path: root/drivers/gpu/drm/drm_syncobj.c

/*
 * Copyright 2017 Red Hat
 * Parts ported from amdgpu (fence wait code).
 * Copyright 2016 Advanced Micro Devices, Inc.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 * Authors:
 *
 */

/**
 * DOC: Overview
 *
 * 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:
 *
 *  - 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
 *
 * 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.
 *
 *
 * 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