On 2018-10-29 7:03 p.m., Ville Syrjälä wrote: > On Mon, Oct 29, 2018 at 05:37:49PM +0100, Michel Dänzer wrote: >> On 2018-10-26 7:59 p.m., Ville Syrjälä wrote: >>> On Fri, Oct 26, 2018 at 05:34:15PM +0000, Kazlauskas, Nicholas wrote: >>>> On 10/26/18 10:53 AM, Ville Syrjälä wrote: >>>>> >>>>> Speaking of timestamps. What is the expected behaviour of vblank >>>>> timestamps when vrr is enabled? >>>> >>>> When vrr is enabled the duration of the vertical front porch will be >>>> extended until flip or timeout occurs. The vblank timestamp will vary >>>> based on duration of the vertical front porch. The min/max duration for >>>> the front porch can be specified by the driver via the min/max range. >>>> >>>> No changes to vblank timestamping handling should be necessary to >>>> accommodate variable refresh rate. >>> >>> The problem is that the timestamp is supposed to correspond to the first >>> active pixel. And since we don't know how long the front porch will be >>> we can't realistically report the true value. So I guess just assuming >>> min front porch length is as good as anything else? >> >> That (and documenting that the timestamp corresponds to the earliest >> possible first active pixel, not necessarily the actual one, with VRR) >> might be good enough for the actual vblank event timestamps. >> >> >> However, I'm not so sure about the timestamps of page flip completion >> events. Those could be very misleading if the flip completes towards the >> timeout, which could result in bad behaviour of applications which use >> them for animation timing. >> >> Maybe the timestamp could be updated appropriately (yes, I'm hand-waving >> :) in drm_crtc_send_vblank_event? > > Hmm. Updated how? Whether it's a page flip event or vblank even we won't > know when the first active pixel will come. Although I suppose if > there is some kind of vrr slew rate limit we could at least account > for that to report a more correct "this is the earliest you migth be > able to see your frame" timestamp. > > Oh, or are you actually saying that shceduling a new flip before the > timeout is actually going to latch that flip immediately? I figured > that the flip would get latched on the next start of vblank regardless, > and the act of scheduling a flip will just kick the hardware to start > scanning the previously latched frame earlier. > > scanout A | ... vblank | scanout A | ... vblank | scanout B | ... vblank > ^ ^ ^ ^ > | | flip C latch C > flip B latch B This would kind of defeat the point of VRR, wouldn't it? If a flip was scheduled after the start of vblank, the vblank would always time out, resulting in the minimum refresh rate. > scanout A | ... vblank | scanout B | ... vblank | scanout C | ... vblank > ^ ^ ^ ^ > | latch B | latch C > flip B flip C So this is what happens. Regardless, when the flip is latched, AMD hardware generates a "pflip" interrupt, and its handler calls drm_crtc_send_vblank_event (and in the case of DC drm_crtc_accurate_vblank_count before that). So the time when drm_crtc_send_vblank_event is called might be a good approximation of when scanout of the next frame starts. Another possibility might be to wait for the hardware vline counter to wrap around to 0 before calling drm_crtc_accurate_vblank_count, then the calculations should be based on 0 instead of crtc_vtotal. -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Mesa and X developer _______________________________________________ amd-gfx mailing list amd-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/amd-gfx
