drm/syncobj: Add better overview documentation for syncobj (v2)

/**

 * 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>