linux/drivers/gpu/drm/xe/xe_exec.c

// SPDX-License-Identifier: MIT
/*
 * Copyright © 2022 Intel Corporation
 */

#include "xe_exec.h"

#include <drm/drm_device.h>
#include <drm/drm_exec.h>
#include <drm/drm_file.h>
#include <drm/xe_drm.h>
#include <linux/delay.h>

#include "xe_bo.h"
#include "xe_device.h"
#include "xe_exec_queue.h"
#include "xe_macros.h"
#include "xe_ring_ops_types.h"
#include "xe_sched_job.h"
#include "xe_sync.h"
#include "xe_vm.h"

/**
 * DOC: Execbuf (User GPU command submission)
 *
 * Execs have historically been rather complicated in DRM drivers (at least in
 * the i915) because a few things:
 *
 * - Passing in a list BO which are read / written to creating implicit syncs
 * - Binding at exec time
 * - Flow controlling the ring at exec time
 *
 * In XE we avoid all of this complication by not allowing a BO list to be
 * passed into an exec, using the dma-buf implicit sync uAPI, have binds as
 * seperate operations, and using the DRM scheduler to flow control the ring.
 * Let's deep dive on each of these.
 *
 * We can get away from a BO list by forcing the user to use in / out fences on
 * every exec rather than the kernel tracking dependencies of BO (e.g. if the
 * user knows an exec writes to a BO and reads from the BO in the next exec, it
 * is the user's responsibility to pass in / out fence between the two execs).
 *
 * Implicit dependencies for external BOs are handled by using the dma-buf
 * implicit dependency uAPI (TODO: add link). To make this works each exec must
 * install the job's fence into the DMA_RESV_USAGE_WRITE slot of every external
 * BO mapped in the VM.
 *
 * We do not allow a user to trigger a bind at exec time rather we have a VM
 * bind IOCTL which uses the same in / out fence interface as exec. In that
 * sense, a VM bind is basically the same operation as an exec from the user
 * perspective. e.g. If an exec depends on a VM bind use the in / out fence
 * interface (struct drm_xe_sync) to synchronize like syncing between two
 * dependent execs.
 *
 * Although a user cannot trigger a bind, we still have to rebind userptrs in
 * the VM that have been invalidated since the last exec, likewise we also have
 * to rebind BOs that have been evicted by the kernel. We schedule these rebinds
 * behind any pending kernel operations on any external BOs in VM or any BOs
 * private to the VM. This is accomplished by the rebinds waiting on BOs
 * DMA_RESV_USAGE_KERNEL slot (kernel ops) and kernel ops waiting on all BOs
 * slots (inflight execs are in the DMA_RESV_USAGE_BOOKING for private BOs and
 * in DMA_RESV_USAGE_WRITE for external BOs).
 *
 * Rebinds / dma-resv usage applies to non-compute mode VMs only as for compute
 * mode VMs we use preempt fences and a rebind worker (TODO: add link).
 *
 * There is no need to flow control the ring in the exec as we write the ring at
 * submission time and set the DRM scheduler max job limit SIZE_OF_RING /
 * MAX_JOB_SIZE. The DRM scheduler will then hold all jobs until space in the
 * ring is available.
 *
 * All of this results in a rather simple exec implementation.
 *
 * Flow
 * ~~~~
 *
 * .. code-block::
 *
 *	Parse input arguments
 *	Wait for any async VM bind passed as in-fences to start
 *	<----------------------------------------------------------------------|
 *	Lock global VM lock in read mode                                       |
 *	Pin userptrs (also finds userptr invalidated since last exec)          |
 *	Lock exec (VM dma-resv lock, external BOs dma-resv locks)              |
 *	Validate BOs that have been evicted                                    |
 *	Create job                                                             |
 *	Rebind invalidated userptrs + evicted BOs (non-compute-mode)           |
 *	Add rebind fence dependency to job                                     |
 *	Add job VM dma-resv bookkeeping slot (non-compute mode)                |
 *	Add job to external BOs dma-resv write slots (non-compute mode)        |
 *	Check if any userptrs invalidated since pin ------ Drop locks ---------|
 *	Install in / out fences for job
 *	Submit job
 *	Unlock all
 */

/*
 * Add validation and rebinding to the drm_exec locking loop, since both can
 * trigger eviction which may require sleeping dma_resv locks.
 */
static int xe_exec_fn(struct drm_gpuvm_exec *vm_exec)
{ … }

int xe_exec_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
{ … }