Re: [PATCH v3 2/2] drm/tegra: output: rgb: Support LVDS encoder bridge

From: Dmitry Osipenko <digetx@gmail.com>
To: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: linux-tegra@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Sam Ravnborg <sam@ravnborg.org>,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v3 2/2] drm/tegra: output: rgb: Support LVDS encoder bridge
Date: Fri, 17 Apr 2020 00:15:33 +0300	[thread overview]
Message-ID: <fbafa641-f2ed-22b5-eaeb-bd2726b53d0a@gmail.com> (raw)
In-Reply-To: <20200416205012.GA28162@pendragon.ideasonboard.com>

16.04.2020 23:50, Laurent Pinchart пишет:
> Hi Dmitry,
> 
> On Thu, Apr 16, 2020 at 11:21:40PM +0300, Dmitry Osipenko wrote:
>> 16.04.2020 21:52, Dmitry Osipenko пишет:
>> ...
>>>> May I also recommend switching to the DRM panel bridge helper ? It will
>>>> simplify the code.
>>>
>>> Could you please clarify what is the "DRM panel bridge helper"?
>>>
>>> I think we won't need any additional helpers after switching to the
>>> bridge connector helper, no?
>>
>> Actually, I now see that the panel needs to be manually attached to the
>> connector.
> 
> The DRM panel bridge helper creates a bridge from a panel (with
> devm_drm_panel_bridge_add()). You can then attach that bridge to the
> chain, like any other bridge, and the enable/disable operations will be
> called automatically without any need to call the panel enable/disable
> manually as done currently.
> 
>> Still it's not apparent to me how to get panel out of the bridge. It
>> looks like there is no such "panel helper" for the bridge API or I just
>> can't find it.
> 
> You don't need to get a panel out of the bridge. You should get the
> bridge as done today,

You mean "get the panel", correct?

> and wrap it in a bridge with
> devm_drm_panel_bridge_add().
> 

The lvds-codec already wraps panel into the bridge using
devm_drm_panel_bridge_add() and chains it for us, please see
lvds_codec_probe() / lvds_codec_attach().

Does it mean that lvds-codec is not supporting the new model?

Everything works nicely using the old model, where bridge creates
connector and attaches panel to it for us.

[I'm still not sure what is the point to use the new model in a case of
a simple chain of bridges]

Using the new model, the connector isn't created by the bridge, so I
need to duplicate that creation effort in the driver. Once the bridge
connector is manually created, I need to attach panel to this connector,
but panel is reachable only via the remote bridge (which wraps the panel).

driver connector -> LVDS bridge -> panel bridge -> panel
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