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

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

On Mon, Feb 12, 2024 at 05:53:48PM +0100, Maxime Ripard wrote:
> On Mon, Feb 12, 2024 at 05:49:33PM +0200, Ville Syrjälä wrote:
> > On Mon, Feb 12, 2024 at 11:01:07AM +0100, Maxime Ripard wrote:
> > > 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.
> > 
> > No it is not.
> 
> The property is compatible with i915 interpretation of it, whether you
> like it or not. And that's what Sebastian was referring to.
> 
> > Eg. what happens if you set the thing to full range for RGB (which you
> > must on many broken monitors), and then the kernel automagically
> > switches to YCbCr (for whatever reason) but the monitor doesn't
> > support full range YCbCr? Answer: you get crap output.
> 
> And that part is just moving goalposts.

No. Allowing users to get correct colors with broken displays
is the sole reason why this property even exists.

-- 
Ville Syrjälä
Intel