[PATCH] drm/syncobj: Add better overview documentation for syncobj

[Jason Ekstrand]

This patch only brings the syncobj documentation up-to-date for the
original form of syncobj.  It does not contain any information about the
design of timeline syncobjs.
---
 drivers/gpu/drm/drm_syncobj.c | 81 +++++++++++++++++++++++++++++++----
 1 file changed, 73 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 1438dcb3ebb1..03cc888744fb 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -29,17 +29,82 @@
 /**
  * DOC: Overview
  *
- * DRM synchronisation objects (syncobj, see struct &drm_syncobj) are
- * persistent objects that contain an optional fence. The fence can be updated
- * with a new fence, or be NULL.
+ * DRM synchronisation objects (syncobj, see struct &drm_syncobj) provide a
+ * synchronization primitive which can be used by userspace to explicitly
+ * synchronize GPU commands, can be shared between userspace processes, and
+ * can be shared between different DRM drivers.
+ * Their primary use-case is to implement Vulkan fences and semaphores.
+ * The syncobj userspace API provides ioctls for several operations:
  *
- * syncobj's can be waited upon, where it will wait for the underlying
- * fence.
+ *  - Creation and destruction of syncobjs
+ *  - Import and export of syncobjs to/from a syncobj file descriptor
+ *  - Import and export a syncobj's underlying fence to/from a sync file
+ *  - Reset a syncobj (set its fence to NULL)
+ *  - Signal a syncobj (set a trivially signaled fence)
+ *  - Wait a syncobj's fence to be signaled
  *
- * syncobj's can be export to fd's and back, these fd's are opaque and
- * have no other use case, except passing the syncobj between processes.
+ * At it's core, a syncobj simply a wrapper around a pointer to a struct
+ * &dma_fence which may be NULL.
+ * When GPU work which signals a syncobj is enqueued in a DRM driver,
+ * the syncobj fence is replaced with a fence which will be signaled by the
+ * completion of that work.
+ * When GPU work which waits on a syncobj is enqueued in a DRM driver, the
+ * driver retrieves syncobj's current fence at the time the work is enqueued
+ * waits on that fence before submitting the work to hardware.
+ * If the syncobj's fence is NULL, the enqueue operation is expected to fail.
+ * All manipulation of the syncobjs's fence happens in terms of the current
+ * fence at the time the ioctl is called by userspace regardless of whether
+ * that operation is an immediate host-side operation (signal or reset) or
+ * or an operation which is enqueued in some driver queue.
  *
- * Their primary use-case is to implement Vulkan fences and semaphores.
+ *
+ * Host-side wait on syncobjs
+ * --------------------------
+ *
+ * The userspace syncobj API provides an ioctl for waiting on a set of
+ * syncobjs.
+ * The wait ioctl takes an array of syncobj handles and a flag indicating
+ * whether it should return immediately once one syncobj has been signaled
+ * or if it should wait for all the syncobjs to be signaled.
+ * Unlike the enqueued GPU work dependencies, the host-side wait ioctl
+ * will also optionally wait for the syncobj to first receive a fence and
+ * then wait on that fence.
+ * Assuming the syncobj starts off with a NULL fence, this allows a client
+ * to do a host wait in one thread (or process) which waits on GPU work
+ * submitted in another thread (or process) without having to manually
+ * synchronize between the two.
+ * This requirement is inherited from the Vulkan fence API.
+ *
+ *
+ * Import/export of syncobjs
+ * -------------------------
+ *
+ * The userspace syncobj API provides two mechanisms for import/export of
+ * syncobjs.
+ *
+ * The first lets the client import or export an entire syncobj to a file
+ * descriptor.
+ * These fd's are opaque and have no other use case, except passing the
+ * syncobj between processes.
+ * All syncobj handles which are created as the result of such an import
+ * operation refer to the same underlying syncobj as was used for the
+ * export and the syncobj can be used persistently across all the processes
+ * with which it is shared.
+ * Unlike dma-buf, importing a syncobj creates a new handle for every
+ * import instead of de-duplicating.
+ * The primary use-case of this persistent import/export is for shared
+ * Vulkan fences and semaphores.
+ *
+ * The second import/export mechanism lets the client export the syncobj's
+ * current fence to/from a &sync_file.
+ * When a syncobj is exported to a sync file, that sync file wraps the
+ * sycnobj's fence at the time of export and any later signal or reset
+ * operations on the syncobj will not affect the exported sync file.
+ * When a sync file is imported into a syncobj, the syncobj's fence is set
+ * to the fence wrapped by that sync file.
+ * Because sync files are immutable, resetting or signaling the syncobj
+ * will not affect any sync files whose fences have been imported into the
+ * syncobj.
  *
  * syncobj have a kref reference count, but also have an optional file.
  * The file is only created once the syncobj is exported.
-- 
2.21.0