linux-kernel - Re: [RFC 1/4] video: panel: add CLAA101WA01A panel support

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Date:	Fri, 01 Feb 2013 12:19:39 +0800
From:	Mark Zhang <nvmarkzhang@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Alexandre Courbot <acourbot@...dia.com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Mark Zhang <markz@...dia.com>, linux-kernel@...r.kernel.org,
	linux-fbdev@...r.kernel.org, linux-tegra@...r.kernel.org,
	gnurou@...il.com
Subject: Re: [RFC 1/4] video: panel: add CLAA101WA01A panel support

On 02/01/2013 01:20 AM, Stephen Warren wrote:
> On 01/30/2013 08:51 PM, Mark Zhang wrote:
>> On 01/31/2013 04:19 AM, Stephen Warren wrote:
>>> On 01/30/2013 12:20 AM, Mark Zhang wrote:
>>>> On 01/30/2013 11:02 AM, Alexandre Courbot wrote:
>>>>> Add support for the Chunghwa CLAA101WA01A display panel.
>>>
>>>>> +static int panel_claa101_get_modes(struct display_entity *entity,
>>>>> +				   const struct videomode **modes)
>>>>> +{
>>>>> +	/* TODO get modes from EDID? */
>>>>
>>>> Why not move the "nvidia,ddc" from encoder's DT to panel's DT? In that
>>>> case, you can get EDID here. I know drm has some helpers to fetch EDID
>>>> but I recall there are some other functions which has no drm
>>>> dependencies which may be suitable for you.
>>>
>>> DDC access is a property of the display controller, not the panel
>>> itself. The panel might be hooked up to a display controller's DDC/I2C
>>> channel as the target, but it isn't the host/controller of the DDC/I2C
>>> channel. As such, placing the nvidia,ddc property into the display
>>> controller node makes sense.
>>
>> Yes, DC triggers the DDC access and is the host of the DDC/I2C channel.
>> So I think it's reasonable to put nvidia,ddc property into the display
>> controller node. But the video mode info in EDID which be fetched via
>> DDC is the property of the panel, so this info should be provided by
>> panel driver.
> 
> No, that makes absolutely no sense at all in the EDID case.
> 
> By the same argument, we'd need a panel driver for every external
> monitor which implemented EDID, just to transfer the EDID results from
> the display controller's DDC channel into the panel driver and back into
> the display controller code, which wants the mode list.
> 

Ah, yes, this is right. We can't write driver for every external monitor
which implements EDID. Although I think it's more reasonable that panel
decides whether the DDC probe is necessary.

> Again, if the mode list is coming from DDC, the display controller
> should retrieve it in exactly the same way it retrieves it for any
> external monitor - by direct control of the DDC channel to read the
> EDID. The only time it makes sense for the panel driver to get involved
> in supplying the mode list is when there's no EDID, so the list must be
> hard-coded into the driver.
> 
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