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

[Daniel Vetter]

On Thu, Jul 5, 2018 at 6:45 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> 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.

Yeah I got confused and Chris corrected hat. _REL seems fine, just
don't duplicate the timestamp computation.
-Daniel

>> > 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



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch