[PATCH] drm/fb_helper: Determine proper crtc_index within drm_fb_helper_fb_dirty

[H. Nikolaus Schaller]

The call to drm_client_modeset_wait_for_vblank() currently hardcodes the
crtc_index to 0. This is incorrect when CRTC 0 is inactive and not
generating vblank interrupts.

When this occurs, the console is flooded with vblank timeout warnings:

[   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 behavior was observed on a PandaBoard-ES when only the HDMI port
(CRTC 1) is connected, while the LCD (CRTC 0) and DVI port (CRTC 2)
remain disconnected. The root cause is the hardcoded crtc_index = 0.

Fix this by locating the matching CRTCs. If multiple CRTCs match, the
last active one is selected. If no matching CRTC is found, the vblank
wait is skipped entirely.

Fixes: d8c4bddcd8bcb ("drm/fb-helper: Synchronize dirty worker with vblank")
Signed-off-by: H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/drm_fb_helper.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 1541fc8a9ac2d..70999cdc98b32 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -232,9 +232,23 @@ static void drm_fb_helper_fb_dirty(struct drm_fb_helper *helper)
 	struct drm_clip_rect clip_copy;
 	unsigned long flags;
 	int ret;
+	unsigned int crtc_index = 0;
+	struct drm_crtc *crtc;
 
 	mutex_lock(&helper->lock);
-	drm_client_modeset_wait_for_vblank(&helper->client, 0);
+
+	drm_for_each_crtc(crtc, dev) {
+		if (crtc->primary && crtc->primary->state &&
+		    crtc->primary->state->fb == helper->fb &&
+		    crtc->state && crtc->state->active) {
+			crtc_index = drm_crtc_index(crtc) + 1;
+			break;
+		}
+	}
+
+	if (crtc_index > 0)
+		drm_client_modeset_wait_for_vblank(&helper->client, crtc_index - 1);
+
 	mutex_unlock(&helper->lock);
 
 	if (drm_WARN_ON_ONCE(dev, !helper->funcs->fb_dirty))
-- 
2.50.1 (Apple Git-155)