Message-ID: <2mih3humepuedtli7ge52ncom4uffkqravdpalncgfyucmwdzc@bp5o7i3ky77a>
Date: Thu, 15 Feb 2024 12:00:01 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Sebastian Wick <sebastian.wick@...hat.com>
Cc: Ville Syrjälä <ville.syrjala@...ux.intel.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann <tzimmermann@...e.de>, 
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, Emma Anholt <emma@...olt.net>, 
	Jonathan Corbet <corbet@....net>, Sandy Huang <hjc@...k-chips.com>, 
	Heiko Stübner <heiko@...ech.de>, Chen-Yu Tsai <wens@...e.org>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Samuel Holland <samuel@...lland.org>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	Hans Verkuil <hverkuil@...all.nl>, linux-rockchip@...ts.infradead.org, linux-sunxi@...ts.linux.dev, 
	linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: Re: Re: Re: Re: Re: Re: [PATCH v5 08/44] drm/connector: hdmi:
 Add Broadcast RGB property

On Mon, Feb 12, 2024 at 06:06:18PM +0100, Sebastian Wick wrote:
> 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.
> 
> But it's really not.

It really is. This whole discussion started by "well it would be nice if
we made that property handled by the core", and we're now at the "we
need to deal with broken YCbCr displays and i915 opinion about them"
stage. After creating documentation, unit tests, etc. It's the textbook
definition of moving goalposts. And while i915 won't be affected by all
that work.

That series has been stuck for multiple iterations on pointless and
mundane debates while the biggest part and whole point of it is not
getting any review. So yeah, sorry, it's frustrating.

> The Broadcast RGB property kind of works from a user space perspective
> because it's a workaround for broken sinks. If a sink expects limited
> range we can force full range. If this however affects YCbCr modes as
> well, then this isn't a workaround for broken RGB range anymore
> because it now breaks YCbCr.

Or, you know, it's a workaround for broken YCbCr display.

> Sorry, but vc4 just has to change.
> 
> And again: let's please stop trying to improve the property.

I'm not. I'm super close to just dropping that patch entirely and keep
the current situation that seems to work fine for everyone.

Maxime

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