Messages in this thread Patch in this message | | | Date | Mon, 11 Mar 2019 15:20:09 +0100 | From | Boris Brezillon <> | Subject | Re: [PATCH 1/5] drm: don't block fb changes for async plane updates |
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On Mon, 11 Mar 2019 13:15:23 +0000 "Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com> wrote:
> On 3/11/19 6:06 AM, Boris Brezillon wrote: > > Hello Nicholas, > > > > On Mon, 4 Mar 2019 15:46:49 +0000 > > "Kazlauskas, Nicholas" <Nicholas.Kazlauskas@amd.com> wrote: > > > >> On 3/4/19 9:49 AM, Helen Koike wrote: > >>> In the case of a normal sync update, the preparation of framebuffers (be > >>> it calling drm_atomic_helper_prepare_planes() or doing setups with > >>> drm_framebuffer_get()) are performed in the new_state and the respective > >>> cleanups are performed in the old_state. > >>> > >>> In the case of async updates, the preparation is also done in the > >>> new_state but the cleanups are done in the new_state (because updates > >>> are performed in place, i.e. in the current state). > >>> > >>> The current code blocks async udpates when the fb is changed, turning > >>> async updates into sync updates, slowing down cursor updates and > >>> introducing regressions in igt tests with errors of type: > >>> > >>> "CRITICAL: completed 97 cursor updated in a period of 30 flips, we > >>> expect to complete approximately 15360 updates, with the threshold set > >>> at 7680" > >>> > >>> Fb changes in async updates were prevented to avoid the following scenario: > >>> > >>> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, cleanup fb1 > >>> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb2 > >>> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 (wrong) > >>> Where we have a single call to prepare fb2 but double cleanup call to fb2. > >>> > >>> To solve the above problems, instead of blocking async fb changes, we > >>> place the old framebuffer in the new_state object, so when the code > >>> performs cleanups in the new_state it will cleanup the old_fb and we > >>> will have the following scenario instead: > >>> > >>> - Async update, oldfb = NULL, newfb = fb1, prepare fb1, no cleanup > >>> - Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb1 > >>> - Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 > >>> > >>> Where calls to prepare/cleanup are ballanced. > >>> > >>> Cc: <stable@vger.kernel.org> # v4.14+: 25dc194b34dd: drm: Block fb changes for async plane updates > >>> Fixes: 25dc194b34dd ("drm: Block fb changes for async plane updates") > >>> Suggested-by: Boris Brezillon <boris.brezillon@collabora.com> > >>> Signed-off-by: Helen Koike <helen.koike@collabora.com> > >>> > >>> --- > >>> Hello, > >>> > >>> As mentioned in the cover letter, > >>> I tested on the rockchip and on i915 (with a patch I am still working on for > >>> replacing cursors by async update), with igt plane_cursor_legacy and > >>> kms_cursor_legacy and I didn't see any regressions. > >>> I couldn't test on MSM and AMD because I don't have the hardware (and I am > >>> having some issues testing on vc4) and I would appreciate if anyone could help > >>> me testing those. > >>> > >>> I also think it would be a better solution if, instead of having async > >>> to do in-place updates in the current state, the async path should be > >>> equivalent to a syncronous update, i.e., modifying new_state and > >>> performing a flip > >>> IMHO, the only difference between sync and async should be that async update > >>> doesn't wait for vblank and applies the changes immeditally to the hw, > >>> but the code path could be almost the same. > >>> But for now I think this solution is ok (swaping new_fb/old_fb), and > >>> then we can adjust things little by little, what do you think? > >>> > >>> Thanks! > >>> Helen > >>> > >>> drivers/gpu/drm/drm_atomic_helper.c | 20 ++++++++++---------- > >>> 1 file changed, 10 insertions(+), 10 deletions(-) > >>> > >>> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c > >>> index 540a77a2ade9..e7eb96f1efc2 100644 > >>> --- a/drivers/gpu/drm/drm_atomic_helper.c > >>> +++ b/drivers/gpu/drm/drm_atomic_helper.c > >>> @@ -1608,15 +1608,6 @@ int drm_atomic_helper_async_check(struct drm_device *dev, > >>> old_plane_state->crtc != new_plane_state->crtc) > >>> return -EINVAL; > >>> > >>> - /* > >>> - * FIXME: Since prepare_fb and cleanup_fb are always called on > >>> - * the new_plane_state for async updates we need to block framebuffer > >>> - * changes. This prevents use of a fb that's been cleaned up and > >>> - * double cleanups from