linux-kernel - Re: [PATCH v4 1/3] drm/panel: Add RGB666 variant of Innolux AT070TN90

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Message-ID: <20180511085938.wjmdsdjfvdxd4shk@flea>
Date:   Fri, 11 May 2018 10:59:38 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Paul Kocialkowski <contact@...lk.fr>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-sunxi@...glegroups.com,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Chen-Yu Tsai <wens@...e.org>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>
Subject: Re: [PATCH v4 1/3] drm/panel: Add RGB666 variant of Innolux AT070TN90

On Wed, May 09, 2018 at 01:31:23PM +0200, Paul Kocialkowski wrote:
> On Wed, 2018-05-09 at 09:12 +0200, Maxime Ripard wrote:
> > On Tue, May 08, 2018 at 12:04:11AM +0200, Paul Kocialkowski wrote:
> > > This adds timings for the RGB666 variant of the Innolux AT070TN90 panel,
> > > as found on the Ainol AW1 tablet.
> > > 
> > > The panel also supports RGB888 output. When RGB666 mode is used instead,
> > > the two extra lanes per component are grounded.
> > > 
> > > In the future, it might become necessary to introduce a dedicated
> > > device-tree property to specify the bus format to use instead of the
> > > default one for the panel. This will allow supporting different bus
> > > formats for the same panel modes.
> > > 
> > > Signed-off-by: Paul Kocialkowski <contact@...lk.fr>
> > > ---
> > >  drivers/gpu/drm/panel/panel-simple.c | 26 ++++++++++++++++++++++++++
> > >  1 file changed, 26 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> > > index cbf1ab404ee7..32e30d5a8f08 100644
> > > --- a/drivers/gpu/drm/panel/panel-simple.c
> > > +++ b/drivers/gpu/drm/panel/panel-simple.c
> > > @@ -1063,6 +1063,29 @@ static const struct panel_desc innolux_at043tn24 = {
> > >  	.bus_flags = DRM_BUS_FLAG_DE_HIGH | DRM_BUS_FLAG_PIXDATA_POSEDGE,
> > >  };
> > >  
> > > +static const struct drm_display_mode innolux_at070tn90_mode = {
> > > +	.clock = 40000,
> > > +	.hdisplay = 800,
> > > +	.hsync_start = 800 + 112,
> > > +	.hsync_end = 800 + 112 + 1,
> > > +	.htotal = 800 + 112 + 1 + 87,
> > > +	.vdisplay = 480,
> > > +	.vsync_start = 480 + 141,
> > > +	.vsync_end = 480 + 141 + 1,
> > > +	.vtotal = 480 + 141 + 1 + 38,
> > > +	.vrefresh = 60,
> > > +};
> > > +
> > > +static const struct panel_desc innolux_at070tn90 = {
> > > +	.modes = &innolux_at070tn90_mode,
> > > +	.num_modes = 1,
> > > +	.size = {
> > > +		.width = 154,
> > > +		.height = 86,
> > > +	},
> > > +	.bus_format = MEDIA_BUS_FMT_RGB666_1X18,
> > > +};
> > > +
> > 
> > I'm not really convinced this is the right approach. You said it
> > yourself, the panel is using a 24-bits interface, and you just happen
> > to have a tablet that routed it using a 18-bits interface instead.
> > 
> > That doesn't belong in the driver, especially associated to the
> > compatible, but where the routing is described: in the device
> > tree. And given that the panel interface is a 24 bits panel, if we
> > were to have a default, we should have this one, and not the one
> > fitting your use case.
> 
> I fully agree, this is why I suggested introducing a dedicated dt
> property for selecting the bus format (in the commit message). I still
> proposed this patch as a temporary solution, but I'm definitely willing
> to craft a proper solution as well.
> 
> Here is an initial proposition:
> 1. Making bus_format an array in struct panel_desc and listing all the
> relevant bus formats that the panel can support there;

I'm not sure this is needed, the input format is always the same in
your case, the panel will always take a 24 bits RGB value. What you
want to change is the encoder output format (and I guess you want that
to be meaningful to enable or not the dithering).

> 2. Introducing an optional "bus-format" dt property that indicates which
> bus format to use, and using the first index of the bus formats array if
> the property is not present;

I guess the width would be enough, and that way we can take the
bus-width format that is already defined (but used in the v4l2
framework, not in DRM yet).

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com

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