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

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Date:	Thu, 31 Jan 2013 12:54:46 +0800
From:	Mark Zhang <nvmarkzhang@...il.com>
To:	Alexandre Courbot <gnurou@...il.com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Mark Zhang <markz@...dia.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [RFC 1/4] video: panel: add CLAA101WA01A panel support

On 01/31/2013 12:24 PM, Alexandre Courbot wrote:
> On Thu, Jan 31, 2013 at 12:51 PM, Mark Zhang <nvmarkzhang@...il.com> wrote:
>>> 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. Actually display controller cares about the video modes,
>> not the way to get these info. So I think it's better to do the whole
>> work like this:
>>
>> 1) display controller relied on CDF to call "display_entity_get_modes"
>> 2) panel driver calls the function which is hooked to display controller
>> to get the video modes via DDC.
>> 3) if the video modes can't be fetched via DDC, panel driver provides
>> the info(either by hard-coded or defined in DT) anyway
> 
> This would require a callback dedicated to this in display_entity and
> would break its simple design.

We just need to add a function pointer param which can be used by panel
driver in "display_entity_get_modes", don't we?

> 
>> Just like I said above, display controller should not query DDC
>> directly. Since CDF already defines these for us, display controller
>> should call CDF's functions, like: display_entity_get_modes,
>> display_entity_get_size... I think this is the key difference I wanna
>> talk about. And also in this way, display controller doesn't need to
>> care about this 2 steps logics, just call "display_entity_get_modes" is OK.
> 
> Well the fact is that it *is* the display controller that queries DDC
> directly. The panel is just a bus here, and the EDID EEPROM could
> perfectly work without it. If you model what you described with DT
> nodes, I'm afraid you will end up with something both complex (with DC
> node referencing panel node and back again for the EDID bus) and that
> does not picture reality accurately.

Display controller don't know whether the panel has EDID EEPROM but the
panel driver knows. So why we need to make display controller queries
EDID blindly? Since panel driver knows about all "video modes/panel
size" stuffs, why not we let it handle all these things?

> 
> Alex.
> 
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