[PATCH v3 4/6] drm: allow DRM_MODE_PAGE_FLIP_ASYNC for atomic commits

[Simon Ser]

If the driver supports it, allow user-space to supply the
DRM_MODE_PAGE_FLIP_ASYNC flag to request an async page-flip.
Set drm_crtc_state.async_flip accordingly.

Document that drivers will reject atomic commits if an async
flip isn't possible. This allows user-space to fall back to
something else. For instance, Xorg falls back to a blit.
Another option is to wait as close to the next vblank as
possible before performing the page-flip to reduce latency.

v2: document new uAPI

v3: add comment about Intel hardware which needs one last
sync page-flip before being able to switch to async (Ville, Pekka)

Signed-off-by: Simon Ser <contact at emersion.fr>
Co-developed-by: André Almeida <andrealmeid at igalia.com>
Signed-off-by: André Almeida <andrealmeid at igalia.com>
Reviewed-by: André Almeida <andrealmeid at igalia.com>
Reviewed-by: Alex Deucher <alexander.deucher at amd.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Joshua Ashton <joshua at froggi.es>
Cc: Melissa Wen <mwen at igalia.com>
Cc: Harry Wentland <hwentlan at amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/drm_atomic_uapi.c | 28 +++++++++++++++++++++++++---
 include/uapi/drm/drm_mode.h       |  9 +++++++++
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
index 79730fa1dd8e..ee24ed7e2edb 100644
--- a/drivers/gpu/drm/drm_atomic_uapi.c
+++ b/drivers/gpu/drm/drm_atomic_uapi.c
@@ -1278,6 +1278,18 @@ static void complete_signaling(struct drm_device *dev,
 	kfree(fence_state);
 }
 
+static void
+set_async_flip(struct drm_atomic_state *state)
+{
+	struct drm_crtc *crtc;
+	struct drm_crtc_state *crtc_state;
+	int i;
+
+	for_each_new_crtc_in_state(state, crtc, crtc_state, i) {
+		crtc_state->async_flip = true;
+	}
+}
+
 int drm_mode_atomic_ioctl(struct drm_device *dev,
 			  void *data, struct drm_file *file_priv)
 {
@@ -1318,9 +1330,16 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
 	}
 
 	if (arg->flags & DRM_MODE_PAGE_FLIP_ASYNC) {
-		drm_dbg_atomic(dev,
-			       "commit failed: invalid flag DRM_MODE_PAGE_FLIP_ASYNC\n");
-		return -EINVAL;
+		if (!dev->mode_config.async_page_flip) {
+			drm_dbg_atomic(dev,
+				       "commit failed: DRM_MODE_PAGE_FLIP_ASYNC not supported\n");
+			return -EINVAL;
+		}
+		if (dev->mode_config.atomic_async_page_flip_not_supported) {
+			drm_dbg_atomic(dev,
+				       "commit failed: DRM_MODE_PAGE_FLIP_ASYNC not supported with atomic\n");
+			return -EINVAL;
+		}
 	}
 
 	/* can't test and expect an event at the same time. */
@@ -1418,6 +1437,9 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
 	if (ret)
 		goto out;
 
+	if (arg->flags & DRM_MODE_PAGE_FLIP_ASYNC)
+		set_async_flip(state);
+
 	if (arg->flags & DRM_MODE_ATOMIC_TEST_ONLY) {
 		ret = drm_atomic_check_only(state);
 	} else if (arg->flags & DRM_MODE_ATOMIC_NONBLOCK) {
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 86a292c3185a..b39e78117b18 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -942,6 +942,15 @@ struct hdr_output_metadata {
  * Request that the page-flip is performed as soon as possible, ie. with no
  * delay due to waiting for vblank. This may cause tearing to be visible on
  * the screen.
+ *
+ * When used with atomic uAPI, the driver will return an error if the hardware
+ * doesn't support performing an asynchronous page-flip for this update.
+ * User-space should handle this, e.g. by falling back to a regular page-flip.
+ *
+ * Note, some hardware might need to perform one last synchronous page-flip
+ * before being able to switch to asynchronous page-flips. As an exception,
+ * the driver will return success even though that first page-flip is not
+ * asynchronous.
  */
 #define DRM_MODE_PAGE_FLIP_ASYNC 0x02
 #define DRM_MODE_PAGE_FLIP_TARGET_ABSOLUTE 0x4
-- 
2.37.3