[Intel-wired-lan] [PATCH v1] e1000e: allow non-monotonic SYSTIM readings

Keller, Jacob E jacob.e.keller at intel.com
Wed Oct 24 17:51:49 UTC 2018

> -----Original Message-----
> From: Miroslav Lichvar [mailto:mlichvar at redhat.com]
> Sent: Wednesday, October 24, 2018 2:46 AM
> To: Keller, Jacob E <jacob.e.keller at intel.com>
> Cc: intel-wired-lan at lists.osuosl.org; Richard Cochran <richardcochran at gmail.com>
> Subject: Re: [PATCH v1] e1000e: allow non-monotonic SYSTIM readings
> > Weird issue, but I think this is a better solution than returning garbage time data
> like we were before.
> It is indeed a weird issue. I think one explanation could be a double
> overflow of SYSTIML with the unreliable latching of SYSTIMH.

Makes some sense.

> If my math is right, depending on the frequency of the clock the
> SYSTIML register overflows about every 8, 16, or 262 microseconds.
> That seems too short to reliably contain reading of two registers.

Right. In theory the hardware is supposed to be latching the values, but we know that's problematic on some of the parts in this driver.

> Let's say the first reading of SYSMTIML is 0xffff0000 and the second
> reading is 0xff000000. An overflow is detected. But before SYSTIMH is
> read for the second time, another overflow may happen, which will
> cause the returned time to be ahead of the true PHC time and the next
> correct reading may be out-of-order.


> I'm wondering whether the commit 37b12910 ("e1000e: Fix tight loop
> implementation of systime read algorithm") made this more likely to
> happen (if it really is what happens).

If your analysis is correct, that makes sense.

> The best fix might be to use a much smaller INCVALUE, so that the
> double overflow cannot happen, and implement the frequency adjustment
> in software, similarly to the system clock. This could be reused in
> other drivers that don't support a one-step clock in order to simplify
> their code.

This makes sense. Especially since we already use a timecounter, we already don't report exactly what the hardware register indicates. This can be confusing if using hardware timer controls, or if some setup tries to read timestamps out-of-band from the PTP clock interface. But I don't think that's a major concern if we're already using a timecounter.


> --
> Miroslav Lichvar

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