[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 <tzimmerm...@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