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

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Date:   Tue, 18 Jul 2017 09:35:03 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     Chen-Yu Tsai <wens@...e.org>,
        "open list:ARM/SHMOBILE ARM..." <linux-renesas-soc@...r.kernel.org>,
        "moderated list:ARM/SAMSUNG EXYNO..." 
        <linux-samsung-soc@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH 4/4] drm/sun4i: make sure we don't have a commit pending

On Tue, Jul 18, 2017 at 9:07 AM, Maxime Ripard
<maxime.ripard@...e-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@...e-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@...e-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).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch