On Fri, Jun 30, 2017 at 08:18:19PM +0200, Daniel Vetter wrote: > On Fri, Jun 30, 2017 at 5:26 PM, Daniel Vetter <daniel@xxxxxxxx> wrote: > > On Fri, Jun 30, 2017 at 05:18:41PM +0300, ville.syrjala@xxxxxxxxxxxxxxx wrote: > >> From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > >> > >> Add drm_crtc_vblank_get_accurate() which is the same as > >> drm_crtc_vblank_get() except that it will forcefully update the > >> the current vblank counter/timestamp value even if the interrupt > >> is already enabled. > >> > >> And we'll switch drm_wait_one_vblank() over to using it to make sure it > >> will really wait at least one vblank even if it races with the irq > >> handler. > >> > >> Cc: Daniel Vetter <daniel@xxxxxxxx> > >> Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > > > > Hm, I just thought of doing an > > s/drm_vblank_count/drm_crtc_accurate_vblank_count/ in > > drm_crtc_arm_vblank_event. > > > > What does this buy us above&beyond the other patch? And why isn't > > vblank_get accurate always? > > This also seems to have the risk of not fixing the arm_vblank_event > race if someone puts the vblank_get_accurate outside of the critical > section. Then we're back to the same old race. And since that means > you need to have a vblank_get_accurate right before the > arm_vblank_event in all cases, we might as well just put it into the > helper. You wrote in the other thread putting it into arm_vblank_event > might be wasteful, but I don't see any scenario where that could be > the case ... > > Or do I miss something? The interrupt most likely will be on already when pipe_update_end() gets called since pipe_update_start() will enable it already, and Chris's disable_immediate optimization will keep it on until the next interrupt. Prior to that optimization the drm_vblank_get() in pipe_update_end() would have had to turn on the interrupt and perform the vblank counter update, and thus the second update from drm_crtc_accurate_vblank_count() would have been wasteful. I'm not sure I like relying on the fact that pipe_update_start() already turned the interrupt on and it's going to be kept on magically. One solution for that would be to hang on to the reference from pipe_update_start() until we've armed the event. Then we know the vblank_get() won't have to enable the interrupt. However, if we start to sample the counter for some other purpose (watermarks, fb unpinning etc.) during pipe_update_end() then we'll again be faced with another potentially pointless update if we decide to use drm_crtc_accurate_vblank_count() again. -- Ville Syrjälä Intel OTC _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
