[PATCH v3 3/6] dt-bindings: Add panel-timing subnode to simple-panel

[Thierry Reding]

On Thu, Feb 08, 2018 at 12:48:50PM -0500, Sean Paul wrote:
> This patch adds a new subnode to simple-panel allowing us to override
> the typical timing expressed in the panel's display_timing.
> 
> Changes in v2:
>  - Split out the binding into a new patch (Rob)
>  - display-timings is a new section (Rob)
>  - Use the full display-timings subnode instead of picking the timing
>    out (Rob/Thierry)
> Changes in v3:
>  - Go back to using the timing subnode directly, but rename to
>    panel-timing (Rob)
> 
> Cc: Doug Anderson <dianders at chromium.org>
> Cc: Eric Anholt <eric at anholt.net>
> Cc: Heiko Stuebner <heiko at sntech.de>
> Cc: Jeffy Chen <jeffy.chen at rock-chips.com>
> Cc: Rob Herring <robh+dt at kernel.org>
> Cc: Stéphane Marchesin <marcheu at chromium.org>
> Cc: Thierry Reding <thierry.reding at gmail.com>
> Cc: devicetree at vger.kernel.org
> Cc: dri-devel at lists.freedesktop.org
> Cc: linux-rockchip at lists.infradead.org
> Signed-off-by: Sean Paul <seanpaul at chromium.org>
> ---
>  .../bindings/display/panel/simple-panel.txt        | 30 ++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/display/panel/simple-panel.txt b/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> index 45a457ad38f0..7788b9ce160b 100644
> --- a/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> +++ b/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> @@ -12,6 +12,24 @@ Optional properties:
>  - enable-gpios: GPIO pin to enable or disable the panel
>  - backlight: phandle of the backlight device attached to the panel
>  
> +panel-timing subnode
> +--------------------
> +
> +This optional subnode is for devices which require a mode differing from the
> +panel's "typical" display timing as programmed in the simple-panel driver.
> +Overriding the driver mode must only be done in the following scenario:
> + - The restrictions motivating the override cannot be applied to the platform's
> +   display driver (ie: it must be specific to the device not the platform)
> + - The panel must not have a fixed mode attributed to it in the driver

I think this is somewhat ambiguous. One could argue that panels only
have a fixed mode. What you means if obviously the fixed DRM display
mode, but that's not something that should go into the binding. I'd
simply drop this requirement.

> + - The panel must provide at list one display_timing range by which the override
> +   mode can be validated against

This is the important bit and kind of obsoletes the above anyway. Oh,
and you probably meant "... provide at _least_ one..."

> + - The override mode will use the 'typ' values from the panel-timings subnode

The properties for display timings in DT don't list anything other than
the 'typ' values, right? I'm not sure I understand this requirement.

> + - You must provide all required properties for the panel-timing subnode

Isn't this redundant? required properties are always required.

Other than the unclarities, the above does reflect what we had discussed
on IRC. I wonder, though, how much of this really belongs in the device
tree bindings. The above are almost all restrictions that apply to the
driver, and are very specific to Linux. Perhaps this is something that
belongs in the GPU documentation rather than DT bindings. They are more
like guidelines for what panel drivers should look like rather than what
the DT should look like.

Thierry
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