linux-next/Documentation/driver-api/dma-buf.rst

Buffer Sharing and Synchronization
==================================

The dma-buf subsystem provides the framework for sharing buffers for
hardware (DMA) access across multiple device drivers and subsystems, and
for synchronizing asynchronous hardware access.

This is used, for example, by drm "prime" multi-GPU support, but is of
course not limited to GPU use cases.

The three main components of this are: (1) dma-buf, representing a
sg_table and exposed to userspace as a file descriptor to allow passing
between devices, (2) fence, which provides a mechanism to signal when
one device has finished access, and (3) reservation, which manages the
shared or exclusive fence(s) associated with the buffer.

Shared DMA Buffers
------------------

This document serves as a guide to device-driver writers on what is the dma-buf
buffer sharing API, how to use it for exporting and using shared buffers.

Any device driver which wishes to be a part of DMA buffer sharing, can do so as
either the 'exporter' of buffers, or the 'user' or 'importer' of buffers.

Say a driver A wants to use buffers created by driver B, then we call B as the
exporter, and A as buffer-user/importer.

The exporter

 - implements and manages operations in :c:type:`struct dma_buf_ops
   <dma_buf_ops>` for the buffer,
 - allows other users to share the buffer by using dma_buf sharing APIs,
 - manages the details of buffer allocation, wrapped in a :c:type:`struct
   dma_buf <dma_buf>`,
 - decides about the actual backing storage where this allocation happens,
 - and takes care of any migration of scatterlist - for all (shared) users of
   this buffer.

The buffer-user

 - is one of (many) sharing users of the buffer.
 - doesn't need to worry about how the buffer is allocated, or where.
 - and needs a mechanism to get access to the scatterlist that makes up this
   buffer in memory, mapped into its own address space, so it can access the
   same area of memory. This interface is provided by :c:type:`struct
   dma_buf_attachment <dma_buf_attachment>`.

Any exporters or users of the dma-buf buffer sharing framework must have a
'select DMA_SHARED_BUFFER' in their respective Kconfigs.

Userspace Interface Notes
~~~~~~~~~~~~~~~~~~~~~~~~~

Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
and hence the generic interface exposed is very minimal. There's a few things to
consider though:

- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
  llseek operation will report -EINVAL.

  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
  cases. Userspace can use this to detect support for discovering the dma-buf
  size using llseek.

- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
  on the file descriptor.  This is not just a resource leak, but a
  potential security hole.  It could give the newly exec'd application
  access to buffers, via the leaked fd, to which it should otherwise
  not be permitted access.

  The problem with doing this via a separate fcntl() call, versus doing it
  atomically when the fd is created, is that this is inherently racy in a
  multi-threaded app[3].  The issue is made worse when it is library code
  opening/creating the file descriptor, as the application may not even be
  aware of the fd's.

  To avoid this problem, userspace must have a way to request O_CLOEXEC
  flag be set when the dma-buf fd is created.  So any API provided by
  the exporting driver to create a dmabuf fd must provide a way to let
  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().

- Memory mapping the contents of the DMA buffer is also supported. See the
  discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.

- The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for
  details.

Basic Operation and Device DMA Access
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-buf.c
   :doc: dma buf device access

CPU Access to DMA Buffer Objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-buf.c
   :doc: cpu access

Implicit Fence Poll Support
~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-buf.c
   :doc: implicit fence polling

Kernel Functions and Structures Reference
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-buf.c
   :export:

.. kernel-doc:: include/linux/dma-buf.h
   :internal:

Reservation Objects
-------------------

.. kernel-doc:: drivers/dma-buf/dma-resv.c
   :doc: Reservation Object Overview

.. kernel-doc:: drivers/dma-buf/dma-resv.c
   :export:

.. kernel-doc:: include/linux/dma-resv.h
   :internal:

DMA Fences
----------

.. kernel-doc:: drivers/dma-buf/dma-fence.c
   :doc: DMA fences overview

DMA Fence Cross-Driver Contract
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-fence.c
   :doc: fence cross-driver contract

DMA Fence Signalling Annotations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-fence.c
   :doc: fence signalling annotation

DMA Fences Functions Reference
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-fence.c
   :export:

.. kernel-doc:: include/linux/dma-fence.h
   :internal:

Seqno Hardware Fences
~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: include/linux/seqno-fence.h
   :internal:

DMA Fence Array
~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
   :export:

.. kernel-doc:: include/linux/dma-fence-array.h
   :internal:

DMA Fence uABI/Sync File
~~~~~~~~~~~~~~~~~~~~~~~~

.. kernel-doc:: drivers/dma-buf/sync_file.c
   :export:

.. kernel-doc:: include/linux/sync_file.h
   :internal:

Indefinite DMA Fences
~~~~~~~~~~~~~~~~~~~~~

At various times &dma_fence with an indefinite time until dma_fence_wait()
finishes have been proposed. Examples include:

* Future fences, used in HWC1 to signal when a buffer isn't used by the display
  any longer, and created with the screen update that makes the buffer visible.
  The time this fence completes is entirely under userspace's control.

* Proxy fences, proposed to handle &drm_syncobj for which the fence has not yet
  been set. Used to asynchronously delay command submission.

* Userspace fences or gpu futexes, fine-grained locking within a command buffer
  that userspace uses for synchronization across engines or with the CPU, which
  are then imported as a DMA fence for integration into existing winsys
  protocols.

* Long-running compute command buffers, while still using traditional end of
  batch DMA fences for memory management instead of context preemption DMA
  fences which get reattached when the compute job is rescheduled.

Common to all these schemes is that userspace controls the dependencies of these
fences and controls when they fire. Mixing indefinite fences with normal
in-kernel DMA fences does not work, even when a fallback timeout is included to
protect against malicious userspace:

* Only the kernel knows about all DMA fence dependencies, userspace is not aware
  of dependencies injected due to memory management or scheduler decisions.

* Only userspace knows about all dependencies in indefinite fences and when
  exactly they will complete, the kernel has no visibility.

Furthermore the kernel has to be able to hold up userspace command submission
for memory management needs, which means we must support indefinite fences being
dependent upon DMA fences. If the kernel also support indefinite fences in the
kernel like a DMA fence, like any of the above proposal would, there is the
potential for deadlocks.

.. kernel-render:: DOT
   :alt: Indefinite Fencing Dependency Cycle
   :caption: Indefinite Fencing Dependency Cycle

   digraph "Fencing Cycle" {
      node [shape=box bgcolor=grey style=filled]
      kernel [label="Kernel DMA Fences"]
      userspace [label="userspace controlled fences"]
      kernel -> userspace [label="memory management"]
      userspace -> kernel [label="Future fence, fence proxy, ..."]

      { rank=same; kernel userspace }
   }

This means that the kernel might accidentally create deadlocks
through memory management dependencies which userspace is unaware of, which
randomly hangs workloads until the timeout kicks in. Workloads, which from
userspace's perspective, do not contain a deadlock.  In such a mixed fencing
architecture there is no single entity with knowledge of all dependencies.
Thefore preventing such deadlocks from within the kernel is not possible.

The only solution to avoid dependencies loops is by not allowing indefinite
fences in the kernel. This means:

* No future fences, proxy fences or userspace fences imported as DMA fences,
  with or without a timeout.

* No DMA fences that signal end of batchbuffer for command submission where
  userspace is allowed to use userspace fencing or long running compute
  workloads. This also means no implicit fencing for shared buffers in these
  cases.