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