Hi
Am 22.06.26 um 15:34 schrieb Jani Nikula:
On Mon, 22 Jun 2026, Thomas Zimmermann <[email protected]> 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
[ 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.
v2:
- move look-up code into separate helper
- support drivers with legacy modesetting
v1:
- see
https://lore.kernel.org/dri-devel/[email protected]/
Co-authored-by: H. Nikolaus Schaller <[email protected]>
Signed-off-by: Thomas Zimmermann <[email protected]>
Fixes: d8c4bddcd8bcb ("drm/fb-helper: Synchronize dirty worker with vblank")
Tested-by: Icenowy Zheng <[email protected]>
Closes: https://bugs.debian.org/1138033
Cc: <[email protected]> # v6.19+
---
drivers/gpu/drm/drm_fb_helper.c | 71 ++++++++++++++++++++++++++++++++-
1 file changed, 70 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 7b11a582f8ec..cbf0a9a7b8e5 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -225,16 +225,85 @@ 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;
+ struct drm_plane *plane;
+
+ drm_for_each_plane(plane, dev) {
+ const struct drm_plane_state *plane_state;
+ const struct drm_crtc *crtc;
+
+ if (plane->type != DRM_PLANE_TYPE_PRIMARY)
+ continue;
+
+ plane_state = plane->state;
+ if (plane_state->fb != helper->fb || !plane_state->crtc)
+ continue; /* plane doesn't display fbdev emulation */
+
+ crtc = plane_state->crtc;
+ if (!crtc->state->active)
+ continue;
+ if (drm_WARN_ON_ONCE(dev, crtc->index > INT_MAX))
+ continue; /* driver bug */
I take it this is here because crtc->index is unsigned, and this
function returns int so you can have negative error codes. Ditto the
other function below.
I feel like this is something that should be checked once somewhere, if
that. I think adding arbitrary checks like this invites more arbitrary
checks everywhere. crtc->index is supposed to be invariant over the
lifetime of the CRTC.
Ok, makes sense.
Best regards
Thomas
BR,
Jani.
+
+ return crtc->index;
+ }
+
+ return -EINVAL;
+}
+
+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 */
+ if (drm_WARN_ON_ONCE(dev, crtc->index > INT_MAX))
+ continue; /* driver bug */
+
+ 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))
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)
