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

[maxime.ripard@free-electrons.com (Maxime Ripard)]

Hi Daniel,

On Tue, Jul 18, 2017 at 09:35:03AM +0200, Daniel Vetter wrote:
> On Tue, Jul 18, 2017 at 9:07 AM, Maxime Ripard
> <maxime.ripard@free-electrons.com> wrote:
> > 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@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@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).
> 
> You can upgrade any property change to an atomic modeset by e.g.
> setting connector->mode_changed (and then making sure to call
> check_modeset() helper again perhaps). This is for cases where your hw
> can't handle a property change within 1 vblank. The default is just
> the solution for most common hw.
> 
> The other way round works too, you can clear these flags in your
> atomic_check callbacks. But that requires a bit more care (to make
> sure you never clear it when there's something else also changing that
> still needs a full modeset sequence to commit to hw).

Hmm, that's good to know, but that would imply disabling the CRTC each
time we change even a small property, with all the visual artifacts it
might imply, right?

Maxime

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