Re: [PATCH v3 5/8] net: dsa: hellcreek: Add TAPRIO offloading support

On Tue, Aug 25, 2020 at 11:33:53AM +0200, Kurt Kanzenbach wrote:

On Tue Aug 25 2020, Vladimir Oltean wrote:

quoted

On Thu, Aug 20, 2020 at 10:11:15AM +0200, Kurt Kanzenbach wrote:

Explain again how this works, please? The hrtimer measures the CLOCK_TAI
of the CPU, but you are offloading the CLOCK_TAI domain of the NIC? So
you are assuming that the CPU and the NIC PHC are synchronized? What if
they aren't?

Yes, I assume that's synchronized with e.g. phc2sys.

My intuition tells me that this isn't the user's expectation, and that
it should do the right thing even if it's not synchronized to the system
clock.

quoted

And what if the base-time is in the past, do you deal with that (how
does the hardware deal with a base-time in the past)?
A base-time in the past (example: 0) should work: you should advance the
base-time into the nearest future multiple of the cycle-time, to at
least preserve phase correctness of the schedule.

If the hrtimer is programmed with a value in the past, it fires
instantly.

Yes, it does.

The callback is executed and the start time is programmed.

With a valid value from the hardware's perspective?

Thanks,
-Vladimir