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, 3 insertions, 0 deletions

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 8e3cb08241c8..2915118436ce 100644
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -733,6 +733,7 @@ static int drm_dev_init(struct drm_device *dev,
 	INIT_LIST_HEAD(&dev->filelist);
 	INIT_LIST_HEAD(&dev->filelist_internal);
 	INIT_LIST_HEAD(&dev->clientlist);
+	INIT_LIST_HEAD(&dev->client_sysrq_list);
 	INIT_LIST_HEAD(&dev->vblank_event_list);
 
 	spin_lock_init(&dev->event_lock);
@@ -1100,6 +1101,7 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
 			goto err_unload;
 	}
 	drm_panic_register(dev);
+	drm_client_sysrq_register(dev);
 
 	DRM_INFO("Initialized %s %d.%d.%d for %s on minor %d\n",
 		 driver->name, driver->major, driver->minor,
@@ -1144,6 +1146,7 @@ void drm_dev_unregister(struct drm_device *dev)
 {
 	dev->registered = false;
 
+	drm_client_sysrq_unregister(dev);
 	drm_panic_unregister(dev);
 
 	drm_client_dev_unregister(dev);