@...adcom.com>, Andrew Lunn
 <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>, Russell King
 <linux@...linux.org.uk>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo
 Abeni <pabeni@...hat.com>, Richard Cochran <richardcochran@...il.com>, Radu
 Pirea <radu-nicolae.pirea@....nxp.com>, Jay Vosburgh
 <j.vosburgh@...il.com>, Andy Gospodarek <andy@...yhouse.net>, Nicolas Ferre
 <nicolas.ferre@...rochip.com>, Claudiu Beznea <claudiu.beznea@...on.dev>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, Jonathan Corbet
 <corbet@....net>, Horatiu Vultur <horatiu.vultur@...rochip.com>,
 UNGLinuxDriver@...rochip.com, Simon Horman <horms@...nel.org>, Vladimir
 Oltean <vladimir.oltean@....com>, Thomas Petazzoni
 <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, Maxime Chevallier
 <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH RFC net-next v8 04/13] net: Change the API of PHY
 default timestamp to MAC

On Fri, 16 Feb 2024 10:09:36 -0800
Rahul Rameshbabu <rrameshbabu@...dia.com> wrote:

> On Fri, 16 Feb, 2024 16:52:22 +0100 Kory Maincent <kory.maincent@...tlin.com>
> wrote:
> > Change the API to select MAC default time stamping instead of the PHY.
> > Indeed the PHY is closer to the wire therefore theoretically it has less
> > delay than the MAC timestamping but the reality is different. Due to lower
> > time stamping clock frequency, latency in the MDIO bus and no PHC hardware
> > synchronization between different PHY, the PHY PTP is often less precise
> > than the MAC. The exception is for PHY designed specially for PTP case but
> > these devices are not very widespread. For not breaking the compatibility
> > default_timestamp flag has been introduced in phy_device that is set by
> > the phy driver to know we are using the old API behavior.
> >
> > Signed-off-by: Kory Maincent <kory.maincent@...tlin.com>
> > ---  
> 
> Overall, I agree with the motivation and reasoning behind the patch. It
> takes dedicated effort to build a good phy timestamping mechanism, so
> this approach is good. I do have a question though. In this patch if we
> set the phy as the default timestamp mechanism, does that mean for even
> non-PTP applications, the phy will be used for timestamping when
> hardware timestamping is enabled? If so, I think this might need some
> thought because there are timing applications in general when a
> timestamp closest to the MAC layer would be best.

This patch comes from a request from Russell due to incompatibility between MAC
and PHY timestamping when both were supported.
https://lore.kernel.org/netdev/Y%2F4DZIDm1d74MuFJ@shell.armlinux.org.uk/

His point was adding PTP support to a PHY driver would select timestamp from it
by default even if we had a better timestamp with the MAC which is often the
case. This is an unwanted behavior.
https://lore.kernel.org/netdev/Y%2F6Cxf6EAAg22GOL@shell.armlinux.org.uk/

In fact, with the new support of NDOs hwtstamp and the
dev_get/set_hwtstamp_phylib functions, alongside this series which make
timestamp selectable, changing the default timestamp may be not necessary
anymore.

Russell any thought about it? 

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

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