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@fooishbar.org Cc: Pekka Paalanen pekka.paalanen@collabora.co.uk Cc: stable@vger.kernel.org Reviewed-by: Daniel Stone daniels@collabora.com Cc: Ville Syrjälä ville.syrjala@linux.intel.com Signed-off-by: Daniel Vetter daniel.vetter@intel.com -- Resending because last attempt failed CI and meanwhile the results are lost :-/ -Daniel --- 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 965173fd0ac2..4f140ff6fb98 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); + + DRM_DEBUG_ATOMIC("demoting %p to blocking mode to avoid EBUSY\n", state); + nonblocking = false; + } else { + DRM_DEBUG_ATOMIC("committing %p nonblocking\n", state); + }
- return config->funcs->atomic_commit(state->dev, state, true); + return config->funcs->atomic_commit(state->dev, state, nonblocking); } EXPORT_SYMBOL(drm_atomic_nonblocking_commit);
On Wed, 8 Apr 2020 at 17:24, Daniel Vetter daniel.vetter@ffwll.ch wrote:
Resending because last attempt failed CI and meanwhile the results are lost :-/
Did anything happen with this?
Cheers, Daniel
On Thu, May 14, 2020 at 8:42 AM Daniel Stone daniel@fooishbar.org wrote:
On Wed, 8 Apr 2020 at 17:24, Daniel Vetter daniel.vetter@ffwll.ch wrote:
Resending because last attempt failed CI and meanwhile the results are lost :-/
Did anything happen with this?
Nope. There's an igt now that fails with this, and I'm not sure whether changing the igt is the right idea or not.
I'm kinda now thinking about changing this to instead document under which exact situations you can get a spurious EBUSY, and enforcing that in the code with some checks. Essentially only possible if you do a ALLOW_MODESET | NONBLOCKING on the other crtc. And then tell userspace you get to eat that. We've been shipping with this for so long by now that's defacto the uapi anyway :-/
Thoughts? Too horrible? -Daniel
Hi,
On Thu, 14 May 2020 at 08:08, Daniel Vetter daniel.vetter@ffwll.ch wrote:
Did anything happen with this?
Nope. There's an igt now that fails with this, and I'm not sure whether changing the igt is the right idea or not.
I'm kinda now thinking about changing this to instead document under which exact situations you can get a spurious EBUSY, and enforcing that in the code with some checks. Essentially only possible if you do a ALLOW_MODESET | NONBLOCKING on the other crtc. And then tell userspace you get to eat that. We've been shipping with this for so long by now that's defacto the uapi anyway :-/
Thoughts? Too horrible?
I've been trying to avoid that, to be honest. Taking a random delay because the kernel needs to do global things is fine. But making userspace either do an expensive/complicated cross-CRTC synchronisation is less easy; for some compositors, that means reaching across threads to make sure all CRTCs are quiescent. Either that, or deferring your ALLOW_MODESET to somewhere else, like an idle handler, far away from where you were originally trying to do it, which wouldn't be pleasant. The other option is that we teach people to ignore EBUSY as random noise which can just sometimes happen and to try again (when? how often? and you still have cross-CRTC synchronisation issues), which doesn't scream compositor best practice to me.
I'd be very much in favour of putting the blocking down in the kernel at least until the kernel can give us a clear indication to tell us what's going on, and ideally which other resources need to be dragged in, in a way which is distinguishable from your compositor having broken synchronisation.
Cheers, Daniel
On Thu, May 14, 2020 at 9:18 AM Daniel Stone daniel@fooishbar.org wrote:
Hi,
On Thu, 14 May 2020 at 08:08, Daniel Vetter daniel.vetter@ffwll.ch wrote:
Did anything happen with this?
Nope. There's an igt now that fails with this, and I'm not sure whether changing the igt is the right idea or not.
I'm kinda now thinking about changing this to instead document under which exact situations you can get a spurious EBUSY, and enforcing that in the code with some checks. Essentially only possible if you do a ALLOW_MODESET | NONBLOCKING on the other crtc. And then tell userspace you get to eat that. We've been shipping with this for so long by now that's defacto the uapi anyway :-/
Thoughts? Too horrible?
I've been trying to avoid that, to be honest. Taking a random delay because the kernel needs to do global things is fine. But making userspace either do an expensive/complicated cross-CRTC synchronisation is less easy; for some compositors, that means reaching across threads to make sure all CRTCs are quiescent. Either that, or deferring your ALLOW_MODESET to somewhere else, like an idle handler, far away from where you were originally trying to do it, which wouldn't be pleasant. The other option is that we teach people to ignore EBUSY as random noise which can just sometimes happen and to try again (when? how often? and you still have cross-CRTC synchronisation issues), which doesn't scream compositor best practice to me.
I'd be very much in favour of putting the blocking down in the kernel at least until the kernel can give us a clear indication to tell us what's going on, and ideally which other resources need to be dragged in, in a way which is distinguishable from your compositor having broken synchronisation.
We know, the patch already computes that ... So would be a matter of exporting that to userspace. We have a mask of all additional crtc that will get an event and will -EBUSY until that's done. -Daniel
On Thu, 14 May 2020 at 08:25, Daniel Vetter daniel.vetter@ffwll.ch wrote:
On Thu, May 14, 2020 at 9:18 AM Daniel Stone daniel@fooishbar.org wrote:
On Thu, 14 May 2020 at 08:08, Daniel Vetter daniel.vetter@ffwll.ch wrote: I'd be very much in favour of putting the blocking down in the kernel at least until the kernel can give us a clear indication to tell us what's going on, and ideally which other resources need to be dragged in, in a way which is distinguishable from your compositor having broken synchronisation.
We know, the patch already computes that ... So would be a matter of exporting that to userspace. We have a mask of all additional crtc that will get an event and will -EBUSY until that's done.
Yep, but unless and until that happens, could we please get this in? Given it would require uAPI changes, we'd need to modify all the compositors to work with the old path (random EBUSY) and the new path (predictable and obvious), so at least preserving the promise that per-CRTC updates are really independent would be good.
Cheers, Daniel
On Thu, May 14, 2020 at 08:40:21AM +0100, Daniel Stone wrote:
On Thu, 14 May 2020 at 08:25, Daniel Vetter daniel.vetter@ffwll.ch wrote:
On Thu, May 14, 2020 at 9:18 AM Daniel Stone daniel@fooishbar.org wrote:
On Thu, 14 May 2020 at 08:08, Daniel Vetter daniel.vetter@ffwll.ch wrote: I'd be very much in favour of putting the blocking down in the kernel at least until the kernel can give us a clear indication to tell us what's going on, and ideally which other resources need to be dragged in, in a way which is distinguishable from your compositor having broken synchronisation.
We know, the patch already computes that ... So would be a matter of exporting that to userspace. We have a mask of all additional crtc that will get an event and will -EBUSY until that's done.
Yep, but unless and until that happens, could we please get this in? Given it would require uAPI changes, we'd need to modify all the compositors to work with the old path (random EBUSY) and the new path (predictable and obvious), so at least preserving the promise that per-CRTC updates are really independent would be good.
I haven't found the time to look at the intel-gfx-ci fail in igt nor really think about that. Nor care enough to just hammer this ignoring ci, since I didn't even get around to understand why the igt now fails If someone else takes this over, happy to see it land. -Daniel
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