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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <yx2t7xltxxgsngdsxamsfq6y7dze3wzegxcqwmsb5yrxen73x6@u3vilqhpci4w>
Date: Mon, 12 Feb 2024 17:53:48 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc: Sebastian Wick <sebastian.wick@...hat.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: [PATCH v5 08/44] drm/connector: hdmi: Add
 Broadcast RGB property

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.

Maxime

Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)