linux-kernel - Re: [PATCH v2 3/3] drm/panic: Add a drm_panic/draw

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Message-ID: <20250910-astute-vole-of-kindness-c6f6ce@houat>
Date: Wed, 10 Sep 2025 12:49:48 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Jocelyn Falempe <jfalempe@...hat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Javier Martinez Canillas <javierm@...hat.com>, 
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 3/3] drm/panic: Add a drm_panic/draw_test in debugfs

Hi,

On Mon, Sep 08, 2025 at 11:00:31AM +0200, Jocelyn Falempe wrote:
> This adds a new drm_panic/draw_test file in debugfs.
> This file allows to test the panic screen rendering at different
> resolution and pixel format.
> It's useful only for kernel developers that want to create or
> customize a panic screen.
> 
> If you want to check the result at 1024x768 using XRGB8888:
> 
> cd /sys/kernel/debug/drm_panic/
> exec 3<> draw_test
> echo 1024x768:XR24 >&3
> cat <&3 > ~/panic_screen.raw
> exec 3<&-
> 
> Signed-off-by: Jocelyn Falempe <jfalempe@...hat.com>

I see what you meant in your previous version, and I misunderstood what
you were saying, sorry.

> v2:
>  * Use debugfs instead of sending the framebuffer through the kunit logs. (Thomas Zimmermann).
> 
>  drivers/gpu/drm/Kconfig     |   2 +
>  drivers/gpu/drm/drm_panic.c | 117 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 119 insertions(+)
> 
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index f7ea8e895c0c..0d3146070d9c 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -83,6 +83,8 @@ config DRM_PANIC_DEBUG
>  	  Add dri/[device]/drm_panic_plane_x in the kernel debugfs, to force the
>  	  panic handler to write the panic message to this plane scanout buffer.
>  	  This is unsafe and should not be enabled on a production build.
> +	  Also adds a drm_panic/draw_test file in debugfs, to easily test the
> +	  panic screen rendering.
>  	  If in doubt, say "N".
>  
>  config DRM_PANIC_SCREEN
> diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
> index d89812ff1935..0c01d6067eab 100644
> --- a/drivers/gpu/drm/drm_panic.c
> +++ b/drivers/gpu/drm/drm_panic.c
> @@ -873,6 +873,7 @@ static void drm_panic(struct kmsg_dumper *dumper, struct kmsg_dump_detail *detai
>   */
>  #ifdef CONFIG_DRM_PANIC_DEBUG
>  #include <linux/debugfs.h>
> +#include <linux/vmalloc.h>
>  
>  static ssize_t debugfs_trigger_write(struct file *file, const char __user *user_buf,
>  				     size_t count, loff_t *ppos)
> @@ -901,8 +902,122 @@ static void debugfs_register_plane(struct drm_plane *plane, int index)
>  	debugfs_create_file(fname, 0200, plane->dev->debugfs_root,
>  			    plane, &dbg_drm_panic_ops);
>  }
> +
> +/*
> + * Draw test interface
> + * This can be used to check the panic screen at any resolution/pixel format.
> + * The framebuffer memory is freed when the file is closed, so use this sh
> + * script to write the parameters and read the result without closing the file.
> + * cd /sys/kernel/debug/drm_panic/
> + * exec 3<> draw_test
> + * echo 1024x768:XR24 >&3
> + * cat <&3 > ~/panic_screen.raw
> + * exec 3<&-
> + */

This should be documented properly, and I'm also kind of wondering how
that would fit in the larger testing ecosystem.

Ie, how can someone that just starts contributing to Linux, or is
setting up a CI platform, can have that test running.

kunit is great for that, kselftests to some extent too, but I'm not sure
an ad-hoc interface is.

Unless we create IGT tests for it too maybe?

Maxime

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