Re: [PATCH] tegra: ventana: display and backlight DT entries

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 11/13/2012 10:36 PM, Alex Courbot wrote:
> On Wednesday 14 November 2012 00:46:52 Stephen Warren wrote:
>> I do tend to think that we should use EDID where there is one.
>>
>> 1) If there is an EDID in the panel HW, and the panel's I2C is hooked
>> up to Tegra, we should read it out at runtime.
> 
> According to Ventana' platform design guide the LCD panel is hooked on I2C2. 
> The panel's data sheet lists CLK_EDID and DATA_EDID pins, which I assume are 
> for I2C, but there is no mention of an I2C address in both guides.

EDID would always be on I2C address 0x50.

>> 2) Otherwise, if the panel's documentation provides an EDID, we should
>> use that, since it's the most canonical/common/standard representation
>> of the panel's properties.
> 
> Panel's documentation indeed provides full EDID specification in appendix. Mark 
> sent me an EDID blob which works but I don't know where it comes from - Mark, 
> could you tell us?
>
>> 3) Otherwise, use the videomode DT bindings.
>>
>> Another benefit of (2) is that we can actually support the panel
>> without waiting for the videomode DT bindings to be finalized and merged.
> 
> Is there another incentive for preferring (2) over (3)? EDID specs can easily 
> be turned into videomode bindings, and it would also avoid introducing a new 
> file into the kernel source.

If there's an EDID, it is the canonical representation of the display's
features. As such, we should use it directly rather than encoded it into
a different format. Also, the EDID provides much more information than
the display mode bindings, e.g. vendor/model name, serial number, IIRC
perhaps some (limited?) color space information, etc.

>> Although if Ventana requires the power sequences helpers, that already
>> means we won't be able to support Ventana's panel in 3.8 unless the
>> power sequences code gets merged for 3.8; is that likely?
> 
> Likely, I don't know, possible - maybe. Power seqs work and I could push to 
> get them merged, but the following points need to be considered:
> - DT bindings are likely to change from their current form. I want to take 
> advantage of the gpio API changes that are undergoing, and also probably of 
> your preprocessor patch for dtc (not sure if that is already usable in the 
> kernel?). Considering the feature is young I don't think a DT change would be 
> a big deal, but the general consensus seems to be that DT bindings are 
> immutable - maybe my perception is wrong?

The DT bindings are certainly supposed to be fixed. I'm still not
convinced that there is any mileage at all in changing the DT bindings
for GPIOs as part of the power sequences or similar work; the GPIO
bindings have been around for years and can't change.

> - If I am to take maintainership of the feature, I guess I will have to get 
> the patches sufficiently Ack'ed by enough people, and also have someone else 
> pull from my tree (Linus? Or maybe some other power maintainer?). I am not 
> familiar with the exact procedure here - moreover, my GPG key only has one 
> signature from a trusted kernel dev, I am not sure if this is enough.

It's probably best to go ask whoever maintains the rest of the code in
the directory above where you placed it. See if they'll take your pull
request. If not, you can try sending directly to Linus, CCing enough
other known people to get acks (I think you already collected a few right?)