Atmel HLCDC + Atomic operations: hook for internal atomic state change

[Boris Brezillon]

Hi Daniel,

On Thu, 5 Feb 2015 10:34:20 +0100
Daniel Vetter <daniel at ffwll.ch> wrote:

> On Wed, Feb 04, 2015 at 08:58:40PM +0100, Boris Brezillon wrote:
> > Hi Ville,
> > 
> > On Wed, 4 Feb 2015 20:02:27 +0200
> > Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> > 
> > > On Wed, Feb 04, 2015 at 06:23:15PM +0100, Boris Brezillon wrote:
> > > > Hello,
> > > > 
> > > > I'm currently adding support for atomic operations (or atomic
> > > > modesetting) in the Atmel HLCDC driver.
> > > > Everything is pretty much in place, and all the features provided by the
> > > > current driver are working as expected.
> > > > However, there's one feature I'd like to add (actually I was hoping
> > > > atomic support could help me deal with this feature), and I not sure
> > > > how to do it.
> > > > 
> > > > The HLCDC IP provides a way to discard a specific area on the primary
> > > > plane (in case at least one of the overlay is activated and alpha
> > > > blending is disabled).
> > > > Doing this will reduce the amount of data to transfer from the main
> > > > memory to the Display Controller, and thus alleviate the load on the
> > > > memory bus (since this link is quite limited on such hardware,
> > > > this kind of optimization is really important).
> > > > 
> > > > My problem here is that there is no way, in the current atomic
> > > > implementation, to internally ask for a plane state modification.
> > > > 
> > > > Is there a plan to add such hooks that would be called after the
> > > > requested state modifications (i.e. operations done before the
> > > > drm_atomic_commit call in all helper functions), but before the atomic
> > > > checks begin (i.e. call to drm_atomic_check_only) ?
> > > > Such hooks would let me ask for a primary plane update (modifying the
> > > > discard area property) if needed.
> > > > 
> > > > Maybe I'm totally mistaken in my approach to solve this problem, so
> > > > please let me know if you see other solutions.
> > > 
> > > So this looks pretty much exactly like the overlay optimization feature
> > > in OMAPs. I don't really see why you need to treat is as some kind of
> > > plane property. It's just an internal implementation detail so can't you
> > > just compute the discard area at commit() time based on what planes are
> > > going to be active? Or if you want to take it into account in some
> > > bandwidth calculation you can compute it already at check() time.
> > > 
> > 
> > Okay, I'll have a look at the OMAP driver, but I'd really like to
> > apply the discard area setting as part of the primary plane
> > atomic_update function (the discard area registers are part of the
> > primary plane registers, and plane settings are updated by setting a
> > specific bit to 1).
> > 
> > I tried to update the primary plane discard settings as part of the
> > atomic_update, but when nothing touches the primary plane (an
> > update_plane on one of the overlay planes), the primary plane is kept
> > unchanged, and thus the new primary settings are never applied.
> 
> So I'm not sure whether I understand this correctly, so let me just invent
> some fake hw model and explain with that ;-) Please adjust in your reply.
> 
> Assumption: We have 1 crtc and 2 planes, a primary and an overlay on top.
> Our fancy hw has an optional rect within the primary plane which we can
> tell it not to scan out. The idea is that that rect perfectly matches the
> placement of the 2nd overlay plane.
> 
> Step 1: We need to store this state somewhere of this special rect. So
> let's create a derived plane state for the primary plane.
> 
> struct fhw_primary_plane_state {
> 	struct drm_plane_state base;
> 
> 	bool enable_punchout;
> 	int punchout_x/_y/_h/_w;
> };
> 
> tegra is a nice example of what you all need to do when your driver needs
> derived state objects.

Yep, already created my own state when adding support for atomic
mode-setting (see [1]), and that's exactly what I was planning to do
(add disc_x/y/w/h fields in my plane state) ;-).

> 
> Step 2: We need to update the state of the _primary_ plane every time the
> _overlay_ plane moves around or gets enabled/disable. That must be done
> int the atomic_check hook provided by crtc helpers. Pseudo-code of that
> functions follows with comments inline

That's where I was hesitant, so this should be done in the atomic_check.

> 
> fhw_overlay_plane_atomic_check(struct drm_plane *plane, struct drm_plane_state *state)
> {
> 	/* First we need to get at the state of the primary plane.
> 	 * Grabbing additional state objects as needed is officially how
> 	 * ->atomic_check is supposed to work. The locking will magically
> 	 * work out, as long as you just dutifully pass the unchanged
> 	 * errno so that deadlock handling is still ok. */
> 	
> 	primar_plane = /* exercise for the reader */
	primar_plane = state->crtc->primary;

	Should work, isn't it ?

> 	primary_plane_state = drm_atomic_get_plane_state(state->state,
> 							 primary_plane);
> 	if (IS_ERR(primary_plane_state))
> 		return PTR_ERR(primary_plane_state);
> 
> 	fhw_primary_plane_state = upcast(primary_plane_state);
> 
> 	/* Update punchout, only enable when overlay is on. */
> 	fhw_primary_plane_state.enabel_punchout = !!state->crtc;
> 	fhw_primary_plane_state.punchout_x = state->crtc_x;
> 	...
> 
> 	return 0;
> }

That's exactly what I was planning to do, just wasn't sure if I was
allowed to modify one of the state when in the atomic_check callback
(the primary plane might have already been checked, and here, we're
modifying it afterward).

> 
> Step 3: In your atomic_plane_commit for the _primary_ plane write the
> punchout rect plus enable bit into hw. Atomic helpers will take care of
> everything for you. The assumption is that pure plane updates are cheap,
> so there won't be any optimization for no-op updates. We could add this
> later on.

Yep.

> 
> Summary: You need three pieces for fancy state:
> - Your own state structure(s).
> - Compute that derived state at atomic_check time (totally ok to grab
>   other states to do this if needed, this is how it's designed).
> - Bash your special state into hw at commit time.

Thanks for this detailed answer.

Best Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com