Messages in this thread | | | From | "Kazlauskas, Nicholas" <> | Subject | Re: [PATCH 1/5] drm: don't block fb changes for async plane updates | Date | Mon, 11 Mar 2019 13:15:23 +0000 |
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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.
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); 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.
Nicholas Kazlauskas
> >> >> I do think it would be good to add something to the function docs that >> explains this requirement and the issue that it addresses. It's a little >> unintuitive to require that the old fb is placed into the new state, but >> it makes sense as a workaround to this problem. >> >> Nicholas Kazlauskas >> >>> } >>> } >>> EXPORT_SYMBOL(drm_atomic_helper_async_commit); >>> >> >
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