[PATCH] drm/client: check whether CRTC is active before waiting for vblank

[Icenowy Zheng]

Currently the implementaion of drm_client_modeset_wait_for_vblank()
assumes drm_vblank_get() will fail when the CRTC isn't active. However
it seems that this is not true, and running fbcon on a device with the
first CRTC inactive will lead to kernel warning in some cases (which
could be reproduced with the loongson driver).

Change the implementation to add a check for the active state (atomic) /
enabled state (non-atomic) before calling drm_vblank_get(). As the
assumption of drm_vblank_get() failing for inactive CRTC isn't met, the
error status of drm_vblank_get() can now be exported too.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: d8c4bddcd8bc ("drm/fb-helper: Synchronize dirty worker with vblank")
Signed-off-by: Icenowy Zheng <zhengxingda@xxxxxxxxxxx>
---
 drivers/gpu/drm/drm_client_modeset.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_client_modeset.c b/drivers/gpu/drm/drm_client_modeset.c
index bb49b8361271a..1b03bf351256e 100644
--- a/drivers/gpu/drm/drm_client_modeset.c
+++ b/drivers/gpu/drm/drm_client_modeset.c
@@ -1310,7 +1310,7 @@ int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned i
 {
 	struct drm_device *dev = client->dev;
 	struct drm_crtc *crtc;
-	int ret;
+	int ret = 0;
 
 	/*
 	 * Rate-limit update frequency to vblank. If there's a DRM master
@@ -1326,15 +1326,24 @@ int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned i
 	 * Only wait for a vblank event if the CRTC is enabled, otherwise
 	 * just don't do anything, not even report an error.
 	 */
+	if (drm_drv_uses_atomic_modeset(dev)) {
+		if (!crtc->state || !crtc->state->active)
+			goto out;
+	} else {
+		if (!crtc->enabled)
+			goto out;
+	}
+
 	ret = drm_crtc_vblank_get(crtc);
 	if (!ret) {
 		drm_crtc_wait_one_vblank(crtc);
 		drm_crtc_vblank_put(crtc);
 	}
 
+out:
 	drm_master_internal_release(dev);
 
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL(drm_client_modeset_wait_for_vblank);
 
-- 
2.52.0