drm/client: Support emergency restore via sysrq for all clients

Move the sysrq functionality from DRM fbdev helpers to the DRM device
and in-kernel clients, so that it becomes available on all clients.

DRM fbdev helpers support emergency restoration of the console output
via a special key combination. Press SysRq+v to replace the current
compositor with the kernel's output on the framebuffer console. This
allows users to see the log messages during system emergencies.

By moving the functionality from fbdev helpers to the DRM device, any
in-kernel client can serve as emergency output. This can be used to
bring up drm_log, for example.

Each DRM device registers itself to the list of possible sysrq handlers.
On receiving SysRq+v, the DRM core goes over all registered devices and
restores an in-kernel DRM client for each of them.

See Documentation/admin-guide/sysrq.rst on how to invoke SysRq. Switch
VTs to bring back the user-space compositor.

v2:
- declare placeholders as 'static inline' (kernel test robot)
- fix grammar in commit description

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20251110154616.539328-3-tzimmermann@suse.de

1 files changed, 8 insertions, 0 deletions

diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
index 778b2cca6c49..5af49c5c3778 100644
--- a/include/drm/drm_device.h
+++ b/include/drm/drm_device.h
@@ -239,6 +239,14 @@ struct drm_device {
 	struct list_head clientlist;
 
 	/**
+	 * @client_sysrq_list:
+	 *
+	 * Entry into list of devices registered for sysrq. Allows in-kernel
+	 * clients on this device to handle sysrq keys.
+	 */
+	struct list_head client_sysrq_list;
+
+	/**
 	 * @vblank_disable_immediate:
 	 *
 	 * If true, vblank interrupt will be disabled immediately when the