linux-kernel - Re: [PATCH 2/7] drm/uAPI: Add "active color format" drm property as feedback for userspace

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Date: Tue, 9 Jan 2024 23:21:38 +0000
From: Andri Yngvason <andri@...vason.is>
To: Daniel Stone <daniel@...ishbar.org>
Cc: Harry Wentland <harry.wentland@....com>, Leo Li <sunpeng.li@....com>, 
	Rodrigo Siqueira <Rodrigo.Siqueira@....com>, Alex Deucher <alexander.deucher@....com>, 
	Christian König <christian.koenig@....com>, 
	"Pan, Xinhui" <Xinhui.Pan@....com>, David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, Jani Nikula <jani.nikula@...ux.intel.com>, 
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>, Rodrigo Vivi <rodrigo.vivi@...el.com>, 
	Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>, amd-gfx@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, 
	intel-gfx@...ts.freedesktop.org, Simon Ser <contact@...rsion.fr>, 
	Werner Sembach <wse@...edocomputers.com>
Subject: Re: [PATCH 2/7] drm/uAPI: Add "active color format" drm property as
 feedback for userspace

Hi Daniel,

Please excuse my misconfigured email client. HTML was accidentally
enabled in my previous messages, so I'll re-send it for the benefit of
mailing lists.

þri., 9. jan. 2024 kl. 22:32 skrifaði Daniel Stone <daniel@...ishbar.org>:
>
> On Tue, 9 Jan 2024 at 18:12, Andri Yngvason <andri@...vason.is> wrote:
> > + * active color format:
> > + *     This read-only property tells userspace the color format actually used
> > + *     by the hardware display engine "on the cable" on a connector. The chosen
> > + *     value depends on hardware capabilities, both display engine and
> > + *     connected monitor. Drivers shall use
> > + *     drm_connector_attach_active_color_format_property() to install this
> > + *     property. Possible values are "not applicable", "rgb", "ycbcr444",
> > + *     "ycbcr422", and "ycbcr420".
>
> How does userspace determine what's happened without polling? Will it
> only change after an `ALLOW_MODESET` commit, and be guaranteed to be
> updated after the commit has completed and the event being sent?
> Should it send a HOTPLUG event? Other?

Userspace does not determine what's happened without polling. The
purpose of this property is not for programmatic verification that the
preferred property was applied. It is my understanding that it's
mostly intended for debugging purposes. It should only change as a
consequence of modesetting, although I didn't actually look into what
happens if you set the "preferred color format" outside of a modeset.

The way I've implemented things in sway, calling the
"preferred_signal_format" command triggers a modeset with the
"preferred color format" set and calling "get_outputs", immediately
queries the "actual color format" and displays it.

Regards,
Andri