occuring. > >>> - */ > >>> - if (old_plane_state->fb != new_plane_state->fb) > >>> - return -EINVAL; > >>> - > >>> funcs = plane->helper_private; > >>> if (!funcs->atomic_async_update) > >>> return -EINVAL; > >>> @@ -1657,6 +1648,9 @@ void drm_atomic_helper_async_commit(struct drm_device *dev, > >>> int i; > >>> > >>> for_each_new_plane_in_state(state, plane, plane_state, i) { > >>> + struct drm_framebuffer *new_fb = plane_state->fb; > >>> + struct drm_framebuffer *old_fb = plane->state->fb; > >>> + > >>> funcs = plane->helper_private; > >>> funcs->atomic_async_update(plane, plane_state); > >>> > >>> @@ -1665,11 +1659,17 @@ void drm_atomic_helper_async_commit(struct drm_device *dev, > >>> * plane->state in-place, make sure at least common > >>> * properties have been properly updated. > >>> */ > >>> - WARN_ON_ONCE(plane->state->fb != plane_state->fb); > >>> + WARN_ON_ONCE(plane->state->fb != new_fb); > >>> WARN_ON_ONCE(plane->state->crtc_x != plane_state->crtc_x); > >>> WARN_ON_ONCE(plane->state->crtc_y != plane_state->crtc_y); > >>> WARN_ON_ONCE(plane->state->src_x != plane_state->src_x); > >>> WARN_ON_ONCE(plane->state->src_y != plane_state->src_y); > >>> + > >>> + /* > >>> + * Make sure the FBs have been swapped so that cleanups in the > >>> + * new_state performs a cleanup in the old FB. > >>> + */ > >>> + WARN_ON_ONCE(plane_state->fb != old_fb); > >> > >> I personally think this approach is fine and the WARN_ON s are good for > >> catching drivers that want to use these in the future. > > > > Well, I agree this change is the way to go for a short-term solution > > to relax the old_fb == new_fb constraint, but I keep thinking this whole > > "update plane_state in place" is a recipe for trouble and just make > > things more complicated for drivers for no obvious reasons. Look at the > > VC4 implem [1] if you need a proof that things can get messy pretty > > quickly. > > > > All this state-fields-copying steps could be skipped if the core was > > simply swapping the old/new states as is done in the sync update path. > > > > [1]https://elixir.bootlin.com/linux/v5.0-rc7/source/drivers/gpu/drm/vc4/vc4_plane.c#L878 > > I completely agree with this view FWIW. I had a discussion with Daniel > about this when I had posted the original block FB changes patch. > > - The plane object needs to be locked in order for async state to be updated > - Blocking commit work holds the lock for the plane, async update won't > happen > - Non-blocking commit work that's still ongoing won't have hw_done > signaled and drm_atomic_helper_async_check will block the async update > > So this looks safe in theory, with the exception of the call to > drm_atomic_helper_cleanup_planes occuring after hw_done is signaled.
Isn't it also the case in the sync update path?
> > I believe that the behavior of this function still remains the same even > if plane->state is swapped to something else during the call (since > old_plane_state should never be equal to plane->state if the commit > succeeded and the plane is in the commit), but I'm not sure that's > something we'd want to rely on. > > I think other than that issue, you could probably just: > > drm_atomic_helper_prepare_planes(...); > drm_atomic_helper_swap_state(...); > drm_atomic_state_get(state);
Why do we need a state_get() here? AFAICT, it's done this way in the sync update path because of the non-blocking semantic where the state might be released by the caller before it's been applied by the commit worker.
> drm_atomic_helper_async_commit(...); > drm_atomic_helper_cleanup_planes(dev, state); > > and it would work as expected. But there still may be other things I'm > missing or haven't considered here.
Actually, when I said we could swap states, I was not necessarily thinking about re-using drm_atomic_helper_swap_state(), but instead swap states directly in drm_atomic_helper_async_commit():
for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) { WARN_ON(plane->state != old_plane_state); old_plane_state->state = state; new_plane_state->state = NULL; state->planes[i].state = old_plane_state; plane->state = new_plane_state;
funcs = plane->helper_private; funcs->atomic_async_update(plane, new_plane_state); }
This way we would avoid the WARN_ON() lines we have in drm_atomic_helper_async_commit() to check that things have been properly updated in-place, and we would also get rid of the driver code copying the plane_state property that can change during an async update.
But, as you said, I might be missing other potential issues.
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