linux-kernel - Re: [PATCH v5 05/16] drm/vkms: Add dummy pixel_read/pixel

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Message-ID: <9c12e240-271a-4bf5-a946-7a2a25d1b18a@infradead.org>
Date: Wed, 13 Mar 2024 12:08:12 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Louis Chauvet <louis.chauvet@...tlin.com>,
 Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
 Melissa Wen <melissa.srw@...il.com>, Maíra Canal
 <mairacanal@...eup.net>, Haneen Mohammed <hamohammed.sa@...il.com>,
 Daniel Vetter <daniel@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, arthurgrillo@...eup.net,
 Jonathan Corbet <corbet@....net>, pekka.paalanen@...oniitty.fi
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 jeremie.dautheribes@...tlin.com, miquel.raynal@...tlin.com,
 thomas.petazzoni@...tlin.com, seanpaul@...gle.com, marcheu@...gle.com,
 nicolejadeyee@...gle.com
Subject: Re: [PATCH v5 05/16] drm/vkms: Add dummy pixel_read/pixel_write
 callbacks to avoid NULL pointers



On 3/13/24 10:44, Louis Chauvet wrote:
> Introduce two callbacks which does nothing. They are used in replacement
> of NULL and it avoid kernel OOPS if this NULL is called.
> 
> If those callback are used, it means that there is a mismatch between
> what formats are announced by atomic_check and what is realy supported by
> atomic_update.
> 
> Signed-off-by: Louis Chauvet <louis.chauvet@...tlin.com>
> ---
>  drivers/gpu/drm/vkms/vkms_formats.c | 43 +++++++++++++++++++++++++++++++------
>  1 file changed, 37 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_formats.c b/drivers/gpu/drm/vkms/vkms_formats.c
> index 55a4365d21a4..b57d85b8b935 100644
> --- a/drivers/gpu/drm/vkms/vkms_formats.c
> +++ b/drivers/gpu/drm/vkms/vkms_formats.c


> @@ -261,8 +286,8 @@ void vkms_writeback_row(struct vkms_writeback_job *wb,
>  
>  /**

Please mak this comment conform to kernel-doc format or don't use "/**" to
begin the comment.

>   * Retrieve the correct read_pixel function for a specific format.
> - * The returned pointer is NULL for unsupported pixel formats. The caller must ensure that the
> - * pointer is valid before using it in a vkms_plane_state.
> + * If the format is not supported by VKMS a warn is emitted and a dummy "always read black"
> + * function is returned.
>   *
>   * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
>   */
> @@ -285,18 +310,21 @@ pixel_read_t get_pixel_read_function(u32 format)
>  		 * format must:
>  		 * - Be listed in vkms_formats in vkms_plane.c
>  		 * - Have a pixel_read callback defined here
> +		 *
> +		 * To avoid kernel crash, a dummy "always read black" function is used. It means
> +		 * that during the composition, this plane will always be black.
>  		 */
>  		WARN(true,
>  		     "Pixel format %p4cc is not supported by VKMS planes. This is a kernel bug, atomic check must forbid this configuration.\n",
>  		     &format);
> -		return (pixel_read_t)NULL;
> +		return &black_to_argb_u16;
>  	}
>  }
>  
>  /**

Same here.

>   * Retrieve the correct write_pixel function for a specific format.
> - * The returned pointer is NULL for unsupported pixel formats. The caller must ensure that the
> - * pointer is valid before using it in a vkms_writeback_job.
> + * If the format is not supported by VKMS a warn is emitted and a dummy "don't do anything"
> + * function is returned.
>   *
>   * @format: DRM_FORMAT_* value for which to obtain a conversion function (see [drm_fourcc.h])
>   */

thanks.
-- 
#Randy