Re: [PATCH V6 0/8] drm/exynos: few patches to enhance bridge chip support

[Thierry Reding <thierry.reding@gmail.com>]

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On Wed, Jul 30, 2014 at 11:54:00AM +0530, Ajay kumar wrote:
> Hi Thierry,
> 
> On Tue, Jul 29, 2014 at 5:17 PM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> > On Tue, Jul 29, 2014 at 01:42:09PM +0200, Andreas Färber wrote:
> >> Am 29.07.2014 13:36, schrieb Thierry Reding:
> >> > On Tue, Jul 29, 2014 at 01:21:48PM +0200, Andreas Färber wrote:
> >> >> Hi Ajay,
> >> >>
> >> >> Am 28.07.2014 08:13, schrieb Ajay kumar:
> >> >>> On 7/27/14, Andreas Färber <afaerber@suse.de> wrote:
> >> >>>> Am 25.07.2014 21:22, schrieb Ajay Kumar:
> >> >>>>> This series is based on exynos-drm-next branch of Inki Dae's tree at:
> >> >>>>> git://git.kernel.org/pub/scm/linux/kernel/git/daeinki/drm-exynos.git
> >> >>>>>
> >> >>>>> I have tested this after adding few DT changes for exynos5250-snow,
> >> >>>>> exynos5420-peach-pit and exynos5800-peach-pi boards.
> >> >>>>
> >> >>>> I'm trying to test this with a modified exynos5250-spring DT
> >> [...]
> >> >> Unfortunately the most I got on Spring with attached DT was a blank
> >> >> screen with a white horizontal line in the middle.
> >> >>
> >> >> Do I need to specify a specific panel model for Spring?
> >> [...]
> >> >> From 9172a26a8f0d0f0d170bd27e1c150ad204d8086a Mon Sep 17 00:00:00 2001
> >> >> From: =?UTF-8?q?Andreas=20F=C3=A4rber?= <afaerber@suse.de>
> >> >> Date: Sun, 27 Jul 2014 21:58:06 +0200
> >> >> Subject: [PATCH] ARM: dts: exynos5250: Add eDP/LVDS bridge to Spring
> >> >> MIME-Version: 1.0
> >> >> Content-Type: text/plain; charset=UTF-8
> >> >> Content-Transfer-Encoding: 8bit
> >> >>
> >> >> Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
> >> >> [AF: Redone for v6]
> >> >> Signed-off-by: Andreas F??rber <afaerber@suse.de>
> >> >> ---
> >> >>  arch/arm/boot/dts/exynos5250-spring.dts | 32 +++++++++++++++++++++++++++++++-
> >> >>  1 file changed, 31 insertions(+), 1 deletion(-)
> >> >>
> >> >> diff --git a/arch/arm/boot/dts/exynos5250-spring.dts b/arch/arm/boot/dts/exynos5250-spring.dts
> >> >> index 687dfab86bc8..517b1ff2bfdf 100644
> >> >> --- a/arch/arm/boot/dts/exynos5250-spring.dts
> >> >> +++ b/arch/arm/boot/dts/exynos5250-spring.dts
> >> >> @@ -64,10 +64,14 @@
> >> >>            vdd_pll-supply = <&s5m_ldo8_reg>;
> >> >>    };
> >> >>
> >> >> +  panel: panel {
> >> >> +          compatible = "simple-panel";
> >> >> +  };
> >> >
> >> > You can't do this. "simple-panel" isn't a valid panel model. It should
> >> > probably be removed from the platform_of_match table in the driver.
> >>
> >> Okay, that means the Snow DT is wrong, too:
> >> https://patchwork.kernel.org/patch/4625441/
> >>
> >> And the others specify it as fallback:
> >> https://patchwork.kernel.org/patch/4625461/
> >> https://patchwork.kernel.org/patch/4625451/
> >
> > A quick grep shows that many (all?) devices that use DRM panels provide
> > simple-panel as fallback. That's probably fine as long as they also do
> > provide the specific model. But given that simple-panel does not have a
> > mode or physical size, I don't think even that makes sense.
> On snow, the bridge chip provides the display mode instead of the panel.
> That is why display was working for me.

Okay, I suppose under some circumstances that might make sense. Although
it's still always the panel that dictates the display timings, so the
panel node needs to have a panel model specific compatible value with a
matching entry in the panel-simple driver so that it can even be used in
setups without a bridge.

One other thing: how does the bridge know which mode to drive? I suspect
that it can drive more than one mode? Can it freely be configured or
does it have a predefined set of modes? If the latter, then according to
what you said above there needs to be a way to configure the bridge (via
DT?) so that it reports the mode matching the panel. I wonder if that
should be handled completely in code, so that for example a bridge has a
panel attached it can use the panel's .get_modes() and select a matching
mode among the set that it supports.

Thierry

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