linux-kernel - Re: [PATCH v3 13/33] drm/vkms: Introduce configfs for plane color encoding

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Message-Id: <DF5M2A4RT005.KU8DTLA3CGI0@bootlin.com>
Date: Tue, 23 Dec 2025 13:56:09 +0100
From: "Luca Ceresoli" <luca.ceresoli@...tlin.com>
To: "Louis Chauvet" <louis.chauvet@...tlin.com>, "Haneen Mohammed"
 <hamohammed.sa@...il.com>, "Simona Vetter" <simona@...ll.ch>, "Melissa Wen"
 <melissa.srw@...il.com>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "David Airlie"
 <airlied@...il.com>, <jose.exposito89@...il.com>, "Jonathan Corbet"
 <corbet@....net>
Cc: <victoria@...tem76.com>, <sebastian.wick@...hat.com>,
 <thomas.petazzoni@...tlin.com>, <dri-devel@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v3 13/33] drm/vkms: Introduce configfs for plane color
 encoding

On Mon Dec 22, 2025 at 11:11 AM CET, Louis Chauvet wrote:
> To allows the userspace to test many hardware configuration, introduce a
> new interface to configure the available color encoding per planes. VKMS
> supports multiple color encoding, so the userspace can choose any
> combination.
>
> The supported color encoding are configured by writing a color encoding
> bitmask to the file `supported_color_encoding` and the default color
> encoding is chosen by writing a color encoding bitmask to
> `default_color_encoding`.
>
> Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>

> --- a/Documentation/ABI/testing/configfs-vkms
> +++ b/Documentation/ABI/testing/configfs-vkms
> @@ -124,6 +124,20 @@ Description:
>          Default rotation presented to userspace, same values as
>          possible_rotations.
>
> +What:		/sys/kernel/config/vkms/<device>/planes/<plane>/supported_color_encoding
                                                                                        ^
supported_color_encodings (final 's').

> --- a/Documentation/gpu/vkms.rst
> +++ b/Documentation/gpu/vkms.rst

> +static ssize_t plane_supported_color_encodings_store(struct config_item *item,
> +						     const char *page, size_t count)
> +{
> +	struct vkms_configfs_plane *plane = plane_item_to_vkms_configfs_plane(item);
> +	int ret, val = 0;
> +
> +	ret = kstrtouint(page, 10, &val);
> +	if (ret)
> +		return ret;
> +
> +	/* Should be a supported value */
> +	if (val & ~(VKMS_SUPPORTED_COLOR_ENCODINGS))
> +		return -EINVAL;
> +	/* Should at least provide one color range */
                                             ^ encoding

> +static ssize_t plane_default_color_encoding_show(struct config_item *item, char *page)
> +{
> +	struct vkms_configfs_plane *plane;
> +	unsigned int default_color_encoding;
> +
> +	plane = plane_item_to_vkms_configfs_plane(item);

Set on declare, for consistency (and conciseness).

> +static ssize_t plane_default_color_encoding_store(struct config_item *item,
> +						  const char *page, size_t count)
> +{
> +	struct vkms_configfs_plane *plane = plane_item_to_vkms_configfs_plane(item);
> +	int ret, val = 0;
> +
> +	ret = kstrtouint(page, 10, &val);
> +	if (ret)
> +		return ret;
> +
> +	/* Should be a supported value */
> +	if (val & ~VKMS_SUPPORTED_COLOR_ENCODINGS)
> +		return -EINVAL;
> +	/* Should at least provide one color range */
> +	if ((val & VKMS_SUPPORTED_COLOR_ENCODINGS) == 0)
> +		return -EINVAL;

This if() is redundant, the is_power_of_2() implies it because you already
ruled out any unsupported bit. Also, the comment is wrong. You can drop
these 3 lines entirely.

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com