On Fri, Apr 05, 2024 at 01:42:49PM +0000, Kulkarni, Vandita wrote: > > -----Original Message----- > > From: Intel-gfx <intel-gfx-boun...@lists.freedesktop.org> On Behalf Of Ville > > Syrjala > > Sent: Friday, April 5, 2024 3:04 AM > > To: intel-gfx@lists.freedesktop.org > > Subject: [PATCH v2 01/17] drm/i915: Update pipes in reverse order for > > bigjoiner > > > > From: Ville Syrjälä <ville.syrj...@linux.intel.com> > > > > With bigjoiner the master crtc is the one that will send out the uapi > > event/etc. We want that to happen after all the slaves are done, so let's > > try > > to do the commits in reverse order so that the master comes last. > > > > Even worse, the modeset helper will simply complete the commit on the > > slave pipe immediately as it consider the crtc to be inactive (it can't see > > our > > crtc_state->hw.active/etc.). > > > > With regular sync updates this generally doesn't matter all that much as the > > slave pipe should typically finish its work during the same frame as the > > master pipe. However in case the slave pipe's commit slips into the next > > frame we end up in a bit of trouble. This is most visible with either async > > flips > > (currently disabled with bigjoiner exactly for this reason), and DSB gamma > > updates. With DSB the problem happens because the DSB itself will wait until > > the next start vblank before starting to execute. So if the master pipe > > already > > finished its commit and the DSB on the slave pipe is still waiting for the > > next > > vblank we will assume the DSB as gotten stuck and terminate it. > > > > Reversing the commit order should ameliarate this for the most part as the > > master pipe is guaranteed to start its commit after the slave pipe started. > > The > > one thing that can still screw us over is the fact that we aren't > > necessarily > > going to commit the pipes in the reverse order as the actual order is > > dictated > > by the DDB overlap avoidance. > > But that can only happen while other pipes are being enabled/disabled, and > > so in the normal steady state we should be safe. > > > > The full fix will involve making the commit machinery aware of the slave > > pipes and not finish their commits prematurely. But that will involve a bit > > more work than this. And this commit order reversal will still be > > beneficial to > > avoid userspace getting an -EBUSY from the following page flip if the second > > pipe's commit does stretch into the next frame. > > > > LGTM. > Reviewed-by: Vandita Kulkarni <vandita.kulka...@intel.com> > > I had just one query though, > Will there be a case where we can get vblank between slave update and master > update, > if so do you think there will be any problem due to that?
It can happen, in which case you may observe a vertical tear. Since we've disabled all the fancy transcoder level stuff (vrr/lrr/etc.) that should be the worst of it. -- Ville Syrjälä Intel
