[PATCH v2] drm/vc4: Return -EBUSY if there's already a pending flip event.

[robert.foss@collabora.com]

From: Robert Foss <robert.foss@collabora.com>

As per the documentation in drm_crtc.h, atomic_commit should return
-EBUSY if an asycnhronous update is requested and there is an earlier
update pending.

Note: docs cited here are drm_crtc.h, and the whole quote is:

    *  - -EBUSY, if an asynchronous updated is requested and there is
    *    an earlier updated pending. Drivers are allowed to support a queue
    *    of outstanding updates, but currently no driver supports that.
    *    Note that drivers must wait for preceding updates to complete if a
    *    synchronous update is requested, they are not allowed to fail the
    *    commit in that case.

Signed-off-by: Robert Foss <robert.foss@collabora.com>
---

Changes since v1:
- Corrected and simplified patch to piggyback on a previously existing check.

 drivers/gpu/drm/vc4/vc4_kms.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_kms.c b/drivers/gpu/drm/vc4/vc4_kms.c
index 4718ae5..7c7188d 100644
--- a/drivers/gpu/drm/vc4/vc4_kms.c
+++ b/drivers/gpu/drm/vc4/vc4_kms.c
@@ -117,10 +117,18 @@ static int vc4_atomic_commit(struct drm_device *dev,
 		return -ENOMEM;
 
 	/* Make sure that any outstanding modesets have finished. */
-	ret = down_interruptible(&vc4->async_modeset);
-	if (ret) {
-		kfree(c);
-		return ret;
+	if (async) {
+		ret = down_trylock(&vc4->async_modeset);
+		if (ret) {
+			kfree(c);
+			return -EBUSY;
+		}
+	} else {
+		ret = down_interruptible(&vc4->async_modeset);
+		if (ret) {
+			kfree(c);
+			return ret;
+		}
 	}
 
 	ret = drm_atomic_helper_prepare_planes(dev, state);
-- 
2.5.0