Re: [PATCH 1/5] drm: don't block fb changes for async plane updates

Daniel Vetter
2019-03-11T19:51:35Z

On Mon, Mar 11, 2019 at 03:20:09PM +0100, Boris Brezillon wrote:
> 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.

Ok I dug around again, and I think I reconstructed the problem again.

The issue is the lifetimes of state structs. The nonblocking commit worker
doesn't hold a reference onto the new states at all. The only reason those
new states cannot disappear is that the next atomic comit touching the
same states waits for crtc_commit.hw_done before it pushes its own update
through (and then goes and releases those state structures).

The old state has no such issue, since each commit takes ownership of the
old state and then releases it. And can do that any time after hw_done.

Now with the current async code that's no issue, because we do check for
hw_done. The trouble is that hw_done is a kernel-internal implementation
detail. The only think userspace can observe is flip_done, and that's
what's used for -EBUSY for normal page-flips. For cursor this kinda
doesn't matter, because these two should be fairly close together (in most
cases hw_done even happens before flip_done, but that depends upon the
driver). So the occasional silent fallback to a synchronous commit doesn't
really matter.

What we could do is just wait for hw_done for async commits, but that's
kinda not cool either since it blocks (again cursor is ill-defined enough
that it doesn't matter). And pushing async updates to a worker means we
need to greatly extend the crtc_commit tracking (at least to each plane
state). I think most of that exist now, since we had to add it anyway for
planes which can be reassigned between crtc.

tldr; maybe we can do the full swapping now?

I agree it feels like the cleaner solution, but definitely need a pile of
igt tests to make sure we can mix&match between async and sync commits and
nothing blows up. And sync commits need to use reassignment of planes to
different crtcs plus nonblocking commit (I think amd hw can do all that,
or at least I've seen prep patches).
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation