drm/syncobj: Add documentation for timeline syncobj

 *  - Signal a syncobj (set a trivially signaled fence)

 *  - Wait for a syncobj's fence to appear and be signaled

 *

 * The syncobj userspace API also provides operations to manipulate a syncobj

 * in terms of a timeline of struct &dma_fence_chain rather than a single

 * struct &dma_fence, through the following operations:

 *

 *   - Signal a given point on the timeline

 *   - Wait for a given point to appear and/or be signaled

 *   - Import and export from/to a given point of a timeline

 *

 * 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

 *

 * If the syncobj is considered as a binary (its state is either signaled or

 * unsignaled) primitive, when GPU work is enqueued in a DRM driver to signal

 * the syncobj, the syncobj's fence is replaced with a fence which will be

 * signaled by the completion of that work.

 * If the syncobj is considered as a timeline primitive, when GPU work is

 * enqueued in a DRM driver to signal the a given point of the syncobj, a new

 * struct &dma_fence_chain pointing to the DRM driver's fence and also

 * pointing to the previous fence that was in the syncobj. The new struct

 * &dma_fence_chain fence replace the syncobj's fence and will be signaled by

 * completion of the DRM driver's work and also any work associated with the

 * fence previously in the syncobj.

 *

 * When GPU work which waits on a syncobj is enqueued in a DRM driver, at the

 * time the work is enqueued, it waits on the syncobj's fence before

 * submitting the work to hardware. That fence is either :

 *

 *    - The syncobj's current fence if the syncobj is considered as a binary

 *      primitive.

 *    - The struct &dma_fence associated with a given point if the syncobj is

 *      considered as a timeline primitive.

 *

 * If the syncobj's fence is NULL or not present in the syncobj's timeline,

 * the enqueue operation is expected to fail.

 *

 * With binary syncobj, 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.

 *

 * With a timeline syncobj, all manipulation of the synobj's fence happens in

 * terms of a u64 value referring to point in the timeline. See

 * dma_fence_chain_find_seqno() to see how a given point is found in the

 * timeline.

 *

 * Note that applications should be careful to always use timeline set of

 * ioctl() when dealing with syncobj considered as timeline. Using a binary

 * set of ioctl() with a syncobj considered as timeline could result incorrect

 * synchronization. The use of binary syncobj is supported through the

 * timeline set of ioctl() by using a point value of 0, this will reproduce

 * the behavior of the binary set of ioctl() (for example replace the

 * syncobj's fence when signaling).

 *

 *

 * Host-side wait on syncobjs

 * --------------------------

 * synchronize between the two.

 * This requirement is inherited from the Vulkan fence API.

 *

 * Similarly, &DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT takes an array of syncobj

 * handles as well as an array of u64 points and does a host-side wait on all

 * of syncobj fences at the given points simultaneously.

 *

 * &DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT also adds the ability to wait for a given

 * fence to materialize on the timeline without waiting for the fence to be

 * signaled by using the &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE flag. This

 * requirement is inherited from the wait-before-signal behavior required by

 * the Vulkan timeline semaphore API.

 *

 *

 * Import/export of syncobjs

 * -------------------------

 * Because sync files are immutable, resetting or signaling the syncobj

 * will not affect any sync files whose fences have been imported into the

 * syncobj.

 *

 *

 * Import/export of timeline points in timeline syncobjs

 * -----------------------------------------------------

 *

 * &DRM_IOCTL_SYNCOBJ_TRANSFER provides a mechanism to transfer a struct

 * &dma_fence_chain of a syncobj at a given u64 point to another u64 point

 * into another syncobj.

 *

 * Note that if you want to transfer a struct &dma_fence_chain from a given

 * point on a timeline syncobj from/into a binary syncobj, you can use the

 * point 0 to mean take/replace the fence in the syncobj.

 */

#include <linux/anon_inodes.h>