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, 1 insertions, 44 deletions

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 1392738ce2fe..9a734017756b 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -32,7 +32,6 @@
 #include <linux/console.h>
 #include <linux/export.h>
 #include <linux/pci.h>
-#include <linux/sysrq.h>
 #include <linux/vga_switcheroo.h>
 
 #include <drm/drm_atomic.h>
@@ -270,42 +269,6 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper, b
 }
 EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode_unlocked);
 
-#ifdef CONFIG_MAGIC_SYSRQ
-/* emergency restore, don't bother with error reporting */
-static void drm_fb_helper_restore_work_fn(struct work_struct *ignored)
-{
-	struct drm_fb_helper *helper;
-
-	mutex_lock(&kernel_fb_helper_lock);
-	list_for_each_entry(helper, &kernel_fb_helper_list, kernel_fb_list) {
-		struct drm_device *dev = helper->dev;
-
-		if (dev->switch_power_state == DRM_SWITCH_POWER_OFF)
-			continue;
-
-		mutex_lock(&helper->lock);
-		drm_client_modeset_commit_locked(&helper->client);
-		mutex_unlock(&helper->lock);
-	}
-	mutex_unlock(&kernel_fb_helper_lock);
-}
-
-static DECLARE_WORK(drm_fb_helper_restore_work, drm_fb_helper_restore_work_fn);
-
-static void drm_fb_helper_sysrq(u8 dummy1)
-{
-	schedule_work(&drm_fb_helper_restore_work);
-}
-
-static const struct sysrq_key_op sysrq_drm_fb_helper_restore_op = {
-	.handler = drm_fb_helper_sysrq,
-	.help_msg = "force-fb(v)",
-	.action_msg = "Restore framebuffer console",
-};
-#else
-static const struct sysrq_key_op sysrq_drm_fb_helper_restore_op = { };
-#endif
-
 static void drm_fb_helper_dpms(struct fb_info *info, int dpms_mode)
 {
 	struct drm_fb_helper *fb_helper = info->par;
@@ -602,11 +565,8 @@ void drm_fb_helper_fini(struct drm_fb_helper *fb_helper)
 	drm_fb_helper_release_info(fb_helper);
 
 	mutex_lock(&kernel_fb_helper_lock);
-	if (!list_empty(&fb_helper->kernel_fb_list)) {
+	if (!list_empty(&fb_helper->kernel_fb_list))
 		list_del(&fb_helper->kernel_fb_list);
-		if (list_empty(&kernel_fb_helper_list))
-			unregister_sysrq_key('v', &sysrq_drm_fb_helper_restore_op);
-	}
 	mutex_unlock(&kernel_fb_helper_lock);
 
 	if (!fb_helper->client.funcs)
@@ -1840,9 +1800,6 @@ __drm_fb_helper_initial_config_and_unlock(struct drm_fb_helper *fb_helper)
 		 info->node, info->fix.id);
 
 	mutex_lock(&kernel_fb_helper_lock);
-	if (list_empty(&kernel_fb_helper_list))
-		register_sysrq_key('v', &sysrq_drm_fb_helper_restore_op);
-
 	list_add(&fb_helper->kernel_fb_list, &kernel_fb_helper_list);
 	mutex_unlock(&kernel_fb_helper_lock);