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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Nov 17, 2016 at 12:58:00PM +0100, Maarten Lankhorst wrote:
> Op 16-11-16 om 17:32 schreef Ville Syrjälä:
> > 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@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.
> > What is this so called __ function exactly?
> __drm_atomic_get_current_plane_state, which is only used by drm_atomic_crtc_state_for_each_plane_state.
> 
> It iterates over crtc_state->plane_mask and then gets new_plane_state if available, or plane->state if the plane is not part of the state.
> This is mostly used for watermark calculations.

Sounds like we should kill that sucker and make things clearer by
enforcing the "want to access foo->state? then grab foo->lock first"
rule.

> >> 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. :)
> >>
> >> ~Maarten
> 

-- 
Ville Syrjälä
Intel OTC
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