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

Daniel Vetter
2019-03-11T19:53:59Z

On Mon, Mar 04, 2019 at 03:46:49PM +0000, Kazlauskas, Nicholas 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.
>
> 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.

Agreed.

And yeah this looks like a reasonable short-term fix.
-Daniel

>
> Nicholas Kazlauskas
>
> > }
> > }
> > EXPORT_SYMBOL(drm_atomic_helper_async_commit);
> >
>

--
Daniel Vetter
Software Engineer, Intel Corporation