Re: [PATCH v3 3/5] net: Let the active time stamping layer be selectable.

[Michael Walle]

[+ Horatiu]

Am 2023-03-10 12:35, schrieb Vladimir Oltean:
> On Fri, Mar 10, 2023 at 11:48:52AM +0100, Köry Maincent wrote:
> > > From previous discussions, I believe that a device tree property was
> > > added in order to prevent perceived performance regressions when
> > > timestamping support is added to a PHY driver, correct?
> >
> > Yes, i.e. to select the default and better timestamp on a board.
>
> Is there a way to unambiguously determine the "better" timestamping on 
> a board?
>
> Is it plausible that over time, when PTP timestamping matures and,
> for example, MDIO devices get support for PTP_SYS_OFFSET_EXTENDED
> (an attempt was here: https://lkml.org/lkml/2019/8/16/638), the
> relationship between PTP clock qualities changes, and so does the
> preference change?
>
> > > I have a dumb question: if updating the device trees is needed in order
> > > to prevent these behavior changes, then how is the regression problem
> > > addressed for those device trees which don't contain this new property
> > > (all device trees)?
> >
> > On that case there is not really solution,
>
> If it's not really a solution, then doesn't this fail at its primary
> purpose of preventing regressions?
>
> > but be aware that CONFIG_PHY_TIMESTAMPING need to be activated to
> > allow timestamping on the PHY. Currently in mainline only few (3)
> > defconfig have it enabled so it is really not spread,
>
> Do distribution kernels use the defconfigs from the kernel, or do they
> just enable as many options that sound good as possible?
>
> > maybe I could add more documentation to prevent further regression
> > issue when adding support of timestamp to a PHY driver.
>
> My opinion is that either the problem was not correctly identified,
> or the proposed solution does not address that problem.
>
> What I believe is the problem is that adding support for PHY 
> timestamping
> to a PHY driver will cause a behavior change for existing systems which
> are deployed with that PHY.
>
> If I had a multi-port NIC where all ports share the same PHC, I would
> want to create a boundary clock with it. I can do that just fine when
> using MAC timestamping. But assume someone adds support for PHY
> timestamping and the kernel switches to using PHY timestamps by 
> default.
> Now I need to keep in sync the PHCs of the PHYs, something which was
> implicit before (all ports shared the same PHC). I have done nothing
> incorrectly, yet my deployment doesn't work anymore. This is just an
> example. It doesn't sound like a good idea in general for new features
> to cause a behavior change by default.
>
> Having identified that as the problem, I guess the solution should be
> to stop doing that (and even though a PHY driver supports timestamping,
> keep using the MAC timestamping by default).
>
> There is a slight inconvenience caused by the fact that there are
> already PHY drivers using PHY timestamping, and those may have been
> introduced into deployments with PHY timestamping. We cannot change the
> default behavior for those either. There are 5 such PHY drivers today
> (I've grepped for mii_timestamper in drivers/net/phy).
>
> I would suggest that the kernel implements a short whitelist of 5
> entries containing PHY driver names, which are compared against
> netdev->phydev->drv->name (with the appropriate NULL pointer checks).
> Matches will default to PHY timestamping. Otherwise, the new default
> will be to keep the behavior as if PHY timestamping doesn't exist
> (MAC still provides the timestamps), and the user needs to select the
> PHY as the timestamping source explicitly.
>
> Thoughts?

While I agree in principle (I have suggested to make MAC timestamping
the default before), I see a problem with the recent LAN8814 PHY
timestamping support, which will likely be released with 6.3. That
would now switch the timestamping to PHY timestamping for our board
(arch/arm/boot/dts/lan966x-kontron-kswitch-d10-mmt-8g.dts). I could
argue that is a regression for our board iff NETWORK_PHY_TIMESTAMPING
is enabled. Honestly, I don't know how to proceed here and haven't
tried to replicate the regression due to limited time. Assuming,
that I can show it is a regression, what would be the solution then,
reverting the commit? Horatiu, any ideas?

I digress from the original problem a bit. But if there would be such
a whitelist, I'd propose that it won't contain the lan8814 driver.

Other than that, I guess I have to put some time into testing
before it's too late.

-michael