Re: [PATCH RFC 3/6] drm: Plane YCbCr to RGB conversion related properties

[Brian Starkey <brian.starkey@arm.com>]

On Fri, Apr 21, 2017 at 06:21:48PM +0300, Ville Syrjälä wrote:
>On Fri, Apr 21, 2017 at 03:53:42PM +0100, Brian Starkey wrote:
>> Hi,
>>
>> Thanks for picking this up Jyri.
>>
>> On Fri, Apr 21, 2017 at 04:52:03PM +0300, Ville Syrjälä wrote:
>> >On Fri, Apr 21, 2017 at 04:39:16PM +0300, Jyri Sarha wrote:
>> >> On 04/21/17 14:17, Ville Syrjälä wrote:
>> >> >> +static char *ycbcr_to_rgb_mode_name[] = {
>> >> >> +	[DRM_PLANE_YCBCR_BT601_LIM_TO_RGB_BT601_FULL] =
>> >> >> +		"YCbCr BT.601 limited range TO RGB BT.601 full range",
>> >> >> +	[DRM_PLANE_YCBCR_BT601_FULL_TO_RGB_BT601_FULL] =
>> >> >> +		"YCbCr BT.601 full range TO RGB BT.601 full range",
>> >> >> +	[DRM_PLANE_YCBCR_BT709_LIM_TO_RGB_BT709_FULL] =
>> >> >> +		"YCbCr BT.709 limited range TO RGB BT.709 full range",
>> >> >> +	[DRM_PLANE_YCBCR_BT2020_LIM_TO_RGB_BT2020_FULL] =
>> >> >> +		"YCbCr BT.2020 limited range TO RGB BT.2020 full range",
>> >> >> +	[DRM_PLANE_YCBCR_BT601_LIM_TO_RGB_BT709_FULL] =
>> >> >> +		"YCbCr BT.601 limited range TO RGB BT.709 full range",
>> >> > We probably don't want to conflate the YCbCr->RGB part with the colorspace
>> >> > conversion because the YCbCr->RGB part should be performed on gamma encoded
>> >> > data and the colorspace conversion on linear data. So we need a degamma
>> >> > stage in between. At least that seemed to be the general concencus after
>> >> > the last round of mails on this topic.
>> >> >
>>
>> I thought the conclusion of the last round was that on some hardware
>> this would be conflated as a conscious choice. If the HW doesn't have
>> the required degamma->csc->gamma stages, and the implementor/user
>> doesn't care about being a little incorrect, then it can all be
>> described in this single property.
>
>I was proposing the single prop approach initially, but now I think it
>might just lead to more confusion. So a dedicated property for each stage
>is the clearer design I think. We do lose potentially a bit of
>discoverability when not all combinations are supported, but we have
>that problem in many other places as well, so not a big deal I think.
>

Yeah you lose discoverability, but do you also lose the ability to do
the non-perfect, single-stage conversions?

For HW that only has a matrix, is the driver expected to combine all
of the separated stages down into a single matrix? Or it wouldn't
expose the other properties, only a matrix, and userspace has to come
up with a blob that does the (approximate) right thing?

>>
>> If there is more capable hardware with the additional stages, then
>> they should expose additional properties (in pipeline order):
>>
>> YCBCR_TO_RGB_*:
>>    Does YCbCr->RGB conversion on non-linear YCbCr data,
>>    only the enum values which don't have a CSC are exposed.
>
>Not sure what you mean but that last part.
>

Just that the enum list should only contain things that are:
YCbCr BT.601  -> RGB BT.601
YCbCr BT.709  -> RGB BT.709
YcbCr BT.2020 -> RGB BT.2020

and not a those including a color space change, e.g.:
YCbCr BT.601  -> RGB BT.709

because an RGB BT.601 -> RGB BT.709 conversion can be performed
later with the RGB_TO_RGB/CTM/whatever property.

>>
>> DEGAMMA:
>>    Does the non-linear to linear conversion on the RGB data output from
>>    the YCBCR_TO_RGB stage.
>>
>> RGB_TO_RGB_*:
>>    Similar set of properties to YCBCR_TO_RGB_*, allowing RGB->RGB CSC
>>    on the linear data.
>
>We may want to call this just CTM to match the matching crtc prop.
>

Good call.

-Brian

>>
>> GAMMA:
>>    Convert the CSC'd RGB data back in to non-linear for blending (if
>>    blending is to be done with non-linear data).
>>
>> Drivers can expose as many or as few of the above properties as their
>> hardware supports.
>>
>> >>
>> >> I do not really have the expertise to argue with that. I merely copied
>> >> the idea from the mail thread I referred to in the cover letter.
>> >> However, there are several display HWs out there that do not have all
>> >> bolts and knobs to make the color-space conversion in exactly the ideal
>> >> order, omap DSS being one of them.
>> >
>> >Yeah. Intel hardware is in the same boat for the time being. On current
>> >hw I think we can only really expose the YCbCr->RGB and degamma stages.
>> >
>> >On some limited set of platforms we could expose a blob as well, and I
>> >suppose it would then be possible to use it for color space conversion
>> >if you ignore gamma and/or only deal with linear RGB data. But it's such
>> >a limited subset of hardware for us that I don't think I'm interested
>> >in exposing it.
>> >
>>
>> I'm not sure the custom blob is worth having either. It can easily be
>> added later if we decide we want it after all.
>>
>> Thanks,
>> -Brian
>>
>> >In the future we should be getting a more fully fleged pipeline.
>> >
>> >>
>> >> > After staring at the v4l docs on this stuff I kinda like their
>> >> > "encoding" terminology to describe the YCbCr->RGB part, so I'm now a
>> >> > little partial to calling the prop something like YCBCR_ENCODING. OTOH
>> >>
>> >> I guess this property should be called YCBCR_DECODING.
>> >
>> >Only if you think of it as a verb.
>> >
>> >>
>> >> > if we want to expose the raw matrix as a blob then maybe calling it a
>> >> > CSC might be better. Not sure there's much point in exposing it though.
>> >>
>> >> In my first version it was called just CSC, but then I wanted to make it
>> >> explicit what this CSC was used for to avoid mixing the YCbCr decoding
>> >> matrix with YCbCr encoding matrix. At least on OMAP DSS we have pieces
>> >> of HW that can do only one or the other, e.g. the offset calculations
>> >> are supported only to one direction.
>> >
>> >Are you planning to do RGB->YCbCr conversion in the plane as well? I
>> >think we'll be only doing that at crtc/connector level.
>> >
>> >>
>> >> > I don't think most people are in the habit if cooking up new ways to
>> >> > encode their pixel data.
>> >> >
>> >>
>> >> In the embedded side I can imagine there could be some custom appliances
>> >> where one may want to do some custom thing with the CSC and not needing
>> >> a custom kernel for that could make a life easier... but then again I am
>> >> not really an expert in this area.
>> >
>> >I would assume most customy things you'd do in the crtc (eg. color
>> >correction and whatnot). But could be that I just lack imagination.
>> >
>> >--
>> >Ville Syrjälä
>> >Intel OTC
>
>-- 
>Ville Syrjälä
>Intel OTC
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