[PATCH v2 2/4] drm/atomic: Add accessor macros for the current state.

[Daniel Vetter]

On Wed, Nov 16, 2016 at 05:11:56PM +0100, Maarten Lankhorst wrote:
> Op 16-11-16 om 16:04 schreef Daniel Vetter:
> > On Wed, Nov 16, 2016 at 04:35:45PM +0200, Ville Syrjälä wrote:
> >> On Wed, Nov 16, 2016 at 02:58:06PM +0100, Maarten Lankhorst wrote:
> >>> With checks! This will allow safe access to the current state,
> >>> while ensuring that the correct locks are held.
> >>>
> >>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> >>> ---
> >>>  include/drm/drm_atomic.h       | 66 ++++++++++++++++++++++++++++++++++++++++++
> >>>  include/drm/drm_modeset_lock.h | 21 ++++++++++++++
> >>>  2 files changed, 87 insertions(+)
> >>>
> >>> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> >>> index e527684dd394..462408a2d1b8 100644
> >>> --- a/include/drm/drm_atomic.h
> >>> +++ b/include/drm/drm_atomic.h
> >>> @@ -334,6 +334,72 @@ __drm_atomic_get_current_plane_state(struct drm_atomic_state *state,
> >>>  	return plane->state;
> >>>  }
> >>>  
> >>> +
> >>> +/**
> >>> + * drm_atomic_get_current_plane_state - get current plane state
> >>> + * @plane: plane to grab
> >>> + *
> >>> + * This function returns the current plane state for the given plane,
> >>> + * with extra locking checks to make sure that the plane state can be
> >>> + * retrieved safely.
> >>> + *
> >>> + * Returns:
> >>> + *
> >>> + * Pointer to the current plane state.
> >>> + */
> >>> +static inline struct drm_plane_state *
> >>> +drm_atomic_get_current_plane_state(struct drm_plane *plane)
> >>> +{
> >>> +	struct drm_plane_state *plane_state = plane->state;
> >>> +	struct drm_crtc *crtc = plane_state ? plane_state->crtc : NULL;
> >>> +
> >>> +	if (crtc)
> >>> +		drm_modeset_lock_assert_one_held(&plane->mutex, &crtc->mutex);
> >>> +	else
> >>> +		drm_modeset_lock_assert_held(&plane->mutex);
> >> Hmm. Daniel recently smashed me on the head with a big clue bat to point
> >> out that accessing object->state isn't safe unless you hold the object lock.
> >> So this thing seems suspicious. What's the use case for this?
> >>
> >> I guess in this case it might be safe since a parallel update would lock
> >> the crtc as well. But it does feel like promoting potentially dangerous
> >> behaviour. Also there are no barriers so I'm not sure this would be
> >> guaranteed to give us the correct answer anyway.
> >>
> >> As far as all of these functions go, should they return const*? Presumably
> >> you wouldn't want to allow changes to the current state.
> > Yep, need at least a lockdep check for the plane->mutex. And imo also a
> > check that we're indeed in atomic_check per the idea I dropped on the
> > cover letter.
> >
> > And +1 on const * for these, that seems like a very important check.
> Well I allowed for crtc lock held because the __ function uses crtc->mutex as safety lock.
> 
> I thought of const, but some code like i915_page_flip manipulates the current state with the right locks held.
> Perhaps we should disallow that. :)

Yup, since that code is going to die anyway. And if any driver has a need
for this, then they better open code (and have lots of answers for lots of
questions really).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch