linux-kernel - Re: [PATCH RESEND v2 16/32] drm/vkms: Introduce config for plane format

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Message-Id: <DF2A35683YPN.2TH0WT8JJAJ70@bootlin.com>
Date: Fri, 19 Dec 2025 15:55:03 +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 RESEND v2 16/32] drm/vkms: Introduce config for plane
 format

Hi Louis,

On Wed Oct 29, 2025 at 3:36 PM CET, Louis Chauvet wrote:
> VKMS driver supports all the pixel formats for planes, but for testing it
> can be useful to only advertise few of them. This new configuration
> interface will allow configuring the pixel format per planes.

[...]

> Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>
> +int __must_check vkms_config_plane_add_format(struct vkms_config_plane *plane_cfg, u32 drm_format)
> +{
> +	bool found = false;
> +
> +	for (int i = 0; i < ARRAY_SIZE(vkms_supported_plane_formats); i++) {
> +		if (vkms_supported_plane_formats[i] == drm_format) {
> +			found = true;
> +			break;
> +		}
> +	}
> +
> +	if (!found)
> +		return -EINVAL;
> +	for (unsigned int i = 0; i < plane_cfg->supported_formats_count; i++) {
> +		if (plane_cfg->supported_formats[i] == drm_format)
> +			return 0;
> +	}
> +	u32 *new_ptr = krealloc_array(plane_cfg->supported_formats,
> +				      plane_cfg->supported_formats_count + 1,
> +				      sizeof(*plane_cfg->supported_formats), GFP_KERNEL);
> +	if (!new_ptr)
> +		return -ENOMEM;
> +
> +	plane_cfg->supported_formats = new_ptr;
> +	plane_cfg->supported_formats[plane_cfg->supported_formats_count] = drm_format;
> +	plane_cfg->supported_formats_count++;
> +
> +	return 0;
> +}

This whole logic appears quite complex for what you need here. I suspect
using the facilities in linux/bitmap.h would make your code simpler by
allocating a (multi-)ulong array of
ARRAY_SIZE(vkms_supported_plane_formats) bits. This would surely use less
memory and avoid all reallocations, too.

> --- a/drivers/gpu/drm/vkms/vkms_config.h
> +++ b/drivers/gpu/drm/vkms/vkms_config.h

> +/**
> + * vkms_config_plane_remove_format - Remove a specific format from a plane
> + * @plane_cfg: Plane to remove the format to
> + * @drm_format: Format to remove
> + */
> +void vkms_config_plane_remove_format(struct vkms_config_plane *plane_cfg, u32 drm_format);
> +
> +/**
> + * vkms_config_plane_remove_all_formats - Remove all formast from a plane
                                                        formats

Luca

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