Re: [RFC] drm/atomic+msm: add helper to implement legacy dirtyfb

[Daniel Vetter]

On Wed, Apr 04, 2018 at 12:03:00PM +0200, Daniel Vetter wrote:
> On Tue, Apr 03, 2018 at 06:42:23PM -0400, Rob Clark wrote:
> > Add an atomic helper to implement dirtyfb support.  This is needed to
> > support DSI command-mode panels with x11 userspace (ie. when we can't
> > rely on pageflips to trigger a flush to the panel).
> > 
> > To signal to the driver that the async atomic update needs to
> > synchronize with fences, even though the fb didn't change, the
> > drm_atomic_state::dirty flag is added.
> > 
> > Signed-off-by: Rob Clark <robdclark@xxxxxxxxx>
> > ---
> > Background: there are a number of different folks working on getting
> > upstream kernel working on various different phones/tablets with qcom
> > SoC's.. many of them have command mode panels, so we kind of need a
> > way to support the legacy dirtyfb ioctl for x11 support.
> > 
> > I know there is work on a proprer non-legacy atomic property for
> > userspace to communicate dirty-rect(s) to the kernel, so this can
> > be improved from triggering a full-frame flush once that is in
> > place.  But we kinda needa a stop-gap solution.
> > 
> > I had considered an in-driver solution for this, but things get a
> > bit tricky if userspace ands up combining dirtyfb ioctls with page-
> > flips, because we need to synchronize setting various CTL.FLUSH bits
> > with setting the CTL.START bit.  (ie. really all we need to do for
> > cmd mode panels is bang CTL.START, but is this ends up racing with
> > pageflips setting FLUSH bits, then bad things.)  The easiest soln
> > is to wrap this up as an atomic commit and rely on the worker to
> > serialize things.  Hence adding an atomic dirtyfb helper.
> > 
> > I guess at least the helper, with some small addition to translate
> > and pass-thru the dirty rect(s) is useful to the final atomic dirty-
> > rect property solution.  Depending on how far off that is, a stop-
> > gap solution could be useful.
> > 
> >  drivers/gpu/drm/drm_atomic_helper.c | 66 +++++++++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/msm/msm_atomic.c    |  5 ++-
> >  drivers/gpu/drm/msm/msm_fb.c        |  1 +
> >  include/drm/drm_atomic_helper.h     |  4 +++
> >  include/drm/drm_plane.h             |  9 +++++
> >  5 files changed, 84 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index c35654591c12..a578dc681b27 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -3504,6 +3504,7 @@ void __drm_atomic_helper_plane_duplicate_state(struct drm_plane *plane,
> >  	if (state->fb)
> >  		drm_framebuffer_get(state->fb);
> >  
> > +	state->dirty = false;
> >  	state->fence = NULL;
> >  	state->commit = NULL;
> >  }
> > @@ -3847,6 +3848,71 @@ int drm_atomic_helper_legacy_gamma_set(struct drm_crtc *crtc,
> >  }
> >  EXPORT_SYMBOL(drm_atomic_helper_legacy_gamma_set);
> >  
> > +/**
> > + * drm_atomic_helper_dirtyfb - helper for dirtyfb
> > + *
> > + * A helper to implement drm_framebuffer_funcs::dirty
> > + */
> > +int drm_atomic_helper_dirtyfb(struct drm_framebuffer *fb,
> > +			      struct drm_file *file_priv, unsigned flags,
> > +			      unsigned color, struct drm_clip_rect *clips,
> > +			      unsigned num_clips)
> > +{
> > +	struct drm_modeset_acquire_ctx ctx;
> > +	struct drm_atomic_state *state;
> > +	struct drm_plane *plane;
> > +	int ret = 0;
> > +
> > +	/*
> > +	 * When called from ioctl, we are interruptable, but not when
> > +	 * called internally (ie. defio worker)
> > +	 */
> > +	drm_modeset_acquire_init(&ctx,
> > +		file_priv ? DRM_MODESET_ACQUIRE_INTERRUPTIBLE : 0);
> > +
> > +	state = drm_atomic_state_alloc(fb->dev);
> > +	if (!state) {
> > +		ret = -ENOMEM;
> > +		goto out;
> > +	}
> > +	state->acquire_ctx = &ctx;
> > +
> > +retry:
> > +	drm_for_each_plane(plane, fb->dev) {
> > +		struct drm_plane_state *plane_state;
> > +
> > +		if (plane->state->fb != fb)
> > +			continue;
> > +
> > +		plane_state = drm_atomic_get_plane_state(state, plane);
> > +		if (IS_ERR(plane_state)) {
> > +			ret = PTR_ERR(plane_state);
> > +			goto out;
> > +		}
> > +
> > +		plane_state->dirty = true;
> > +	}
> > +
> > +	ret = drm_atomic_nonblocking_commit(state);
> > +
> > +out:
> > +	if (ret == -EDEADLK) {
> > +		drm_atomic_state_clear(state);
> > +		ret = drm_modeset_backoff(&ctx);
> > +		if (!ret)
> > +			goto retry;
> > +	}
> > +
> > +	drm_atomic_state_put(state);
> > +
> > +	drm_modeset_drop_locks(&ctx);
> > +	drm_modeset_acquire_fini(&ctx);
> > +
> > +	return ret;
> > +
> > +}
> > +EXPORT_SYMBOL(drm_atomic_helper_dirtyfb);
> > +
> >  /**
> >   * __drm_atomic_helper_private_duplicate_state - copy atomic private state
> >   * @obj: CRTC object
> > diff --git a/drivers/gpu/drm/msm/msm_atomic.c b/drivers/gpu/drm/msm/msm_atomic.c
> > index bf5f8c39f34d..bb55a048e98b 100644
> > --- a/drivers/gpu/drm/msm/msm_atomic.c
> > +++ b/drivers/gpu/drm/msm/msm_atomic.c
> > @@ -201,7 +201,10 @@ int msm_atomic_commit(struct drm_device *dev,
> >  	 * Figure out what fence to wait for:
> >  	 */
> >  	for_each_oldnew_plane_in_state(state, plane, old_plane_state, new_plane_state, i) {
> > -		if ((new_plane_state->fb != old_plane_state->fb) && new_plane_state->fb) {
> > +		bool sync_fb = new_plane_state->fb &&
> > +			((new_plane_state->fb != old_plane_state->fb) ||
> > +			 new_plane_state->dirty);
> 
> Why do you have this optimization even here? Imo flipping to the same fb
> should result in the fb getting fully uploaded, whether you're doing a
> legacy page_flip, and atomic one or just a plane update.
> 
> Iirc some userspace does use that as essentially a full-plane frontbuffer
> rendering flush already. IOW I don't think we need your
> plane_state->dirty, it's implied to always be true - why would userspace
> do a flip otherwise?
> 
> The helper itself to map dirtyfb to a nonblocking atomic commit looks
> reasonable, but misses a bunch of the trickery discussed with Noralf and
> others I think.

