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

Brian Starkey brian.starkey at arm.com
Fri Apr 21 14:53:42 UTC 2017


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[] = {
>> >> +		"YCbCr BT.601 limited range TO RGB BT.601 full range",
>> >> +		"YCbCr BT.601 full range TO RGB BT.601 full range",
>> >> +		"YCbCr BT.709 limited range TO RGB BT.709 full range",
>> >> +		"YCbCr BT.2020 limited range TO RGB BT.2020 full range",
>> >> +		"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.

If there is more capable hardware with the additional stages, then
they should expose additional properties (in pipeline order):

   Does YCbCr->RGB conversion on non-linear YCbCr data,
   only the enum values which don't have a CSC are exposed.

   Does the non-linear to linear conversion on the RGB data output from
   the YCBCR_TO_RGB stage.

   Similar set of properties to YCBCR_TO_RGB_*, allowing RGB->RGB CSC
   on the linear data.

   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.


>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

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