Re: [PATCH 3/3] drm/connector: Deprecate split for BT.2020 in drm_colorspace enum

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Feb 03, 2023 at 10:24:52AM -0500, Harry Wentland wrote:
> 
> 
> On 2/3/23 10:19, Ville Syrjälä wrote:
> > On Fri, Feb 03, 2023 at 09:39:42AM -0500, Harry Wentland wrote:
> >>
> >>
> >> On 2/3/23 07:59, Sebastian Wick wrote:
> >>> On Fri, Feb 3, 2023 at 11:40 AM Ville Syrjälä
> >>> <ville.syrjala@linux.intel.com> wrote:
> >>>>
> >>>> On Fri, Feb 03, 2023 at 02:07:44AM +0000, Joshua Ashton wrote:
> >>>>> Userspace has no way of controlling or knowing the pixel encoding
> >>>>> currently, so there is no way for it to ever get the right values here.
> >>>>
> >>>> That applies to a lot of the other values as well (they are
> >>>> explicitly RGB or YCC). The idea was that this property sets the
> >>>> infoframe/MSA/SDP value exactly, and other properties should be
> >>>> added to for use userspace to control the pixel encoding/colorspace
> >>>> conversion(if desired, or userspace just makes sure to
> >>>> directly feed in correct kind of data).
> >>>
> >>> I'm all for getting userspace control over pixel encoding but even
> >>> then the kernel always knows which pixel encoding is selected and
> >>> which InfoFrame has to be sent. Is there a reason why userspace would
> >>> want to control the variant explicitly to the wrong value?
> >>>
> >>
> >> I've asked this before but haven't seen an answer: Is there an existing
> >> upstream userspace project that makes use of this property (other than
> >> what Joshua is working on in gamescope right now)? That would help us
> >> understand the intent better.
> > 
> > The intent was to control the infoframe colorimetry bits,
> > nothing more. No idea what real userspace there was, if any.
> > 
> >>
> >> I don't think giving userspace explicit control over the exact infoframe
> >> values is the right thing to do.
> > 
> > Only userspace knows what kind of data it's stuffing into
> > the pixels (and/or how it configures the csc units/etc.) to
> > generate them.
> > 
> 
> Yes, but userspace doesn't control or know whether we drive
> RGB or YCbCr on the wire. In fact, in some cases our driver
> needs to fallback to YCbCr420 for bandwidth reasons. There
> is currently no way for userspace to know that and I don't
> think it makes sense.

People want that control as well for whatever reason. We've
been asked to allow YCbCr 4:4:4 output many times.

The automagic 4:2:0 fallback I think is rather fundementally
incompatible with fancy color management. How would we even
know whether to use eg. BT.2020 vs. BT.709 matrix? In i915
that stuff is just always BT.709 limited range, no questions
asked.

So I think if userspace wants real color management it's
going to have to set up the whole pipeline. And for that
we need at least one new property to control the RGB->YCbCr
conversion (or to explicitly avoid it).

And given that the proposed patch just swept all the
non-BT.2020 issues under the rug makes me think no
one has actually come up with any kind of consistent
plan for anything else really.

> 
> Userspace needs full control of framebuffer pixel formats,
> as well as control over DEGAMMA, GAMMA, CTM color operations.
> It also needs to be able to select whether to drive the panel
> as sRGB or BT.2020/PQ but it doesn't make sense for it to
> control the pixel encoding on the wire (RGB vs YCbCr).
> 
> > I really don't want a repeat of the disaster of the
> > 'Broadcast RGB' which has coupled together the infoframe 
> > and automagic conversion stuff. And I think this one would
> > be about 100x worse given this property has something
> > to do with actual colorspaces as well.
> >  
> 
> I'm unaware of this disaster. Could you elaborate?

The property now controls both the infoframe stuff (and
whatever super vague stuff DP has for it in MSA) and 
full->limited range compression in the display pipeline. 
And as a result  there is no way to eg. allow already 
limited range input, which is what some people wanted.

And naturally it's all made a lot more terrible by all
the displays that fail to implement the spec correctly,
but that's another topic.

-- 
Ville Syrjälä
Intel