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. v2: Incorporate feedback from Lionel and Christian: - Mention actual ioctl and flag names - Better language around reference counting - Misc. language cleanups Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx> --- drivers/gpu/drm/drm_syncobj.c | 98 +++++++++++++++++++++++++++++++---- 1 file changed, 87 insertions(+), 11 deletions(-) diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c index 1438dcb3ebb1..4b5c7b0ed714 100644 --- a/drivers/gpu/drm/drm_syncobj.c +++ b/drivers/gpu/drm/drm_syncobj.c @@ -29,21 +29,97 @@ /** * 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 + * container for 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 for a syncobj's fence to appear and 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 is simply a wrapper around a pointer to a struct + * &dma_fence which may be NULL. + * When a syncobj is first created, its pointer is either NULL or a pointer + * to an already signaled fence depending on whether the + * &DRM_SYNCOBJ_CREATE_SIGNALED flag is passed to + * &DRM_IOCTL_SYNCOBJ_CREATE. + * 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. + * &DRM_IOCTL_SYNCOBJ_RESET and &DRM_IOCTL_SYNCOBJ_SIGNAL can be used to + * manipulate a syncobj from the host by resetting its pointer to NULL or + * setting its pointer to a fence which is already signaled. * - * Their primary use-case is to implement Vulkan fences and semaphores. * - * syncobj have a kref reference count, but also have an optional file. - * The file is only created once the syncobj is exported. - * The file takes a reference on the kref. + * Host-side wait on syncobjs + * -------------------------- + * + * &DRM_IOCTL_SYNCOBJ_WAIT takes an array of syncobj handles and does a + * host-side wait on all of the syncobj fences simultaneously. + * If &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_ALL is set, the wait ioctl will wait on + * all of the syncobj fences to be signaled before it returns. + * Otherwise, it returns once at least one syncobj fence has been signaled + * and the index of a signaled fence is written back to the client. + * + * Unlike the enqueued GPU work dependencies which fail if they see a NULL + * fence in a syncobj, if &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT is set, + * the host-side wait will first wait for the syncobj to receive a non-NULL + * fence and then wait on that fence. + * If &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT is not set and any one of the + * syncobjs in the array has a NULL fence, -EINVAL will be returned. + * 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 + * ------------------------- + * + * &DRM_IOCTL_SYNCOBJ_FD_TO_HANDLE and &DRM_IOCTL_SYNCOBJ_HANDLE_TO_FD + * provide 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 exported file descriptors and any syncobj handles created as a + * result of importing those file descriptors own a reference to the + * same underlying struct &drm_syncobj and the syncobj can be used + * persistently across all the processes with which it is shared. + * The syncobj is freed only once the last reference is dropped. + * Unlike dma-buf, importing a syncobj creates a new handle (with its own + * reference) 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, which is indicated by + * &DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_IMPORT_SYNC_FILE or + * &DRM_SYNCOBJ_HANDLE_TO_FD_FLAGS_EXPORT_SYNC_FILE lets the client + * import/export the syncobj's current fence from/to 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. */ #include <linux/anon_inodes.h> -- 2.21.0 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
