linux-kernel - Re: [RFC PATCH 2/3] drm: bridge: panel: allow override of the bus format

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Message-ID: <7015227.p44CVkfgjM@avalon>
Date:   Tue, 20 Mar 2018 15:56:52 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Peter Rosin <peda@...ntia.se>
Cc:     linux-kernel@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org
Subject: Re: [RFC PATCH 2/3] drm: bridge: panel: allow override of the bus format

Hi Peter,

Thank you for the patch.

On Sunday, 18 March 2018 00:15:24 EET Peter Rosin wrote:
> Useful if the bridge does some kind of conversion of the bus format.
> 
> Signed-off-by: Peter Rosin <peda@...ntia.se>
> ---
>  drivers/gpu/drm/bridge/panel.c | 22 +++++++++++++++++++++-
>  include/drm/drm_bridge.h       |  1 +
>  2 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/bridge/panel.c b/drivers/gpu/drm/bridge/panel.c
> index 6d99d4a3beb3..ccef0283ff41 100644
> --- a/drivers/gpu/drm/bridge/panel.c
> +++ b/drivers/gpu/drm/bridge/panel.c
> @@ -22,6 +22,7 @@ struct panel_bridge {
>  	struct drm_connector connector;
>  	struct drm_panel *panel;
>  	u32 connector_type;
> +	u32 bus_format;
>  };
> 
>  static inline struct panel_bridge *
> @@ -40,8 +41,15 @@ static int panel_bridge_connector_get_modes(struct
> drm_connector *connector) {
>  	struct panel_bridge *panel_bridge =
>  		drm_connector_to_panel_bridge(connector);
> +	int ret;
> +
> +	ret = drm_panel_get_modes(panel_bridge->panel);
> +
> +	if (panel_bridge->bus_format)
> +		drm_display_info_set_bus_formats(&connector->display_info,
> +						 &panel_bridge->bus_format, 1);

While I agree with the problem statement and, to some extent, the DT bindings, 
I don't think this is the right implementation. You've correctly noted that 
display controller shouldn't blindly use the formats reported by the panel 
through the connector formats, and that hacking the panel driver to override 
the formats isn't a good idea, so I wouldn't override the formats reported by 
the connector. I would instead extend the drm_bridge API to report formats at 
bridge inputs. This would be more generic and allow each bridge to configure 
itself according to the next bridge in the chain.

I'm not sure whether this API extension should be in the form of a new bridge 
function, or if the formats should be stored in the drm_bridge structure 
directly as done for connectors in the display info structure. I'm tempted by 
the former, but I'm open to discussions.

> -	return drm_panel_get_modes(panel_bridge->panel);
> +	return ret;
>  }
> 
>  static const struct drm_connector_helper_funcs
> @@ -203,6 +211,18 @@ void drm_panel_bridge_remove(struct drm_bridge *bridge)
> }
>  EXPORT_SYMBOL(drm_panel_bridge_remove);
> 
> +void drm_panel_bridge_set_bus_format(struct drm_bridge *bridge, u32
> bus_format) +{
> +	struct panel_bridge *panel_bridge;
> +
> +	if (!bridge)
> +		return;
> +
> +	panel_bridge = drm_bridge_to_panel_bridge(bridge);
> +	panel_bridge->bus_format = bus_format;
> +}
> +EXPORT_SYMBOL(drm_panel_bridge_set_bus_format);
> +
>  static void devm_drm_panel_bridge_release(struct device *dev, void *res)
>  {
>  	struct drm_bridge **bridge = res;
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index 682d01ba920c..81903b92f187 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -268,6 +268,7 @@ void drm_bridge_enable(struct drm_bridge *bridge);
>  struct drm_bridge *drm_panel_bridge_add(struct drm_panel *panel,
>  					u32 connector_type);
>  void drm_panel_bridge_remove(struct drm_bridge *bridge);
> +void drm_panel_bridge_set_bus_format(struct drm_bridge *bridge, u32
> bus_format);
> struct drm_bridge *devm_drm_panel_bridge_add(struct device
> *dev,
>  					     struct drm_panel *panel,
>  					     u32 connector_type);

-- 
Regards,

Laurent Pinchart