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

[Boris Brezillon]

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.

Regards,

Boris

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