linux-kernel - Re: [PATCH 1/6] dt-bindings: drm/panel: simple: Add no-hpd property

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Date:   Thu, 25 Oct 2018 14:28:35 -0500
From:   Rob Herring <robh@...nel.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Sean Paul <seanpaul@...omium.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Sandeep Panda <spanda@...eaurora.org>,
        linux-arm-msm@...r.kernel.org,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        jsanka@...eaurora.org, ryandcase@...omium.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH 1/6] dt-bindings: drm/panel: simple: Add no-hpd property

On Mon, Oct 22, 2018 at 01:46:34PM -0700, Douglas Anderson wrote:
> Some eDP panels that are designed to be always connected to a board
> use their HPD signal to signal that they've finished powering on and
> they're ready to be talked to.
> 
> However, for various reasons it's possible that the HPD signal from
> the panel isn't actually hooked up.  In the case where the HPD isn't
> hooked up you can look at the timing diagram on the panel datasheet
> and insert a delay for the maximum amount of time that the HPD might
> take to come up.
> 
> Let's add a property in the device tree for this concept.
> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> 
>  .../devicetree/bindings/display/panel/simple-panel.txt         | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/display/panel/simple-panel.txt b/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> index 45a457ad38f0..b2b872c710f2 100644
> --- a/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> +++ b/Documentation/devicetree/bindings/display/panel/simple-panel.txt
> @@ -11,6 +11,9 @@ 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
> +- no-hpd: This panel is supposed to communicate that it's ready via HPD
> +  (hot plug detect) signal, but the signal isn't hooked up so we should
> +  hardcode the max delay from the panel spec when powering up the panel.

If we have this here, then we should also have hpd-gpios defined here as 
where we describe a connection we should also describe no connection.

Now, hpd-gpios is a bit of a mess being defined in both connector nodes 
and bridge (HDMI/DP) nodes. I think that is just history pre-dating 
connector nodes. Connector nodes are now the preferred place. Connector 
nodes and panel nodes are essentially the same thing (the endpoint of 
display pipeline).

That being said, this patch is fine as is.

Reviewed-by: Rob Herring <robh@...nel.org>

Rob