Re: [PATCH v3 06/11] drm/sun4i: rgb: Add DT property to disable strict clock rate check

From: Vasily Khoruzhick <anarsoul@gmail.com>
To: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree <devicetree@vger.kernel.org>,
	Archit Taneja <architt@codeaurora.org>,
	Andrzej Hajda <a.hajda@samsung.com>,
	David Airlie <airlied@linux.ie>,
	linux-sunxi <linux-sunxi@googlegroups.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Maxime Ripard <maxime.ripard@bootlin.com>,
	Chen-Yu Tsai <wens@csie.org>,
	Thierry Reding <thierry.reding@gmail.com>,
	Sean Paul <seanpaul@chromium.org>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	arm-linux <linux-arm-kernel@lists.infradead.org>,
	Icenowy Zheng <icenowy@aosc.io>
Subject: Re: [PATCH v3 06/11] drm/sun4i: rgb: Add DT property to disable strict clock rate check
Date: Mon, 18 Feb 2019 11:33:05 -0800	[thread overview]
Message-ID: <CA+E=qVfDfDvVmKQDeh2e0SA=tm6VpBMpu9MPKUqxFPObxYFXDg@mail.gmail.com> (raw)
In-Reply-To: <20190218182629.GA14714@bogus>

On Mon, Feb 18, 2019 at 10:26 AM Rob Herring <robh@kernel.org> wrote:
>
> On Thu, Feb 14, 2019 at 09:09:52PM -0800, Vasily Khoruzhick wrote:
> > Clock rate check that was added in commit bb43d40d7c83 ("drm/sun4i: rgb:
> > Validate the clock rate") prevents some panel and bridges from working with
> > sun4i driver.
>
> Sounds lile a regression that should be reverted. The fix is not a
> backwards compatible change either.

anx6345 driver isn't mainlined yet and I'm not sure if this change
breaks any mainlined boards. So likely there's not enough
justification to revert it.

> > Unfortunately, dotclock frequency for some modes are not achievable on
> > sunxi hardware, and there's a slight deviation in rate returned by
> > clk_round_rate(), so they fail this check.
> >
> > Experiments show that panels and bridges work fine with this slight
> > deviation, e.g. Pinebook that uses ANX6345 bridge with 768p eDP panel
> > requests 73 MHz, gets 72.296MHz instead (0.96% difference) and works just
> > fine.
> >
> > This patch adds DT property to disable strict clock rate check
> >
> > Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
> > ---
> >  .../devicetree/bindings/display/sunxi/sun4i-drm.txt          | 2 ++
> >  drivers/gpu/drm/sun4i/sun4i_rgb.c                            | 5 +++++
> >  drivers/gpu/drm/sun4i/sun4i_tcon.c                           | 3 +++
> >  drivers/gpu/drm/sun4i/sun4i_tcon.h                           | 1 +
> >  4 files changed, 11 insertions(+)
> >
> > diff --git a/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt b/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> > index f426bdb42f18..18c8b053a28d 100644
> > --- a/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> > +++ b/Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt
> > @@ -63,6 +63,8 @@ Required properties:
> >      Documentation/devicetree/bindings/media/video-interfaces.txt. The
> >      first port should be the input endpoint. The second should be the
> >      output, usually to an HDMI connector.
> > +  - no-strict-clock-check: don't reject timings if exact dot clock can't be
> > +    reached.
>
> This should be the default IMO. Most panels are a single timing, so if
> we reject it the fallback no display?

As far as I remember the change was introduced to reject some modes
for which dotclock can't be reached when driver is used with VGA
bridge. So if we make it default it'll break boards with VGA bridge
and old DT.

> I thought we had some mechanism already to allow some range of
> frequencies. I think the chromeos guys needed something IIRC.

You can specify frequency range for panels, but there's nothing for
bridges. In my case EDID doesn't specify clock tolerance.

>
> Rob

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