Re: [RFC v2 1/7] drm/atomic: initial support for asynchronous plane update

From: Gustavo Padovan <gustavo@padovan.org>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Gustavo Padovan <gustavo.padovan@collabora.com>,
	dri-devel@lists.freedesktop.org,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [RFC v2 1/7] drm/atomic: initial support for asynchronous plane update
Date: Thu, 27 Apr 2017 16:35:37 -0300	[thread overview]
Message-ID: <20170427193537.GB2568@joana> (raw)
In-Reply-To: <20170427153710.GO30290@intel.com>

Hi Ville,

2017-04-27 Ville Syrjälä <ville.syrjala@linux.intel.com>:

> On Thu, Apr 27, 2017 at 12:15:13PM -0300, Gustavo Padovan wrote:
> > From: Gustavo Padovan <gustavo.padovan@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 any
> > subsequent update until its scan out. In cases like this if we update the
> > cursor synchronously through the atomic API it will cause significant
> > delays that would even 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().
> > 
> > We take this path for legacy cursor updates. Users can also set the
> > DRM_MODE_ATOMIC_ASYNC_UPDATE flag on atomic updates.
> > 
> > For now only single plane updates are supported, but we plan to support
> > multiple planes updates and async PageFlips through this interface as well
> > in the near future.
> > 
> > v2:
> > 	- allow updates even if there is a queued update for the same
> > 	plane.
> >         - fixes on the documentation (Emil Velikov)
> >         - unconditionally call ->atomic_async_update (Emil Velikov)
> >         - check for ->atomic_async_update earlier (Daniel Vetter)
> >         - make ->atomic_async_check() the last step (Daniel Vetter)
> >         - add ASYNC_UPDATE flag (Eric Anholt)
> >         - update state in core after ->atomic_async_update (Eric Anholt)
> > 	- update docs (Eric Anholt)
> > 
> > Cc: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Rob Clark <robdclark@gmail.com>
> > Cc: Eric Anholt <eric@anholt.net>
> > Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
> > ---
> >  drivers/gpu/drm/drm_atomic.c             | 50 ++++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/drm_atomic_helper.c      | 48 ++++++++++++++++++++++++++++++
> >  include/drm/drm_atomic.h                 |  2 ++
> >  include/drm/drm_atomic_helper.h          |  2 ++
> >  include/drm/drm_modeset_helper_vtables.h | 45 ++++++++++++++++++++++++++++
> >  include/uapi/drm/drm_mode.h              |  4 ++-
> >  6 files changed, 150 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> > index 30229ab..7b60cf8 100644
> > --- a/drivers/gpu/drm/drm_atomic.c
> > +++ b/drivers/gpu/drm/drm_atomic.c
> > @@ -77,6 +77,7 @@ drm_atomic_state_init(struct drm_device *dev, struct drm_atomic_state *state)
> >  	 * setting this appropriately?
> >  	 */
> >  	state->allow_modeset = true;
> > +	state->async_update = true;
> >  
> >  	state->crtcs = kcalloc(dev->mode_config.num_crtc,
> >  			       sizeof(*state->crtcs), GFP_KERNEL);
> > @@ -631,6 +632,51 @@ 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_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;
> > +	}
> > +
> > +	/* FIXME: we support single plane updates for now */
> > +	if (!plane || n_planes != 1)
> > +		return false;
> > +
> > +	funcs = plane->helper_private;
> > +	if (!funcs->atomic_async_update)
> > +		return false;
> > +
> > +	if (plane_state->fence)
> > +		return false;
> > +
> > +	if (!plane->state->crtc || (plane->state->crtc != plane_state->crtc))
> > +		return false;
> > +
> > +	/* No configuring new scaling in the async path. */
> 
> Those checks aren't really about scaling. Well, they are also about
> scaling, but they're mainly about changing size.

I just copied the comment from the previous code in the drivers...
I can fix it.

> 
> What I don't really understand is why we're enforcing these restrictions
> in the core but leaving other restrictions up to the driver. I don't see
> size changes as anything really special compared to many of the other
> restrictions that would now be up to each driver.

Because I extracted what was common between msm, vc4 and i915 on their
legacy cursor update calls. I didn't want to enforce anything else in
core for now.

> 
> If you're really after some lowest common denominator as far as
> exposed capabilities are concerned then I think the core should do
> more checking. OTOH if you're not interested limiting what each
> driver exposes then I don't see why the core checks anything at all.

I think a common denominator is what we want but we only have 3 drivers
using it at the moment. We can look for more checks that shoud be done
is core. Any suggestions?

> 
> > +	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;
> > +	}
> > +
> > +	return !funcs->atomic_async_check(plane, plane_state);
> > +}
> > +
> >  static void drm_atomic_crtc_print_state(struct drm_printer *p,
> >  		const struct drm_crtc_state *state)
> >  {
> <snip> 
> >  /**
> > diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> > index 8c67fc0..7c067ca 100644
> > --- a/include/uapi/drm/drm_mode.h
> > +++ b/include/uapi/drm/drm_mode.h
> > @@ -646,13 +646,15 @@ struct drm_mode_destroy_dumb {
> >  #define DRM_MODE_ATOMIC_TEST_ONLY 0x0100
> >  #define DRM_MODE_ATOMIC_NONBLOCK  0x0200
> >  #define DRM_MODE_ATOMIC_ALLOW_MODESET 0x0400
> > +#define DRM_MODE_ATOMIC_ASYNC_UPDATE 0x0800
> 
> What exactly is that flag supposed to mean?

I don't think I provided a good explanation for it, sorry. This flag
tells core to jump ahead the queued update if the conditions in 
drm_atomic_async_check() are met. Useful for cursors and async PageFlips
over the atomic ioctl.

Gustavo
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