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Date: Tue, 20 Feb 2024 17:20:33 +0100
From: =?UTF-8?B?S8O2cnk=?= Maincent <kory.maincent@bootlin.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Rahul Rameshbabu <rrameshbabu@nvidia.com>, Florian Fainelli
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Subject: Re: [PATCH RFC net-next v8 04/13] net: Change the API of PHY
 default timestamp to MAC
Message-ID: <20240220172033.556f321f@kmaincent-XPS-13-7390>
In-Reply-To: <ZdN9pPf3wXwE/9nX@shell.armlinux.org.uk>
References: <20240216-feature_ptp_netnext-v8-0-510f42f444fb@bootlin.com>
	<20240216-feature_ptp_netnext-v8-4-510f42f444fb@bootlin.com>
	<87jzn4gtlv.fsf@nvidia.com>
	<20240219142936.62112d34@kmaincent-XPS-13-7390>
	<ZdN9pPf3wXwE/9nX@shell.armlinux.org.uk>
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On Mon, 19 Feb 2024 16:11:16 +0000
"Russell King (Oracle)" <linux@armlinux.org.uk> wrote:

> On Mon, Feb 19, 2024 at 02:29:36PM +0100, K=C3=B6ry Maincent wrote:
> > On Fri, 16 Feb 2024 10:09:36 -0800
> > Rahul Rameshbabu <rrameshbabu@nvidia.com> wrote:
> >  =20
> > > On Fri, 16 Feb, 2024 16:52:22 +0100 Kory Maincent
> > > <kory.maincent@bootlin.com> wrote: =20
> > > > Change the API to select MAC default time stamping instead of the P=
HY.
> > > > 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 specia=
lly
> > > > for PTP case but these devices are not very widespread. For not
> > > > breaking the compatibility default_timestamp flag has been introduc=
ed
> > > > 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@bootlin.com>
> > > > ---   =20
> > >=20
> > > 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 ev=
en
> > > 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. =20
> >=20
> > This patch comes from a request from Russell due to incompatibility bet=
ween
> > MAC and PHY timestamping when both were supported.
> > https://lore.kernel.org/netdev/Y%2F4DZIDm1d74MuFJ@shell.armlinux.org.uk/
> >=20
> > 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/
> >=20
> > 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 necessa=
ry
> > anymore.
> >=20
> > Russell any thought about it?  =20
>=20
> My position remains: in the case of Marvell PP2 network driver with a
> Marvell PHY, when we add PTP support for the Marvell PHYs (I have
> patches for it for years) then we must _not_ regress the existing
> setup where the PP2 timestamps are the default.

Yes, that's what I thought.
About the Marvell PHYs PTP support I have a few fixes on it, but we will
talk about it when this series gets merged.

Regards,
--=20
K=C3=B6ry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com