[RFC 1/7] drm/atomic: initial support for asynchronous plane update
Daniel Vetter
daniel at ffwll.ch
Tue Apr 11 20:23:39 UTC 2017
On Mon, Apr 10, 2017 at 12:55:33PM -0700, Eric Anholt wrote:
> Gustavo Padovan <gustavo at padovan.org> writes:
>
> > From: Gustavo Padovan <gustavo.padovan at collabora.com>
> >
> > In some cases, like cursor updates, it is interesting to update the
> > plane in an asynchronous fashion to avoid big delays.
> >
> > The current queued update could be still waiting for a fence to signal and
> > thus block and subsequent update until its scan out. In cases like this if
> > we update the cursor synchronously it will cause significant delays on
> > some platforms that would be noticed by the final user.
> >
> > This patch creates a fast path to jump ahead the current queued state and
> > do single planes updates without going through all atomic step in
> > drm_atomic_helper_commit().
> >
> > For now only single plane updates are supported and only if there is no
> > update to that plane in the queued state.
> >
> > We plan to support async PageFlips through this interface as well in the
> > near future.
>
> This looks really nice -- the vc4 code for cursors was really gross to
> write, and I got it wrong a couple of times. Couple of comments inline.
Some more comments below.
-Daniel
>
> > ---
> > drivers/gpu/drm/drm_atomic.c | 75 ++++++++++++++++++++++++++++++++
> > drivers/gpu/drm/drm_atomic_helper.c | 41 +++++++++++++++++
> > include/drm/drm_atomic.h | 4 ++
> > include/drm/drm_atomic_helper.h | 2 +
> > include/drm/drm_modeset_helper_vtables.h | 47 ++++++++++++++++++++
> > 5 files changed, 169 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> > index f32506a..cae0122 100644
> > --- a/drivers/gpu/drm/drm_atomic.c
> > +++ b/drivers/gpu/drm/drm_atomic.c
> > @@ -631,6 +631,79 @@ static int drm_atomic_crtc_check(struct drm_crtc *crtc,
> > return 0;
> > }
> >
> > +static bool drm_atomic_async_check(struct drm_atomic_state *state)
> > +{
> > + struct drm_crtc *crtc;
> > + struct drm_crtc_state *crtc_state;
> > + struct drm_crtc_commit *commit;
> > + struct drm_plane *__plane, *plane = NULL;
> > + struct drm_plane_state *__plane_state, *plane_state = NULL;
> > + const struct drm_plane_helper_funcs *funcs;
> > + int i, n_planes = 0;
> > +
> > + for_each_new_crtc_in_state(state, crtc, crtc_state, i) {
> > + if (drm_atomic_crtc_needs_modeset(crtc_state))
> > + return false;
> > + }
> > +
> > + for_each_new_plane_in_state(state, __plane, __plane_state, i) {
> > + n_planes++;
> > + plane = __plane;
> > + plane_state = __plane_state;
> > + }
> > +
> > + if (!plane || n_planes != 1)
> > + return false;
> > +
> > + if (!plane->state->crtc)
> > + return false;
> > +
> > + if (plane_state->fence)
> > + return false;
> > +
> > + for_each_new_crtc_in_state(state, crtc, crtc_state, i) {
> > + if (plane->crtc != crtc)
> > + continue;
> > +
> > + spin_lock(&crtc->commit_lock);
> > + commit = list_first_entry_or_null(&crtc->commit_list,
> > + struct drm_crtc_commit,
> > + commit_entry);
> > + if (!commit) {
> > + spin_unlock(&crtc->commit_lock);
> > + continue;
> > + }
> > + spin_unlock(&crtc->commit_lock);
> > +
> > + for_each_plane_in_state(crtc_state->state, __plane,
> > + __plane_state, i) {
> > + if (__plane == plane) {
> > + return false;
> > + }
> > + }
> > + }
> > +
> > + /* Not configuring new scaling in the async path. */
>
> s/Not/No/
>
> > + if (plane->state->crtc_w != plane_state->crtc_w ||
> > + plane->state->crtc_h != plane_state->crtc_h ||
> > + plane->state->src_w != plane_state->src_w ||
> > + plane->state->src_h != plane_state->src_h) {
> > + return false;
> > + }
> > +
> > + funcs = plane->helper_private;
> > +
> > + if (!funcs->atomic_async_update)
> > + return false;
I kinda feel like we should check this first, to bail out for all the
drivers that don't implement this before any of the more expensive checks.
> > +
> > + if (funcs->atomic_async_check) {
This is redundant.
> > + if (funcs->atomic_async_check(plane, plane_state) < 0)
> > + return false;
Of course the callback should be still called at the end, so it doesn't
have to check stuff.
> > + }
> > +
> > + return true;
> > +}
> > +
> > static void drm_atomic_crtc_print_state(struct drm_printer *p,
> > const struct drm_crtc_state *state)
> > {
> > @@ -1591,6 +1664,8 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> > }
> > }
> >
> > + state->async_update = drm_atomic_async_check(state);
> > +
> > return ret;
> > }
>
> The promotion of any modeset that passes the async_check test to async
> seems weird -- shouldn't we only be doing that if userspace requested
> async? Right now it seems like we're just getting lucky because only
> cursor planes have it set, and the cursor update ioctls imply async.
