[PATCH 6/6] drm/fb-helper: Synchronize dirty worker with vblank

[Thomas Zimmermann]

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.

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 at suse.de>
---
 drivers/gpu/drm/drm_fb_helper.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 937c3939e502..e0b780634a84 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -364,9 +364,29 @@ static void drm_fb_helper_fb_dirty(struct drm_fb_helper *helper)
 	struct drm_device *dev = helper->dev;
 	struct drm_clip_rect *clip = &helper->damage_clip;
 	struct drm_clip_rect clip_copy;
+	struct drm_crtc *crtc;
 	unsigned long flags;
 	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.
+	 */
+	mutex_lock(&helper->lock);
+	if (!drm_master_internal_acquire(helper->dev)) {
+		goto unlock;
+	}
+	crtc = helper->client.modesets[0].crtc;
+	ret = drm_crtc_vblank_get(crtc);
+	if (!ret) {
+		drm_crtc_wait_one_vblank(crtc);
+		drm_crtc_vblank_put(crtc);
+	}
+	drm_master_internal_release(helper->dev);
+unlock:
+	mutex_unlock(&helper->lock);
+
 	if (drm_WARN_ON_ONCE(dev, !helper->funcs->fb_dirty))
 		return;
 
-- 
2.49.0