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 09:07:32 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Inki Dae <inki.dae@...sung.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mark Yao <mark.yao@...k-chips.com>,
        Heiko Stuebner <heiko@...ech.de>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/SAMSUNG EXYNO..." 
        <linux-samsung-soc@...r.kernel.org>,
        "open list:ARM/SHMOBILE ARM..." <linux-renesas-soc@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>
Subject: Re: [PATCH 4/4] drm/sun4i: make sure we don't have a commit pending

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).

Maxime

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

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