Message-ID: <20240222131210.GA2405678@toolbox>
Date: Thu, 22 Feb 2024 14:12:10 +0100
From: Sebastian Wick <sebastian.wick@...hat.com>
To: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc: Maxime Ripard <mripard@...nel.org>,
	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: [PATCH v5 08/44] drm/connector: hdmi: Add Broadcast RGB property

On Thu, Feb 22, 2024 at 02:58:45PM +0200, Ville Syrjälä wrote:
> On Thu, Feb 22, 2024 at 11:54:04AM +0100, Maxime Ripard wrote:
> > On Mon, Feb 19, 2024 at 03:01:44PM +0100, Sebastian Wick wrote:
> > > On Thu, Feb 15, 2024 at 12:00:01PM +0100, Maxime Ripard wrote:
> > > > 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.
> > > 
> > > Sorry, but what you're saying is just not true.
> > > 
> > > The Broadcast RGB property is an Intel specific property.
> > 
> > No, it's not. vc4 has been using it for a year now.
> > 
> > > It lacked documentation but the user space contract exists and it
> > > based on how i915 implemented it. By changing the semantics you're
> > > breaking user space. The documentation has to document the current
> > > contract between i915 and user space, not whatever you want the
> > > property to be like.
> > > 
> > > I get that you're frustrated that you have to do work while i915 doesn't
> > > but none of that is relevant for what the property is and how user space
> > > expects it to work.
> > 
> > That's not it, really. I don't mind doing the work. I do mind losing
> > functionalities on something that was working fine. And getting the
> > blame for something that is, at best, just as much of an documentation
> > issue on i915 devs.
> 
> We've had a couple of these cases recently where people have taken
> some old property implemented by i915 and implemented it differently
> in some other driver. Dunno if the reason was that people didn't try
> to understand what i915 is doing and why, or they misundestood it,
> or they understood it but decided to ignore it anyway.
> 
> Unfortunately having undocumented corners in the uapi is simply
> a fact of life when dealing with a >15 year old legacy codebase.
> Also there were basically no rules regarding properties in the
> past, so everyone just added random properties whenever they 
> felt like it.
> 
> I think going forward we should probably lay down some extra
> ground rules; if an old undocumented uapi is being extended
> to cover more than one driver we must first figure out what
> the de facto semantics are, and document things properly
> before anything else gets done.

That would be great. The documentation already has requirements for new
properties. Adding the requirement for extending driver-specific
properties to more drivers there would be great and make it "official".

> 
> -- 
> Ville Syrjälä
> Intel
> 

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