[RFC PATCH] Documentation/gpu: Add a VM_BIND async draft document.

[Thomas Hellström]

Add a motivation for and description of asynchronous VM_BIND operation

Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
---
 Documentation/gpu/drm-vm-bind-async.rst | 138 ++++++++++++++++++++++++
 1 file changed, 138 insertions(+)
 create mode 100644 Documentation/gpu/drm-vm-bind-async.rst

diff --git a/Documentation/gpu/drm-vm-bind-async.rst b/Documentation/gpu/drm-vm-bind-async.rst
new file mode 100644
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+====================
+Asynchronous VM_BIND
+====================
+
+Nomenclature:
+=============
+
+* VRAM: On-device memory. Sometimes referred to as device local memory.
+
+* vm: A GPU address space. Typically per process, but can be shared by
+  multiple processes.
+
+* VM_BIND: An operation or a list of operations to modify a vm using
+  an IOCTL. The operations include mapping and unmapping system- or
+  VRAM memory.
+
+* syncobj: A container that abstracts synchronization objects. The
+  synchronization objects can be either generic, like dma-fences or
+  driver specific. A syncobj typically indicates the type of the
+  underlying synchronization object.
+
+* in-syncobj: Argument to a VM_BIND IOCTL, the VM_BIND operation waits
+  for these before starting.
+
+* out-syncbj: Argument to a VM_BIND_IOCTL, the VM_BIND operation
+  signals these when the bind operation is complete.
+
+* memory fence: A synchronization object, different from a dma-fence
+  that uses the value of a specified memory location to determine
+  signaled status. A memory fence can be awaited and signaled by both
+  the GPU and CPU. Memory fences are sometimes referred to as
+  user-fences.
+
+* long-running workload: A workload that may take more than the
+  current stipulated dma-fence maximum signal delay to complete and
+  which therefore needs to set the VM or the GPU execution context in
+  a certain mode that disallows completion dma-fences.
+
+* UMD: User-mode driver.
+
+* KMD: Kernel-mode driver.
+
+
+Synchronous / Asynchronous VM_BIND operation
+============================================
+
+Synchronous VM_BIND
+___________________
+With Synchronous VM_BIND, the VM_BIND operations all complete before the
+ioctl returns. A synchronous VM_BIND takes neither in-fences nor
+out-fences. Synchronous VM_BIND may block and wait for GPU operations;
+for example swapin or clearing, or even previous binds.
+
+Asynchronous VM_BIND
+____________________
+Asynchronous VM_BIND accepts both in-syncobjs and out-syncobjs. While the
+IOCTL may return immediately, the VM_BIND operations wait for the in-syncobjs
+before modifying the GPU page-tables, and signal the out-syncobjs when
+the modification is done in the sense that the next execbuf that
+awaits for the out-syncobjs will see the change. Errors are reported
+synchronously assuming that the asynchronous part of the job never errors.
+In low-memory situations the implementation may block, performing the
+VM_BIND synchronously, because there might not be enough memory
+immediately available for preparing the asynchronous operation.
+
+If the VM_BIND IOCTL takes a list or an array of operations as an argument,
+the in-syncobjs needs to signal before the first operation starts to
+execute, and the out-syncobjs signal after the last operation
+completes. Operations in the operation list can be assumed, where it
+matters, to complete in order.
+
+To aid in supporting user-space queues, the VM_BIND may take a bind context
+AKA bind engine identifier argument. All VM_BIND operations using the same
+bind engine can then be assumed, where it matters, to complete in
+order. No such assumptions can be made between VM_BIND operations
+using separate bind contexts.
+
+The purpose of an Asynchronous VM_BIND operation is for user-mode
+drivers to be able to pipeline interleaved vm modifications and
+execbufs. For long-running workloads, such pipelining of a bind
+operation is not allowed and any in-fences need to be awaited
+synchronously.
+
+Also for VM_BINDS for long-running VMs the user-mode driver should typically
+select memory fences as out-fences since that gives greater flexibility for
+the kernel mode driver to inject other  operations into the bind /
+unbind operations. Like for example inserting breakpoints into batch
+buffers. The workload execution can then easily be pipelined behind
+the bind completion using the memory out-fence as the signal condition
+for a gpu semaphore embedded by UMD in the workload.
+
+Multi-operation VM_BIND IOCTL error handling and interrupts
+========================================
+
+The VM_BIND operations of the ioctl may error due to lack of resources
+to complete and also due to interrupted waits. In both situations UMD
+should preferrably restart the IOCTL after taking suitable action. If
+UMD has overcommited a memory resource, an -ENOSPC error will be
+returned, and UMD may then unbind resources that are not used at the
+moment and restart the IOCTL. On -EINTR, UMD should simply restart the
+IOCTL and on -ENOMEM user-space may either attempt to free known
+system memory resources or abort the operation. If aborting as a
+result of a failed operation in a list of operations, some operations
+may still have completed, and to get back to a known state, user-space
+should therefore attempt to unbind all virtual memory regions touched
+by the failing IOCTL.
+Unbind operations are guaranteed not to cause any errors due to
+resource constraints.
+In between a failed VM_BIND ioctl and a successful restart there may
+be implementation defined restrictions on the use of the VM. For a
+description why, please see KMD implementation details under [error
+state saving]_.
+
+
+KMD implementation details
+==========================
+
+.. [error state saving] Open: When the VM_BIND ioctl returns an error, some
+			or even parts of an operation may have been
+			completed. If the ioctl is restarted, in order
+			to know where to restart, the KMD can
+			either put the VM in an error state and save
+			one instance of the needed restart state
+			internally. In this case, KMD needs to block
+			further modifications of the VM state that may
+			cause additional failures requiring a restart
+			state save, until the error has been fully resolved.
+			If the uAPI instead defines a pointer to a
+			UMD allocated cookie in the IOCTL struct, it
+			could also choose to store the restart state
+			in that cookie.
+
+			The restart state may, for example, be the
+			number of successfully completed operations.
+
+			Easiest for UMD would of course be if KMD did
+			a full unwind on error so that no error state
+			needs to be saved.
-- 
2.39.2