[PATCH 4/4] drm/sun4i: make sure we don't have a commit pending

[Maxime Ripard]

On Mon, Jul 17, 2017 at 02:57:19PM +0800, Chen-Yu Tsai wrote:
> On Mon, Jul 17, 2017 at 2:55 PM, Maxime Ripard
> <maxime.ripard at free-electrons.com> wrote:
> > On Fri, Jul 14, 2017 at 04:56:01PM +0800, Chen-Yu Tsai wrote:
> >> Hi,
> >>
> >> On Thu, Jul 13, 2017 at 10:41 PM, Maxime Ripard
> >> <maxime.ripard at free-electrons.com> wrote:
> >> > In the earlier display engine designs, any register access while a commit
> >> > is pending is forbidden.
> >> >
> >> > One of the symptoms is that reading a register will return another, random,
> >> > register value which can lead to register corruptions if we ever do a
> >> > read/modify/write cycle.
> >>
> >> Alternatively, if changes to the backend (layers) are guaranteed to happen
> >> while the CRTC is disabled (which seems to be the case after looking at
> >> drm_atomic_helper_commit_planes and drm_atomic_helper_commit_tail), we
> >> could just turn on register auto-commit all the time and not deal with
> >> this.
> >
> > As far as I understand, it will only be the case if we need a new
> > modeset or we changed the active CRTC or connectors. But if you change
> > only the format, buffers or properties it won't be the case, and we'll
> > need to commit.
> 
> So in other words, if someone were to use it for actual compositing and
> moved the upper composited layer around, we would need commit support to be
> safe.
>
> Sounds more or less like something a video player would do.

Not only that. A change of buffer will happen every frame or so, and
we can change the format whenever we want too (even if it's usually
going to be in sync with a new buffer). Changing a property can happen
any time too (like zpos for example).

Maxime

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