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authorGustavo Padovan <gustavo.padovan@collabora.co.uk>2016-11-22 09:11:28 +0900
committerDaniel Vetter <daniel.vetter@ffwll.ch>2016-11-22 08:06:36 +0100
commit9a83a71ac0d57ceffa32cbbb438ded055ab77887 (patch)
tree258fda86655d4347bcb3d24170e1b215153f7544
parent8c0b55e22aff84cb6938a993d86c3ce02006236e (diff)
drm/fences: add DOC: for explicit fencing
Document IN_FENCE_FD and OUT_FENCE_PTR properties. v2: incorporate comments from Daniel Vetter Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk> [danvet: s/async/nonblocking/ atomic commits.] Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: http://patchwork.freedesktop.org/patch/msgid/1479773488-15048-1-git-send-email-gustavo@padovan.org
-rw-r--r--Documentation/gpu/drm-kms.rst6
-rw-r--r--drivers/gpu/drm/drm_atomic.c52
2 files changed, 58 insertions, 0 deletions
diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index f19757b..0ef21076 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -293,6 +293,12 @@ Tile Group Property
.. kernel-doc:: drivers/gpu/drm/drm_connector.c
:doc: Tile group
+Explicit Fencing Properties
+---------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_atomic.c
+ :doc: explicit fencing properties
+
Existing KMS Properties
-----------------------
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index ccbffaf..89737e4 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -1816,6 +1816,58 @@ void drm_atomic_clean_old_fb(struct drm_device *dev,
}
EXPORT_SYMBOL(drm_atomic_clean_old_fb);
+/**
+ * DOC: explicit fencing properties
+ *
+ * Explicit fencing allows userspace to control the buffer synchronization
+ * between devices. A Fence or a group of fences are transfered to/from
+ * userspace using Sync File fds and there are two DRM properties for that.
+ * IN_FENCE_FD on each DRM Plane to send fences to the kernel and
+ * OUT_FENCE_PTR on each DRM CRTC to receive fences from the kernel.
+ *
+ * As a contrast, with implicit fencing the kernel keeps track of any
+ * ongoing rendering, and automatically ensures that the atomic update waits
+ * for any pending rendering to complete. For shared buffers represented with
+ * a struct &dma_buf this is tracked in &reservation_object structures.
+ * Implicit syncing is how Linux traditionally worked (e.g. DRI2/3 on X.org),
+ * whereas explicit fencing is what Android wants.
+ *
+ * "IN_FENCE_FD”:
+ * Use this property to pass a fence that DRM should wait on before
+ * proceeding with the Atomic Commit request and show the framebuffer for
+ * the plane on the screen. The fence can be either a normal fence or a
+ * merged one, the sync_file framework will handle both cases and use a
+ * fence_array if a merged fence is received. Passing -1 here means no
+ * fences to wait on.
+ *
+ * If the Atomic Commit request has the DRM_MODE_ATOMIC_TEST_ONLY flag
+ * it will only check if the Sync File is a valid one.
+ *
+ * On the driver side the fence is stored on the @fence parameter of
+ * struct &drm_plane_state. Drivers which also support implicit fencing
+ * should set the implicit fence using drm_atomic_set_fence_for_plane(),
+ * to make sure there's consistent behaviour between drivers in precedence
+ * of implicit vs. explicit fencing.
+ *
+ * "OUT_FENCE_PTR”:
+ * Use this property to pass a file descriptor pointer to DRM. Once the
+ * Atomic Commit request call returns OUT_FENCE_PTR will be filled with
+ * the file descriptor number of a Sync File. This Sync File contains the
+ * CRTC fence that will be signaled when all framebuffers present on the
+ * Atomic Commit * request for that given CRTC are scanned out on the
+ * screen.
+ *
+ * The Atomic Commit request fails if a invalid pointer is passed. If the
+ * Atomic Commit request fails for any other reason the out fence fd
+ * returned will be -1. On a Atomic Commit with the
+ * DRM_MODE_ATOMIC_TEST_ONLY flag the out fence will also be set to -1.
+ *
+ * Note that out-fences don't have a special interface to drivers and are
+ * internally represented by a struct &drm_pending_vblank_event in struct
+ * &drm_crtc_state, which is also used by the nonblocking atomic commit
+ * helpers and for the DRM event handling for existing userspace.
+ */
+
static struct dma_fence *get_crtc_fence(struct drm_crtc *crtc)
{
struct dma_fence *fence;