Yeah, we should only do this when the async flag is set, and set that both
in the page_flip helpers and the cursor helpers.
One thing I wonder is whether we need to differentiate between "pls do
async if you can" and "I want async or let the update fail". Cursors would
be the former, page_flips probably too, but for async through atomic we
might also need the later. But that's just an aside, we can easily wire
this up when we have userspace asking for it (it might come real handy for
VR).
>
> > EXPORT_SYMBOL(drm_atomic_check_only);
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index 8be9719..a79e06c 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -1235,6 +1235,36 @@ static void commit_work(struct work_struct *work)
> > }
> >
> > /**
> > + * drm_atomic_helper_async_commit - commit state asynchronously
> > + *
> > + * This function commits a state asynchronously. It should be used
> > + * on a state only when drm_atomic_async_check() succeds. Async
> > + * commits are not supposed to swap the states like normal sync commits,
> > + * but just do in-place change on the current state.
> > + */
> > +void drm_atomic_helper_async_commit(struct drm_device *dev,
> > + struct drm_atomic_state *state)
> > +{
> > + struct drm_plane *plane;
> > + struct drm_plane_state *plane_state;
> > + const struct drm_plane_helper_funcs *funcs;
> > + int i;
> > +
> > + for_each_new_plane_in_state(state, plane, plane_state, i) {
> > + funcs = plane->helper_private;
> > +
> > + plane->state->src_x = plane_state->src_x;
> > + plane->state->src_y = plane_state->src_y;
> > + plane->state->crtc_x = plane_state->crtc_x;
> > + plane->state->crtc_y = plane_state->crtc_y;
> > +
> > + if (funcs && funcs->atomic_async_update)
> > + funcs->atomic_async_update(plane, plane_state);
> > + }
> > +}
>
> It seems strange to me that the async_update() implementation in the
> driver needs to update the state->fb but not the x/y. Could we move the
> core's x/y update after the call into the driver, and then update
> plane->state->fb in the core instead of the driver?
Yeah, consistency would be real good here ...
> > +EXPORT_SYMBOL(drm_atomic_helper_async_commit);
> > +
> > +/**
> > * drm_atomic_helper_commit - commit validated state object
> > * @dev: DRM device
> > * @state: the driver state object
> > @@ -1258,6 +1288,17 @@ int drm_atomic_helper_commit(struct drm_device *dev,
> > {
> > int ret;
> >
> > + if (state->async_update) {
> > + ret = drm_atomic_helper_prepare_planes(dev, state);
> > + if (ret)
> > + return ret;
> > +
> > + drm_atomic_helper_async_commit(dev, state);
> > + drm_atomic_helper_cleanup_planes(dev, state);
Some async cursor updates are not 100% async, as in the hw might still
scan out the old buffer until the next vblank. Could/should we handle this
in core? Or are we going to shrug this off with "meh, you asked for
tearing, you got tearing"?
> > +
> > + return 0;
> > + }
> > +
> > ret = drm_atomic_helper_setup_commit(state, nonblock);
> > if (ret)
> > return ret;
> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> > index 788daf7..c00c565 100644
> > --- a/include/drm/drm_atomic.h
> > +++ b/include/drm/drm_atomic.h
>
> > diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> > index c01c328..3efa4cc 100644
> > --- a/include/drm/drm_modeset_helper_vtables.h
> > +++ b/include/drm/drm_modeset_helper_vtables.h
>
> > + *
> > + * Check drm_atomic_async_check() to see what is already there
> > + * to discover potential async updates.
> > + *
> > + * RETURNS:
> > + *
> > + * Return 0 on success and -EINVAL if the update can't be async.
> > + * Error return doesn't mean that the update can't be applied,
> > + * it just mean that it can't be an async one.
> > + *
> > + * FIXME:
> > + * - It only works for single plane updates
> > + * - It can't do async update if the plane is already being update
> > + * by the queued state
>
> "already being updated"
>
> > + * - Async Pageflips are not supported yet
> > + */
> > + int (*atomic_async_check)(struct drm_plane *plane,
> > + struct drm_plane_state *state);
> > +
> > + /**
> > + * @atomic_async_update:
> > + *
> > + * Drivers should use this functions to perform asynchronous
>
> "this function"
>
> Also, maybe clarify that "async" means "not vblank synchronized"?
>
> > + * updates of planes, that is jump ahead the currently queued
> > + * state for and update the plane.
> > + *
> > + * This hook is called by drm_atomic_helper_async_commit().
> > + *
> > + * Note that differently from the &drm_plane_helper_funcs.atomic_update
> > + * this hook takes the new &drm_plane_state as parameter. When doing
> > + * async_update drivers shouldn't replace the &drm_plane_state but
> > + * just, update the current one with the new plane configurations in
> > + * the new plane_state.
> > + */
> > + void (*atomic_async_update)(struct drm_plane *plane,
> > + struct drm_plane_state *new_state);
> > };
> >
> > /**
> > --
> > 2.9.3
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--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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