drm/fb-helper: Add locking to sysrq handling

We didn't take the kernel_fb_helper_lock mutex, which protects that
code. While at it, simplify the code
- inline the function (originally shared with kgdb I think)
- drop the error tracking and all the complications
- drop the pointless early out, it served nothing
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007133036.1541639-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_fb_helper.c

@@ -281,18 +281,12 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 EXPORT_SYMBOL(drm_fb_helper_restore_fbdev_mode_unlocked);
 
 #ifdef CONFIG_MAGIC_SYSRQ
-/*
- * restore fbcon display for all kms driver's using this helper, used for sysrq
- * and panic handling.
- */
-static bool drm_fb_helper_force_kernel_mode(void)
+/* emergency restore, don't bother with error reporting */
+static void drm_fb_helper_restore_work_fn(struct work_struct *ignored)
 {
-	bool ret, error = false;
 	struct drm_fb_helper *helper;
 
-	if (list_empty(&kernel_fb_helper_list))
-		return false;
-
+	mutex_lock(&kernel_fb_helper_lock);
 	list_for_each_entry(helper, &kernel_fb_helper_list, kernel_fb_list) {
 		struct drm_device *dev = helper->dev;
 
@@ -300,22 +294,12 @@ static bool drm_fb_helper_force_kernel_mode(void)
 			continue;
 
 		mutex_lock(&helper->lock);
-		ret = drm_client_modeset_commit_locked(&helper->client);
-		if (ret)
-			error = true;
+		drm_client_modeset_commit_locked(&helper->client);
 		mutex_unlock(&helper->lock);
 	}
-	return error;
+	mutex_unlock(&kernel_fb_helper_lock);
 }
 
-static void drm_fb_helper_restore_work_fn(struct work_struct *ignored)
-{
-	bool ret;
-
-	ret = drm_fb_helper_force_kernel_mode();
-	if (ret == true)
-		DRM_ERROR("Failed to restore crtc configuration\n");
-}
 static DECLARE_WORK(drm_fb_helper_restore_work, drm_fb_helper_restore_work_fn);
 
 static void drm_fb_helper_sysrq(int dummy1)