[PATCH V2] drm/vkms: Add vblank events simulated by hrtimers

[Chris Wilson]

Quoting Rodrigo Siqueira (2018-07-05 17:28:20)
> Hi and thanks for all the feedback, I will work on the suggestions you sent,
> but I have some doubts:
> 
> On 07/05, Chris Wilson wrote:
> > Quoting Daniel Vetter (2018-07-05 09:20:13)
> > > On Thu, Jul 05, 2018 at 12:48:43AM -0300, Rodrigo Siqueira wrote:
> > > > +     ktime_t current_timestamp;
> > > > +
> > > > +     hrtimer_init(&out->vblank_hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> > > 
> > > Can't we use absolute timer mode here? That avoids all the timestampt
> > > computations.
> > 
> > Where's your absolute timestamp being computed?
> 
> I did not understand the question. hrtimer_forward_now calculates the
> absolute timestamp from the relative value provided, this is what I am
> using. In any case, it would be easy to switch to absolute mode, but
> the code would not be smaller (or larger).

It was a question to Daniel, trying to illustrate that REL is just
a convenience in the htrimer api over ABS.
 
> > What's being done is recomputing what hrtimer already knows given a
> > relative interval. output->expires should be equivalent to
> > hrtimer->expires, and a lot of this code evaporates.
> 
> Indeed, output->expires can be removed; as for the rest of the code, that
> depends on the answer to question 2 below.
> 
> I have two questions:
> 
> 1. The timestamp that is returned to userspace is (A) the timestamp when the
> interrupt was actually handled, allowing applications to detect when there
> is some irregularities in the interrupt handling timing, or (B) the timestamp
> when the current interrupt was *scheduled* to happen, allowing applications
> to detect overruns but not variations in the interrupt handling timing?

When the previous hrtimer actually occurred rolled back to the actual
start of frame, i.e.

	hrtimer_forward_now(hrtimer, vblank_interval);
	output->last_vblank_timestamp =
		ktime_sub(hrtimer->expires, vblank_interval);

userspace is mainly oblivious to the delivery latency (although it can
measure it as clock_gettim() - vblank->timestamp), but is concerned
about knowing what the current and next vblank HW timing will be.

> 2. If I use hrtimer with a period of 1s and return HRTIMER_RESTART, will I
> be called back (A) 1s after the previous iteration was *scheduled to start*
> (i.e., I will actually be called back at regular intervals, so that after
> 1,000 iterations approximately 1,000s have elapsed) or (B) 1s after the
> previous iteration *ended* (i.e., I will be called back at intervals of
> 1s + the average processing time of the function, so that after 1,000
> iterations significantly more than 1,000s have elapsed)?

No. When you get callback is controlled by the value you set in
hrtimer->expires before you return RESTART. All RESTART does is
immediately requeue the hrtimer, but more efficiently than calling
hrtimer_start yourself. It is the call to hrtimer_forward that computes
the next absolute hrtimer->expires based on the current time and your
vblank  interval.
 
> The code I wrote assumes the answer to both questions is (B). If the
> answer to the second question is A, the code can indeed be made much
> simpler; if the answer to the first question is A, I have not been able
> to keep timing within the expected strict limits of the IGT test in a VM
> (maybe on physical hardware things would go better).

The code you wrote is actually A. hrtimer_forward() rolls the absolute
timer forward such that its expiry is the next interval after now. It is
	while (hrtimer->expires < now)
		hrtimer->expires += interval;
-Chris