Re: [PATCH RFC net-next v8 04/13] net: Change the API of PHY default timestamp to MAC

["Köry Maincent" <kory.maincent@bootlin.com>]

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, Köry Maincent wrote:
> > On Fri, 16 Feb 2024 10:09:36 -0800
> > Rahul Rameshbabu <rrameshbabu@nvidia.com> wrote:
> >   
> > > On Fri, 16 Feb, 2024 16:52:22 +0100 Kory Maincent
> > > <kory.maincent@bootlin.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@bootlin.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?   
> 
> 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,
-- 
Köry Maincent, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com