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:
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 */

/**
 * 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
 *
 * 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.
 *
 * 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
 * --------------------------
 *
 * &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