On Tue, Feb 25, 2020 at 05:34:00PM +0200, Ville Syrjälä wrote: > On Tue, Feb 25, 2020 at 04:09:26PM +0100, Daniel Vetter wrote: > > On Tue, Feb 25, 2020 at 3:48 PM Ville Syrjälä > > <ville.syrjala@xxxxxxxxxxxxxxx> wrote: > > > > > > On Tue, Feb 25, 2020 at 12:50:24PM +0100, Daniel Vetter wrote: > > > > When doing an atomic modeset with ALLOW_MODESET drivers are allowed to > > > > pull in arbitrary other resources, including CRTCs (e.g. when > > > > reconfiguring global resources). > > > > > > > > But in nonblocking mode userspace has then no idea this happened, > > > > which can lead to spurious EBUSY calls, both: > > > > - when that other CRTC is currently busy doing a page_flip the > > > > ALLOW_MODESET commit can fail with an EBUSY > > > > - on the other CRTC a normal atomic flip can fail with EBUSY because > > > > of the additional commit inserted by the kernel without userspace's > > > > knowledge > > > > > > > > For blocking commits this isn't a problem, because everyone else will > > > > just block until all the CRTC are reconfigured. Only thing userspace > > > > can notice is the dropped frames without any reason for why frames got > > > > dropped. > > > > > > > > Consensus is that we need new uapi to handle this properly, but no one > > > > has any idea what exactly the new uapi should look like. As a stop-gap > > > > plug this problem by demoting nonblocking commits which might cause > > > > issues by including CRTCs not in the original request to blocking > > > > commits. > > > > > > > > v2: Add comments and a WARN_ON to enforce this only when allowed - we > > > > don't want to silently convert page flips into blocking plane updates > > > > just because the driver is buggy. > > > > > > > > v3: Fix inverted WARN_ON (Pekka). > > > > > > > > References: https://lists.freedesktop.org/archives/dri-devel/2018-July/182281.html > > > > Bugzilla: https://gitlab.freedesktop.org/wayland/weston/issues/24#note_9568 > > > > Cc: Daniel Stone <daniel@xxxxxxxxxxxxx> > > > > Cc: Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxxxx> > > > > Cc: stable@xxxxxxxxxxxxxxx > > > > Reviewed-by: Daniel Stone <daniels@xxxxxxxxxxxxx> > > > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > > > > --- > > > > drivers/gpu/drm/drm_atomic.c | 34 +++++++++++++++++++++++++++++++--- > > > > 1 file changed, 31 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c > > > > index 9ccfbf213d72..4c035abf98b8 100644 > > > > --- a/drivers/gpu/drm/drm_atomic.c > > > > +++ b/drivers/gpu/drm/drm_atomic.c > > > > @@ -1362,15 +1362,43 @@ EXPORT_SYMBOL(drm_atomic_commit); > > > > int drm_atomic_nonblocking_commit(struct drm_atomic_state *state) > > > > { > > > > struct drm_mode_config *config = &state->dev->mode_config; > > > > - int ret; > > > > + unsigned requested_crtc = 0; > > > > + unsigned affected_crtc = 0; > > > > + struct drm_crtc *crtc; > > > > + struct drm_crtc_state *crtc_state; > > > > + bool nonblocking = true; > > > > + int ret, i; > > > > + > > > > + /* > > > > + * For commits that allow modesets drivers can add other CRTCs to the > > > > + * atomic commit, e.g. when they need to reallocate global resources. > > > > + * > > > > + * But when userspace also requests a nonblocking commit then userspace > > > > + * cannot know that the commit affects other CRTCs, which can result in > > > > + * spurious EBUSY failures. Until we have better uapi plug this by > > > > + * demoting such commits to blocking mode. > > > > + */ > > > > + for_each_new_crtc_in_state(state, crtc, crtc_state, i) > > > > + requested_crtc |= drm_crtc_mask(crtc); > > > > > > > > ret = drm_atomic_check_only(state); > > > > if (ret) > > > > return ret; > > > > > > > > - DRM_DEBUG_ATOMIC("committing %p nonblocking\n", state); > > > > + for_each_new_crtc_in_state(state, crtc, crtc_state, i) > > > > + affected_crtc |= drm_crtc_mask(crtc); > > > > + > > > > + if (affected_crtc != requested_crtc) { > > > > + /* adding other CRTC is only allowed for modeset commits */ > > > > + WARN_ON(!state->allow_modeset); > > > > > > Not sure that's really true. What if the driver needs to eg. > > > redistribute FIFO space or something between the pipes? Or do we > > > expect drivers to now examine state->allow_modeset to figure out > > > if they're allowed to do certain things? > > > > Maybe we need more fine-grained flags here, but adding other states > > (and blocking a commit flow) is exactly the uapi headaches this patch > > tries to solve here. So if our driver currently adds crtc states to > > reallocate fifo between pipes for an atomic flip then yes we're > > breaking userspace. Well, everyone figured out by now that you get > > random EBUSY and dropped frames for no apparent reason at all, and > > work around it. But happy, they are not. > > I don't think we do this currently for the FIFO, but in theory we > could. > > The one thing we might do currently is cdclk reprogramming, but that > can only happen without a full modeset when there's only a single > active pipe. So we shouldn't hit this right now. But that restriction > is going to disappear in the future, at which point we may want to > do this even with multiple active pipes. Looks like we're hitting something like this on some CI systems. After a bit of pondering I guess we could fix that by not sending out any flips events until all crtcs have finished the commit. Not a full solution though as it can't help if there are multiple threads trying to commit independently on different CRTC and one thread happens to need a full modeset on all CRTCs. But seems like it should solve the the single threaded CI fails we're seeing. -- Ville Syrjälä Intel
