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

[Russell King (Oracle)]

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@xxxxxxxxxx> wrote:
> 
> > On Fri, 16 Feb, 2024 16:52:22 +0100 Kory Maincent <kory.maincent@xxxxxxxxxxx>
> > 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@xxxxxxxxxxx>
> > > ---  
> > 
> > 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@xxxxxxxxxxxxxxxxxxxxx/
> 
> 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@xxxxxxxxxxxxxxxxxxxxx/
> 
> 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.

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