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

[Boris Brezillon <boris.brezillon@collabora.com>]

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 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);
> >   
>