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

[Ville Syrjälä]

On Fri, Feb 03, 2023 at 01:28:20PM -0500, Harry Wentland wrote:
> 
> 
> On 2/3/23 11:00, Ville Syrjälä wrote:
> > 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 at 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.
> > 
> 
> We use what we're telling the display, i.e., the value in the
> colorspace property. That way we know whether to use a BT.2020
> or BT.709 matrix.

And given how these things have gone in the past I think
that is likey to bite someone at in the future. Also not
what this property was meant to do nor does on any other
driver AFAIK.

> I don't see how it's fundamentally incompatible with fancy
> color management stuff.
> 
> If we start forbidding drivers from falling back to YCbCr
> (whether 4:4:4 or 4:2:0) we will break existing behavior on
> amdgpu and will see bug reports.

The compositors could deal with that if/when they start doing
the full color management stuff. The current stuff only really
works when the kernel is allowed to do whatever it wants.

> 
> > 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.
> > 
> 
> Does anyone actually use the non-BT.2020 colorspace stuff?

No idea if anyone is using any of it. It's a bit hard to do
right now outside the full passthrough case since we have no
properties to control how the hardware will convert stuff.

Anyways, sounds like what you're basically proposing is
getting rid of this property and starting from scratch.

-- 
Ville Syrjälä
Intel