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

[Maxime Ripard <maxime.ripard@bootlin.com>]

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On Tue, Feb 19, 2019 at 07:44:56AM -0800, Vasily Khoruzhick wrote:
> On Tue, Feb 19, 2019 at 12:56 AM Maxime Ripard
> <maxime.ripard@bootlin.com> wrote:
> >
> > On Mon, Feb 18, 2019 at 11:33:05AM -0800, Vasily Khoruzhick wrote:
> > > 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.
> >
> > I gave it some more though, and came up with the following patch. The
> > basic idea is to leave the boundary check for the bridges that will
> > have EDID and we need to filter out the modes that have no chance of
> > being supported. The tolerancy used is the one defined in VESA specs,
> > but I added a module parameter if you wanted to tune that.
> >
> > And finally, since most of our panels are single timings without any
> > tolerancy, we just try our best in this case and that's it, while
> > leaving the door open to support display_timings and being able to do
> > more once we have an idea of what the tolerancies are.
> >
> > If that works for you, I'll submit it.
> 
> Maxime, thanks for your patch but  it doesn't work for me. Pinebook
> needs 1% tolerance. Having it as a module parameter means that no
> distro will be able to boot on Pinebook out of the box.

I don't really know what to tell you, the VESA spec defines everywhere
that tolerance, and if we're not able to provide that, then we're not
compliant and I don't want us to not be compliant just because one
panel needed to be a bit more flexible, and especially since what
could work on one panel might fail on another one.

If you want alternate solutions, then please answer to:
http://lists.infradead.org/pipermail/linux-arm-kernel/2019-February/630441.html

or provide the EDID blob.

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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