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 <[e-mail]>
Reviewed-by: Jocelyn Falempe <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

drivers/gpu/drm/Makefile

···
 drm-$(CONFIG_DRM_CLIENT) += \
 	drm_client.o \
 	drm_client_event.o \
-	drm_client_modeset.o
+	drm_client_modeset.o \
+	drm_client_sysrq.o
 drm-$(CONFIG_DRM_LIB_RANDOM) += lib/drm_random.o
 drm-$(CONFIG_COMPAT) += drm_ioc32.o
 drm-$(CONFIG_DRM_PANEL) += drm_panel.o

drivers/gpu/drm/drm_client.c

···
 #include <linux/slab.h>
 
 #include <drm/drm_client.h>
+#include <drm/drm_client_event.h>
 #include <drm/drm_device.h>
 #include <drm/drm_drv.h>
 #include <drm/drm_file.h>

drivers/gpu/drm/drm_client_sysrq.c

···
+// SPDX-License-Identifier: GPL-2.0 or MIT
+
+#include <linux/sysrq.h>
+
+#include <drm/drm_client_event.h>
+#include <drm/drm_device.h>
+#include <drm/drm_print.h>
+
+#include "drm_internal.h"
+
+#ifdef CONFIG_MAGIC_SYSRQ
+static LIST_HEAD(drm_client_sysrq_dev_list);
+static DEFINE_MUTEX(drm_client_sysrq_dev_lock);
+
+/* emergency restore, don't bother with error reporting */
+static void drm_client_sysrq_restore_work_fn(struct work_struct *ignored)
+{
+	struct drm_device *dev;
+
+	guard(mutex)(&drm_client_sysrq_dev_lock);
+
+	list_for_each_entry(dev, &drm_client_sysrq_dev_list, client_sysrq_list) {
+		if (dev->switch_power_state == DRM_SWITCH_POWER_OFF)
+			continue;
+
+		drm_client_dev_restore(dev, true);
+	}
+}
+
+static DECLARE_WORK(drm_client_sysrq_restore_work, drm_client_sysrq_restore_work_fn);
+
+static void drm_client_sysrq_restore_handler(u8 ignored)
+{
+	schedule_work(&drm_client_sysrq_restore_work);
+}
+
+static const struct sysrq_key_op drm_client_sysrq_restore_op = {
+	.handler = drm_client_sysrq_restore_handler,
+	.help_msg = "force-fb(v)",
+	.action_msg = "Restore framebuffer console",
+};
+
+void drm_client_sysrq_register(struct drm_device *dev)
+{
+	guard(mutex)(&drm_client_sysrq_dev_lock);
+
+	if (list_empty(&drm_client_sysrq_dev_list))
+		register_sysrq_key('v', &drm_client_sysrq_restore_op);
+
+	list_add(&dev->client_sysrq_list, &drm_client_sysrq_dev_list);
+}
+
+void drm_client_sysrq_unregister(struct drm_device *dev)
+{
+	guard(mutex)(&drm_client_sysrq_dev_lock);
+
+	/* remove device from global restore list */
+	if (!drm_WARN_ON(dev, list_empty(&dev->client_sysrq_list)))
+		list_del(&dev->client_sysrq_list);
+
+	/* no devices left; unregister key */
+	if (list_empty(&drm_client_sysrq_dev_list))
+		unregister_sysrq_key('v', &drm_client_sysrq_restore_op);
+}
+#endif

drivers/gpu/drm/drm_drv.c

···
 	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);
···
 			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,
···
 {
 	dev->registered = false;
 
+	drm_client_sysrq_unregister(dev);
 	drm_panic_unregister(dev);
 
 	drm_client_dev_unregister(dev);

drivers/gpu/drm/drm_fb_helper.c

···
 #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>
···
 	return __drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper, force);
 }
 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)
 {
···
 	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)
···
 		 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);
 

drivers/gpu/drm/drm_internal.h

···
 { }
 #endif
 
+/* drm_client_sysrq.c */
+#if defined(CONFIG_DRM_CLIENT) && defined(CONFIG_MAGIC_SYSRQ)
+void drm_client_sysrq_register(struct drm_device *dev);
+void drm_client_sysrq_unregister(struct drm_device *dev);
+#else
+static inline void drm_client_sysrq_register(struct drm_device *dev)
+{ }
+static inline void drm_client_sysrq_unregister(struct drm_device *dev)
+{ }
+#endif
+
 /* drm_file.c */
 extern struct mutex drm_global_mutex;
 bool drm_dev_needs_global_mutex(struct drm_device *dev);

include/drm/drm_device.h

···
 	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