[PATCH v4] drm/fb-helper: Only consider active CRTCs for vblank sync

[PATCH v4] drm/fb-helper: Only consider active CRTCs for vblank sync

[Thomas Zimmermann]

Only synchronize fbdev output to the vblank of an active CRTC. Go over
the list of CRTCs and pick the first that matches. Fixes warnings as
the one shown below

[   77.201354] WARNING: drivers/gpu/drm/drm_vblank.c:1320 at drm_crtc_wait_one_vblank+0x194/0x1cc [drm], CPU#1: kworker/1:7/1867
[   77.201354] omapdrm omapdrm.0: [drm] vblank wait timed out on crtc 0

This currently happens if the fbdev output is not on CRTC 0.

Atomic and non-atomic drivers require distinct code paths. As for other
fbdev operations, implement both and select the correct one at runtime.

Not finding an active CRTC is not a bug. Do not wait in this case, but
flush the display update as before.

v4:
- avoid possible deadlocks with locking context (Sashiko)
v3:
- drop excessive state validation (Jani)
- acquire plane and CRTC mutices (Sashiko)
v2:
- move look-up code into separate helper
- support drivers with legacy modesetting
v1:
- see https://lore.kernel.org/dri-devel/1c9e0e24-9c4a-4259-8700-cf9e5fd60ca3@suse.de/

Co-authored-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: d8c4bddcd8bcb ("drm/fb-helper: Synchronize dirty worker with vblank")
Tested-by: Icenowy Zheng <zhengxingda@iscas.ac.cn>
Tested-by: H. Nikolaus Schaller <hns@goldelico.com>
Closes: https://bugs.debian.org/1138033
---
 drivers/gpu/drm/drm_fb_helper.c | 92 ++++++++++++++++++++++++++++++++-
 1 file changed, 91 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 3316bb00a662..d4664ed468b2 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -225,16 +225,106 @@ static void drm_fb_helper_resume_worker(struct work_struct *work)
 	console_unlock();
 }
 
+static int find_crtc_index_atomic(struct drm_fb_helper *helper)
+{
+	struct drm_device *dev = helper->dev;
+	int crtc_index = -EINVAL;
+	struct drm_modeset_acquire_ctx ctx;
+	struct drm_plane *plane;
+	int ret = 0;
+
+	drm_modeset_acquire_init(&ctx, 0);
+
+retry:
+	drm_for_each_plane(plane, dev) {
+		const struct drm_plane_state *plane_state;
+
+		if (plane->type != DRM_PLANE_TYPE_PRIMARY)
+			continue;
+
+		ret = drm_modeset_lock(&plane->mutex, &ctx);
+		if (ret)
+			goto err_drm_modeset_lock;
+		plane_state = plane->state;
+
+		if (plane_state->fb == helper->fb && plane_state->crtc) {
+			struct drm_crtc *crtc = plane_state->crtc;
+
+			ret = drm_modeset_lock(&crtc->mutex, &ctx);
+			if (ret)
+				goto err_drm_modeset_lock;
+			if (crtc->state->active)
+				crtc_index = crtc->index;
+			drm_modeset_unlock(&crtc->mutex);
+		}
+		drm_modeset_unlock(&plane->mutex);
+
+		if (crtc_index >= 0)
+			break;
+	}
+
+	drm_modeset_drop_locks(&ctx);
+	drm_modeset_acquire_fini(&ctx);
+
+	return crtc_index;
+
+err_drm_modeset_lock:
+	if (ret == -EDEADLK) {
+		drm_modeset_backoff(&ctx);
+		goto retry;
+	}
+	return ret;
+}
+
+static int find_crtc_index_legacy(struct drm_fb_helper *helper)
+{
+	struct drm_device *dev = helper->dev;
+	struct drm_crtc *crtc;
+
+	drm_for_each_crtc(crtc, dev) {
+		struct drm_plane *plane = crtc->primary;
+
+		if (!crtc->enabled)
+			continue;
+		if (!plane || plane->fb != helper->fb)
+			continue; /* CRTC doesn't display fbdev emulation */
+
+		return crtc->index;
+	}
+
+	return -EINVAL;
+}
+
+static int drm_fb_helper_find_crtc_index(struct drm_fb_helper *helper)
+{
+	struct drm_device *dev = helper->dev;
+	int crtc_index;
+
+	mutex_lock(&dev->mode_config.mutex);
+
+	if (drm_drv_uses_atomic_modeset(dev))
+		crtc_index = find_crtc_index_atomic(helper);
+	else
+		crtc_index = find_crtc_index_legacy(helper);
+
+	mutex_unlock(&dev->mode_config.mutex);
+
+	return crtc_index;
+}
+
 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;
+	int crtc_index;
 	unsigned long flags;
 	int ret;
 
 	mutex_lock(&helper->lock);
-	drm_client_modeset_wait_for_vblank(&helper->client, 0);
+	crtc_index = drm_fb_helper_find_crtc_index(helper);
+	if (crtc_index >= 0)
+		drm_client_modeset_wait_for_vblank(&helper->client, crtc_index);
 	mutex_unlock(&helper->lock);
 
 	if (drm_WARN_ON_ONCE(dev, !helper->funcs->fb_dirty))
-- 
2.54.0

Re: [PATCH v4] drm/fb-helper: Only consider active CRTCs for vblank sync

[Maxime Ripard]

On Thu, 2 Jul 2026 16:50:13 +0200, Thomas Zimmermann wrote:
> Only synchronize fbdev output to the vblank of an active CRTC. Go over
> the list of CRTCs and pick the first that matches. Fixes warnings as
> the one shown below
> 
> [   77.201354] WARNING: drivers/gpu/drm/drm_vblank.c:1320 at drm_crtc_wait_one_vblank+0x194/0x1cc [drm], CPU#1: kworker/1:7/1867
> 
> [ ... ]

Acked-by: Maxime Ripard <mripard@kernel.org>

Thanks!
Maxime