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.