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

[Boris Brezillon]

On Thu, 5 Feb 2015 10:56:30 +0100
Boris Brezillon <boris.brezillon at free-electrons.com> wrote:

> 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) ;-).

[1]https://lkml.org/lkml/2015/2/4/658

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