[RFC PATCH] drm/atomic: add ASYNC_UPDATE flag to the Atomic IOCTL.

[Enric Balletbo i Serra]

From: Gustavo Padovan <gustavo.padovan at collabora.com>

This flag tells core to jump ahead the queued update if the conditions
in drm_atomic_async_check() are met. That means we are only able to do an
async update if no modeset is pending and update for the same plane is
not queued.

It uses the already in place infrastructure for async updates.

It is useful for cursor updates and async PageFlips over the atomic
ioctl, otherwise in some cases updates may be delayed to the point the
user will notice it.

DRM_MODE_ATOMIC_ASYNC_UPDATE should be passed to the Atomic IOCTL to use
this feature.

Signed-off-by: Gustavo Padovan <gustavo.padovan at collabora.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo at collabora.com>
---
Hi,

This is an attempt to introduce the new ASYNC_UPDATE flag for atomic
operations, see the commit message for a more detailed description.

To test this patch we have created an IGT test that we plan to send to
the ML but also was tested using a small program that exercises the uAPI
for easy sanity testing. The program created by Alexandros can be found here
[2]. To test, just build the program and use the --atomic flag to use the
cursor plane in normal (blocking mode), and --atomic-async to use the cursor
plane with the ASYNC_UPDATE flag.E.g.

  drm_cursor --atomic

or

  drm_cursor --atomic-async

The test worked on a Samsung Chromebook Plus on top of mainline plus
the patch to update cursors asynchronously through atomic for the
drm/rockchip driver [3].

Alexandros also did a proof-of-concept to use this flag and draw cursors
using atomic if possible on ozone [1].

Best regards,
 Enric

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1092711
[2] https://gitlab.collabora.com/alf/drm-cursor
[3] https://patchwork.kernel.org/patch/10492693/


 drivers/gpu/drm/drm_atomic.c        | 6 ++++++
 drivers/gpu/drm/drm_atomic_helper.c | 9 ++++++---
 include/uapi/drm/drm_mode.h         | 4 +++-
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index c825c76edc1d..15b799f46982 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -80,6 +80,7 @@ drm_atomic_state_init(struct drm_device *dev, struct drm_atomic_state *state)
 	 * setting this appropriately?
 	 */
 	state->allow_modeset = true;
+	state->async_update = true;
 
 	state->crtcs = kcalloc(dev->mode_config.num_crtc,
 			       sizeof(*state->crtcs), GFP_KERNEL);
@@ -2320,6 +2321,10 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
 			(arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
 		return -EINVAL;
 
+	if ((arg->flags & DRM_MODE_ATOMIC_ALLOW_MODESET) &&
+			(arg->flags & DRM_MODE_ATOMIC_ASYNC_UPDATE))
+		return -EINVAL;
+
 	drm_modeset_acquire_init(&ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE);
 
 	state = drm_atomic_state_alloc(dev);
@@ -2328,6 +2333,7 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
 
 	state->acquire_ctx = &ctx;
 	state->allow_modeset = !!(arg->flags & DRM_MODE_ATOMIC_ALLOW_MODESET);
+	state->async_update = !!(arg->flags & DRM_MODE_ATOMIC_ASYNC_UPDATE);
 
 retry:
 	plane_mask = 0;
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index c35654591c12..aeb0523d3bcf 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -891,7 +891,7 @@ int drm_atomic_helper_check(struct drm_device *dev,
 	if (ret)
 		return ret;
 
-	if (state->legacy_cursor_update)
+	if (state->async_update || state->legacy_cursor_update)
 		state->async_update = !drm_atomic_helper_async_check(dev, state);
 
 	return ret;
@@ -1526,13 +1526,16 @@ int drm_atomic_helper_async_check(struct drm_device *dev,
 	if (new_plane_state->fence)
 		return -EINVAL;
 
+	/* Only do an async update if there is a pending commit. */
+	if (!old_plane_state->commit)
+		return -EINVAL;
+
 	/*
 	 * Don't do an async update if there is an outstanding commit modifying
 	 * the plane.  This prevents our async update's changes from getting
 	 * overridden by a previous synchronous update's state.
 	 */
-	if (old_plane_state->commit &&
-	    !try_wait_for_completion(&old_plane_state->commit->hw_done))
+	if (!try_wait_for_completion(&old_plane_state->commit->hw_done))
 		return -EBUSY;
 
 	return funcs->atomic_async_check(plane, new_plane_state);
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 50bcf4214ff9..772e84f0edeb 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -718,13 +718,15 @@ struct drm_mode_destroy_dumb {
 #define DRM_MODE_ATOMIC_TEST_ONLY 0x0100
 #define DRM_MODE_ATOMIC_NONBLOCK  0x0200
 #define DRM_MODE_ATOMIC_ALLOW_MODESET 0x0400
+#define DRM_MODE_ATOMIC_ASYNC_UPDATE  0x0800
 
 #define DRM_MODE_ATOMIC_FLAGS (\
 		DRM_MODE_PAGE_FLIP_EVENT |\
 		DRM_MODE_PAGE_FLIP_ASYNC |\
 		DRM_MODE_ATOMIC_TEST_ONLY |\
 		DRM_MODE_ATOMIC_NONBLOCK |\
-		DRM_MODE_ATOMIC_ALLOW_MODESET)
+		DRM_MODE_ATOMIC_ALLOW_MODESET |\
+		DRM_MODE_ATOMIC_ASYNC_UPDATE)
 
 struct drm_mode_atomic {
 	__u32 flags;
-- 
2.18.0