drm/fb-helper: Synchronize dirty worker with vblank

Before updating the display from the console's shadow buffer, the dirty
worker now waits for a vblank. This allows several screen updates to pile
up and acts as a rate limiter. If a DRM master is present, it could
interfere with the vblank. Don't wait in this case.

v4:
	* share code with WAITFORVSYNC ioctl (Emil)
	* use lock guard
v3:
	* add back helper->lock
	* acquire DRM master status while waiting for vblank
v2:
	* don't hold helper->lock while waiting for vblank

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20250829091447.46719-1-tzimmermann@suse.de

drivers/gpu/drm/drm_client_modeset.c

@@ -1293,6 +1293,50 @@ int drm_client_modeset_dpms(struct drm_client_dev *client, int mode)
 }
 EXPORT_SYMBOL(drm_client_modeset_dpms);
 
+/**
+ * drm_client_modeset_wait_for_vblank() - Wait for the next VBLANK to occur
+ * @client: DRM client
+ * @crtc_index: The ndex of the CRTC to wait on
+ *
+ * Block the caller until the given CRTC has seen a VBLANK. Do nothing
+ * if the CRTC is disabled. If there's another DRM master present, fail
+ * with -EBUSY.
+ *
+ * Returns:
+ * 0 on success, or negative error code otherwise.
+ */
+int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned int crtc_index)
+{
+	struct drm_device *dev = client->dev;
+	struct drm_crtc *crtc;
+	int ret;
+
+	/*
+	 * Rate-limit update frequency to vblank. If there's a DRM master
+	 * present, it could interfere while we're waiting for the vblank
+	 * event. Don't wait in this case.
+	 */
+	if (!drm_master_internal_acquire(dev))
+		return -EBUSY;
+
+	crtc = client->modesets[crtc_index].crtc;
+
+	/*
+	 * Only wait for a vblank event if the CRTC is enabled, otherwise
+	 * just don't do anything, not even report an error.
+	 */
+	ret = drm_crtc_vblank_get(crtc);
+	if (!ret) {
+		drm_crtc_wait_one_vblank(crtc);
+		drm_crtc_vblank_put(crtc);
+	}
+
+	drm_master_internal_release(dev);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_client_modeset_wait_for_vblank);
+
 #ifdef CONFIG_DRM_KUNIT_TEST
 #include "tests/drm_client_modeset_test.c"
 #endif

drivers/gpu/drm/drm_fb_helper.c

@@ -368,6 +368,10 @@ static void drm_fb_helper_fb_dirty(struct drm_fb_helper *helper)
 	unsigned long flags;
 	int ret;
 
+	mutex_lock(&helper->lock);
+	drm_client_modeset_wait_for_vblank(&helper->client, 0);
+	mutex_unlock(&helper->lock);
+
 	if (drm_WARN_ON_ONCE(dev, !helper->funcs->fb_dirty))
 		return;
 
@@ -1068,15 +1072,9 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
 			unsigned long arg)
 {
 	struct drm_fb_helper *fb_helper = info->par;
-	struct drm_device *dev = fb_helper->dev;
-	struct drm_crtc *crtc;
 	int ret = 0;
 
-	mutex_lock(&fb_helper->lock);
+	guard(mutex)(&fb_helper->lock);
-	if (!drm_master_internal_acquire(dev)) {
-		ret = -EBUSY;
-		goto unlock;
-	}
 
 	switch (cmd) {
 	case FBIO_WAITFORVSYNC:
@@ -1096,28 +1094,12 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
 		 * make. If we're not smart enough here, one should
 		 * just consider switch the userspace to KMS.
 		 */
-		crtc = fb_helper->client.modesets[0].crtc;
+		ret = drm_client_modeset_wait_for_vblank(&fb_helper->client, 0);
-
-		/*
-		 * Only wait for a vblank event if the CRTC is
-		 * enabled, otherwise just don't do anythintg,
-		 * not even report an error.
-		 */
-		ret = drm_crtc_vblank_get(crtc);
-		if (!ret) {
-			drm_crtc_wait_one_vblank(crtc);
-			drm_crtc_vblank_put(crtc);
-		}
-
-		ret = 0;
 		break;
 	default:
 		ret = -ENOTTY;
 	}
 
-	drm_master_internal_release(dev);
-unlock:
-	mutex_unlock(&fb_helper->lock);
 	return ret;
 }
 EXPORT_SYMBOL(drm_fb_helper_ioctl);

include/drm/drm_client.h

@@ -220,6 +220,7 @@ int drm_client_modeset_check(struct drm_client_dev *client);
 int drm_client_modeset_commit_locked(struct drm_client_dev *client);
 int drm_client_modeset_commit(struct drm_client_dev *client);
 int drm_client_modeset_dpms(struct drm_client_dev *client, int mode);
+int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned int crtc_index);
 
 /**
  * drm_client_for_each_modeset() - Iterate over client modesets