Re: [RFC] drm: Parse HDMI 2.0 YCbCr 4:2:0 VDB and VCB

[Jose Abreu]

Hi Ville,


On 04-01-2017 16:36, Ville Syrjälä wrote:
> On Wed, Jan 04, 2017 at 04:15:01PM +0000, Jose Abreu wrote:
>>
 
[snip]

>>> Why does userspace need to know this? My thinking has been that the
>>> driver would do the right thing automagically.
>>>
>>> We do probably want some kind of output colorspace property to allow the
>>> user to select between RGB vs. YCbCr etc. But I think even with that we
>>> should still allow the driver to automagically select YCbCr 4:2:0 output
>>> since that's the only way the mode will work.
>> I agree. When only 4:2:0 is supported there is no need to expose
>> the flag to userspace. How shall then I signal drivers for this
>> 4:2:0'only sampling mode?
>>
>> So, for the remaining modes, you propose a new field in the mode
>> structure called 'colorspace' which contains the list of
>> supported sampling modes for the given mode? I think it would be
>> a nice addition. This way if a mode supports only RGB we only
>> passed RGB flag; if 4:2:0 was also supported we passed the 4:2:0
>> flag, ... And then user could select. We also have to inform user
>> which one is being actually used.
> IIRC there aren't any "RGB only" modes or anything like that. So
> YCbCr 4:2:0 is the special case here. We could just add something to the
> mode struct for it, or do we already have some other flags thing that's
> not exposed to userspace? And I guess drivers should be able to opt into
> supporting these 4:2:0 modes in similar way they opt into
> interlaced/stereo/whatever.

I mean, if a source EDID does not declare support for YCbCr modes
(4:2:2 and 4:4:4 [i think they have to be both supported if sink
supports != RGB]) then only RGB can be used. Or is any YCbCr that
is pre-required? Still, I see your point. When EDID declares
support for YCbCr then all modes can use it, and not only some of
them.

I think for stereo modes the flags can be opt in/out in userspace
exposing. There is a function called
drm_mode_expose_to_userspace() which only exposes stereo modes if
user asks to. We could do something similar for 4:2:0 modes (or
even for all pixel encoding). i.e. expose which encoding can be
used in current video mode. What do you think?

About drivers opting in for 4:2:0 modes, then you propose a new
field in drm_connector (called for example ycbcr_420_allowed)
which only does the parsing of the 4:2:0 modes and adds them to
the list when set to true?

Best regards,
Jose Miguel Abreu

>
>> Best regards,
>> Jose Miguel Abreu
>>
>>>

[snip]