Ok, I've done some history digging:

- i915 and nouveau unconditionally wait for fences, even for same-fb
  flips.
- no idea what amdgpu and vmwgfx are doing, they're not using
  plane_state->fence for implicit fences.
- most arm-soc drivers do have this "optimization" in their code, and it
  even managed to get into the new drm_gem_fb_prepare_fb helper (which I
  reviewed, or well claimed to have ... oops). Afaict it goes back to the
  original msm atomic code, and was then dutifully copypasted all over the
  place.

If folks are ok I'll do a patch series to align drivers with i915/nouveau.
Well, any driver using reservation_object_get_excl_rcu +
drm_atomic_set_fence_for_plane combo, since amdgpu and vmwgfx don't I have
no idea what they're doing or whether they might have the same bug.

>From looking at at least the various prepare_fb callbacks I don't see any
other drivers doing funny stuff around implicit fences.
-Daniel

> > +		if (sync_fb) {
> >  			struct drm_gem_object *obj = msm_framebuffer_bo(new_plane_state->fb, 0);
> >  			struct msm_gem_object *msm_obj = to_msm_bo(obj);
> >  			struct dma_fence *fence = reservation_object_get_excl_rcu(msm_obj->resv);
> > diff --git a/drivers/gpu/drm/msm/msm_fb.c b/drivers/gpu/drm/msm/msm_fb.c
> > index 0e0c87252ab0..a5d882a34a33 100644
> > --- a/drivers/gpu/drm/msm/msm_fb.c
> > +++ b/drivers/gpu/drm/msm/msm_fb.c
> > @@ -62,6 +62,7 @@ static void msm_framebuffer_destroy(struct drm_framebuffer *fb)
> >  static const struct drm_framebuffer_funcs msm_framebuffer_funcs = {
> >  	.create_handle = msm_framebuffer_create_handle,
> >  	.destroy = msm_framebuffer_destroy,
> > +	.dirty = drm_atomic_helper_dirtyfb,
> >  };
> >  
> >  #ifdef CONFIG_DEBUG_FS
> > diff --git a/include/drm/drm_atomic_helper.h b/include/drm/drm_atomic_helper.h
> > index 26aaba58d6ce..9b7a95c2643d 100644
> > --- a/include/drm/drm_atomic_helper.h
> > +++ b/include/drm/drm_atomic_helper.h
> > @@ -183,6 +183,10 @@ int drm_atomic_helper_legacy_gamma_set(struct drm_crtc *crtc,
> >  				       u16 *red, u16 *green, u16 *blue,
> >  				       uint32_t size,
> >  				       struct drm_modeset_acquire_ctx *ctx);
> > +int drm_atomic_helper_dirtyfb(struct drm_framebuffer *fb,
> > +			      struct drm_file *file_priv, unsigned flags,
> > +			      unsigned color, struct drm_clip_rect *clips,
> > +			      unsigned num_clips);
> >  void __drm_atomic_helper_private_obj_duplicate_state(struct drm_private_obj *obj,
> >  						     struct drm_private_state *state);
> >  
> > diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h
> > index f7bf4a48b1c3..296fa22bda7a 100644
> > --- a/include/drm/drm_plane.h
> > +++ b/include/drm/drm_plane.h
> > @@ -76,6 +76,15 @@ struct drm_plane_state {
> >  	 */
> >  	struct drm_framebuffer *fb;
> >  
> > +	/**
> > +	 * @dirty:
> > +	 *
> > +	 * Flag that indicates the fb contents have changed even though the
> > +	 * fb has not.  This is mostly a stop-gap solution until we have
> > +	 * atomic dirty-rect(s) property.
> > +	 */
> > +	bool dirty;
> > +
> >  	/**
> >  	 * @fence:
> >  	 *
> > -- 
> > 2.14.3
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch