[PATCH v8 3/8] drm/panic: Add debugfs entry to test without triggering panic.

[Daniel Vetter]

On Tue, Feb 27, 2024 at 11:04:14AM +0100, Jocelyn Falempe wrote:
> Add a debugfs file, so you can test drm_panic without freezing
> your machine. This is unsafe, and should be enabled only for
> developer or tester.
> 
> to display the drm_panic screen, just run:
> echo 1 > /sys/kernel/debug/drm_panic/trigger
> 
> Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
> ---
>  drivers/gpu/drm/Kconfig     |  9 +++++++
>  drivers/gpu/drm/drm_panic.c | 47 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 56 insertions(+)
> 
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index c17d8a8f6877..8dcea29f595c 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -125,6 +125,15 @@ config DRM_PANIC_BACKGROUND_COLOR
>  	depends on DRM_PANIC
>  	default 0x000000
>  
> +config DRM_PANIC_DEBUG
> +	bool "Add a debug fs entry to trigger drm_panic"
> +	depends on DRM_PANIC && DEBUG_FS
> +	help
> +	  Add drm_panic/trigger in the kernel debugfs, to force the panic
> +	  handler to write the panic message to the scanout buffer. This is
> +	  unsafe and should not be enabled on a production build.
> +	  If in doubt, say "N".
> +
>  config DRM_DEBUG_DP_MST_TOPOLOGY_REFS
>          bool "Enable refcount backtrace history in the DP MST helpers"
>  	depends on STACKTRACE_SUPPORT
> diff --git a/drivers/gpu/drm/drm_panic.c b/drivers/gpu/drm/drm_panic.c
> index c9f386476ef9..c5d3f725c5f5 100644
> --- a/drivers/gpu/drm/drm_panic.c
> +++ b/drivers/gpu/drm/drm_panic.c
> @@ -398,3 +398,50 @@ void drm_panic_unregister(struct drm_plane *plane)
>  }
>  EXPORT_SYMBOL(drm_panic_unregister);
>  
> +
> +/*
> + * DEBUG, This is currently unsafe.
> + * Also it will call all panic_notifier, since there is no way to filter and
> + * only call the drm_panic notifier.
> + */
> +#ifdef CONFIG_DRM_PANIC_DEBUG
> +#include <linux/debugfs.h>
> +
> +static struct dentry *debug_dir;
> +static struct dentry *debug_trigger;
> +
> +static ssize_t dbgfs_trigger_write(struct file *file, const char __user *user_buf,
> +				   size_t count, loff_t *ppos)
> +{
> +	bool run;
> +
> +	if (kstrtobool_from_user(user_buf, count, &run) == 0 && run)
> +		atomic_notifier_call_chain(&panic_notifier_list, 0, "Test drm panic from debugfs");

Since this is just the general panic notifier it feels very misplaced in
the drm subsystem. I think moving that code into the core panic code makes
a lot more sense, then we'd also have all the right people on Cc: to
figure out how we can best recreate the correct calling context (like nmi
context or whatever) for best case simulation of panic code. John Ogness
definitely needs to see this and ack, wherever we put it.
-Sima

> +	return count;
> +}
> +
> +static const struct file_operations dbg_drm_panic_ops = {
> +	.owner = THIS_MODULE,
> +	.write = dbgfs_trigger_write,
> +};
> +
> +static int __init debugfs_start(void)
> +{
> +	debug_dir = debugfs_create_dir("drm_panic", NULL);
> +
> +	if (IS_ERR(debug_dir))
> +		return PTR_ERR(debug_dir);
> +	debug_trigger = debugfs_create_file("trigger", 0200, debug_dir,
> +					    NULL, &dbg_drm_panic_ops);
> +	return 0;
> +}
> +
> +static void __exit debugfs_end(void)
> +{
> +	debugfs_remove_recursive(debug_dir);
> +}
> +
> +module_init(debugfs_start);
> +module_exit(debugfs_end);
> +
> +#endif
> -- 
> 2.43.0
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch