On Thu, Jul 5, 2018 at 6:45 PM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
