[PATCH v5 08/44] drm/connector: hdmi: Add Broadcast RGB property

[Maxime Ripard]

On Fri, Feb 09, 2024 at 09:34:35PM +0100, Sebastian Wick wrote:
> On Mon, Feb 05, 2024 at 10:39:38AM +0100, Maxime Ripard wrote:
> > On Fri, Feb 02, 2024 at 06:37:52PM +0200, Ville Syrjälä wrote:
> > > On Fri, Feb 02, 2024 at 04:59:30PM +0100, Maxime Ripard wrote:
> > > > On Fri, Feb 02, 2024 at 05:40:47PM +0200, Ville Syrjälä wrote:
> > > > > On Fri, Feb 02, 2024 at 02:01:39PM +0100, Maxime Ripard wrote:
> > > > > > Hi,
> > > > > > 
> > > > > > On Mon, Jan 15, 2024 at 03:37:20PM +0100, Sebastian Wick wrote:
> > > > > > > > >  /**
> > > > > > > > >   * DOC: HDMI connector properties
> > > > > > > > >   *
> > > > > > > > > + * Broadcast RGB
> > > > > > > > > + *      Indicates the RGB Quantization Range (Full vs Limited) used.
> > > > > > > > > + *      Infoframes will be generated according to that value.
> > > > > > > > > + *
> > > > > > > > > + *      The value of this property can be one of the following:
> > > > > > > > > + *
> > > > > > > > > + *      Automatic:
> > > > > > > > > + *              RGB Range is selected automatically based on the mode
> > > > > > > > > + *              according to the HDMI specifications.
> > > > > > > > > + *
> > > > > > > > > + *      Full:
> > > > > > > > > + *              Full RGB Range is forced.
> > > > > > > > > + *
> > > > > > > > > + *      Limited 16:235:
> > > > > > > > > + *              Limited RGB Range is forced. Unlike the name suggests,
> > > > > > > > > + *              this works for any number of bits-per-component.
> > > > > > > > > + *
> > > > > > > > > + *      Drivers can set up this property by calling
> > > > > > > > > + *      drm_connector_attach_broadcast_rgb_property().
> > > > > > > > > + *
> > > > > > > > 
> > > > > > > > This is a good time to document this in more detail. There might be two
> > > > > > > > different things being affected:
> > > > > > > > 
> > > > > > > > 1. The signalling (InfoFrame/SDP/...)
> > > > > > > > 2. The color pipeline processing
> > > > > > > > 
> > > > > > > > All values of Broadcast RGB always affect the color pipeline processing
> > > > > > > > such that a full-range input to the CRTC is converted to either full- or
> > > > > > > > limited-range, depending on what the monitor is supposed to accept.
> > > > > > > > 
> > > > > > > > When automatic is selected, does that mean that there is no signalling,
> > > > > > > > or that the signalling matches what the monitor is supposed to accept
> > > > > > > > according to the spec? Also, is this really HDMI specific?
> > > > > > > > 
> > > > > > > > When full or limited is selected and the monitor doesn't support the
> > > > > > > > signalling, what happens?
> > > > > > > 
> > > > > > > Forgot to mention: user-space still has no control over RGB vs YCbCr on
> > > > > > > the cable, so is this only affecting RGB? If not, how does it affect
> > > > > > > YCbCr?
> > > > > > 
> > > > > > So I dug a bit into both the i915 and vc4 drivers, and it looks like if
> > > > > > we're using a YCbCr format, i915 will always use a limited range while
> > > > > > vc4 will follow the value of the property.
> > > > > 
> > > > > The property is literally called "Broadcast *RGB*".
> > > > > That should explain why it's only affecting RGB.
> > > > 
> > > > Right. And the limited range option is called "Limited 16:235" despite
> > > > being usable on bpc > 8 bits. Naming errors occurs, and history happens
> > > > to make names inconsistent too, that's fine and not an argument in
> > > > itself.
> > > > 
> > > > > Full range YCbCr is a much rarer beast so we've never bothered
> > > > > to enable it.
> > > > 
> > > > vc4 supports it.
> > > 
> > > Someone implemented it incorrectly then.
> > 
> > Incorrectly according to what documentation / specification? I'm sorry,
> > but I find it super ironic that i915 gets to do its own thing, not
> > document any of it, and when people try to clean things up they get told
> > that we got it all wrong.
> 
> FWIW, this was an i915 property and if another driver uses the same
> property name it must have the same behavior. Yes, it isn't standardized
> and yes, it's not documented (hence this effort here) but it's still on
> vc4 to make the property compatible.

How is it not compatible? It's a superset of what i915 provides, but
it's strictly compatible with it.

I would argue that i915 is the broken one since userspace could force a
full range output, but since the driver takes the YUV vs RGB decision
itself and only supports limited range for YUV, the driver would
effectively ignore that user-space property, without the user-space
being able to tell it was ignored in the first place.

> Trying to make the property handle YCbCr is very much in the "let's try
> to fix the property" territory that I want to avoid, so I'm in favor of
> adjusting vc4.

Breaking the ABI in the process. For something that is explicitly
supported by the spec, the driver, and the hardware. On a property that
never said it wasn't meant to be used that way, and with semantics based
on a driver that never provided a way to check those restrictions in the
first place.

And it's not like i915 is going to use that code anyway.

Maxime
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