linux-kernel - Re: [PATCH 3/9] drm/panel: simple: make it possible to override LCD bus format

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Date:   Fri, 13 Oct 2017 17:13:39 -0500
From:   Rob Herring <robh@...nel.org>
To:     Lothar Waßmann <LW@...O-electronics.de>
Cc:     David Airlie <airlied@...ux.ie>,
        Mark Rutland <mark.rutland@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/9] drm/panel: simple: make it possible to override LCD
 bus format

On Wed, Oct 11, 2017 at 01:23:35PM +0200, Lothar Waßmann wrote:
> The baseboards for the Ka-Ro electronics series of i.MX modules
> use a 24bit LCD interface, no matter what LCD bus width the SoC on the
> module provides and what the LCD panel expects. LCDs with 6bit per color
> will ignore the 2 LSBs of each color lane, and modules using a SoC
> that provides only 6bit per color, drive the display information on the
> 6 MSBs of each color lane and tie the 2 LSBs of each color lane to GND.
> 
> Thus, no matter what combination of LCD and SoC is used, the LCD port
> can be used without shuffling bit lanes by always configuring the LCD
> output to 24bit mode.

Thanks for providing good reasoning as to why this is needed.

> 
> Add a function to handle certain quirks of the LCD interface to the
> panel driver to be able to override the bus format specified in a
> panel's display_mode.
> 
> Signed-off-by: Lothar Waßmann <LW@...O-electronics.de>
> ---
>  .../bindings/display/panel/simple-panel.txt        |  2 ++
>  drivers/gpu/drm/panel/panel-simple.c               | 40 +++++++++++++++++++++-
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/devicetree/bindings/display/panel/simple-panel.txt b/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> index 1341bbf..e2308c3 100644
> --- a/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> +++ b/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> @@ -7,6 +7,8 @@ Optional properties:
>  - ddc-i2c-bus: phandle of an I2C controller used for DDC EDID probing
>  - enable-gpios: GPIO pin to enable or disable the panel
>  - backlight: phandle of the backlight device attached to the panel
> +- bus-format-override: override the bus_format setting of the panel's
> +  display_mode settings

You need to define valid values here.

However, we also have this proposal[1]. Please align to a common one. 
BTW, we don't need another common panel binding file that [1] added. We 
already have panel-dpi.txt for parallel interface panels. And 
personally, I'd like to see simple-panel.txt disappear.

Rob

[1] https://patchwork.ozlabs.org/patch/823104/