1Buffer Sharing and Synchronization
2==================================
3
4The dma-buf subsystem provides the framework for sharing buffers for
5hardware (DMA) access across multiple device drivers and subsystems, and
6for synchronizing asynchronous hardware access.
7
8This is used, for example, by drm "prime" multi-GPU support, but is of
9course not limited to GPU use cases.
10
11The three main components of this are: (1) dma-buf, representing a
12sg_table and exposed to userspace as a file descriptor to allow passing
13between devices, (2) fence, which provides a mechanism to signal when
14one device as finished access, and (3) reservation, which manages the
15shared or exclusive fence(s) associated with the buffer.
16
17Shared DMA Buffers
18------------------
19
20This document serves as a guide to device-driver writers on what is the dma-buf
21buffer sharing API, how to use it for exporting and using shared buffers.
22
23Any device driver which wishes to be a part of DMA buffer sharing, can do so as
24either the 'exporter' of buffers, or the 'user' or 'importer' of buffers.
25
26Say a driver A wants to use buffers created by driver B, then we call B as the
27exporter, and A as buffer-user/importer.
28
29The exporter
30
31 - implements and manages operations in :c:type:`struct dma_buf_ops
32   <dma_buf_ops>` for the buffer,
33 - allows other users to share the buffer by using dma_buf sharing APIs,
34 - manages the details of buffer allocation, wrapped int a :c:type:`struct
35   dma_buf <dma_buf>`,
36 - decides about the actual backing storage where this allocation happens,
37 - and takes care of any migration of scatterlist - for all (shared) users of
38   this buffer.
39
40The buffer-user
41
42 - is one of (many) sharing users of the buffer.
43 - doesn't need to worry about how the buffer is allocated, or where.
44 - and needs a mechanism to get access to the scatterlist that makes up this
45   buffer in memory, mapped into its own address space, so it can access the
46   same area of memory. This interface is provided by :c:type:`struct
47   dma_buf_attachment <dma_buf_attachment>`.
48
49Any exporters or users of the dma-buf buffer sharing framework must have a
50'select DMA_SHARED_BUFFER' in their respective Kconfigs.
51
52Userspace Interface Notes
53~~~~~~~~~~~~~~~~~~~~~~~~~
54
55Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
56and hence the generic interface exposed is very minimal. There's a few things to
57consider though:
58
59- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
60  with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
61  the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
62  llseek operation will report -EINVAL.
63
64  If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
65  cases. Userspace can use this to detect support for discovering the dma-buf
66  size using llseek.
67
68- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
69  on the file descriptor.  This is not just a resource leak, but a
70  potential security hole.  It could give the newly exec'd application
71  access to buffers, via the leaked fd, to which it should otherwise
72  not be permitted access.
73
74  The problem with doing this via a separate fcntl() call, versus doing it
75  atomically when the fd is created, is that this is inherently racy in a
76  multi-threaded app[3].  The issue is made worse when it is library code
77  opening/creating the file descriptor, as the application may not even be
78  aware of the fd's.
79
80  To avoid this problem, userspace must have a way to request O_CLOEXEC
81  flag be set when the dma-buf fd is created.  So any API provided by
82  the exporting driver to create a dmabuf fd must provide a way to let
83  userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
84
85- Memory mapping the contents of the DMA buffer is also supported. See the
86  discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
87
88- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
89  details.
90
91Basic Operation and Device DMA Access
92~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
93
94.. kernel-doc:: drivers/dma-buf/dma-buf.c
95   :doc: dma buf device access
96
97CPU Access to DMA Buffer Objects
98~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
99
100.. kernel-doc:: drivers/dma-buf/dma-buf.c
101   :doc: cpu access
102
103Fence Poll Support
104~~~~~~~~~~~~~~~~~~
105
106.. kernel-doc:: drivers/dma-buf/dma-buf.c
107   :doc: fence polling
108
109Kernel Functions and Structures Reference
110~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
111
112.. kernel-doc:: drivers/dma-buf/dma-buf.c
113   :export:
114
115.. kernel-doc:: include/linux/dma-buf.h
116   :internal:
117
118Reservation Objects
119-------------------
120
121.. kernel-doc:: drivers/dma-buf/reservation.c
122   :doc: Reservation Object Overview
123
124.. kernel-doc:: drivers/dma-buf/reservation.c
125   :export:
126
127.. kernel-doc:: include/linux/reservation.h
128   :internal:
129
130DMA Fences
131----------
132
133.. kernel-doc:: drivers/dma-buf/dma-fence.c
134   :doc: DMA fences overview
135
136DMA Fences Functions Reference
137~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
138
139.. kernel-doc:: drivers/dma-buf/dma-fence.c
140   :export:
141
142.. kernel-doc:: include/linux/dma-fence.h
143   :internal:
144
145Seqno Hardware Fences
146~~~~~~~~~~~~~~~~~~~~~
147
148.. kernel-doc:: include/linux/seqno-fence.h
149   :internal:
150
151DMA Fence Array
152~~~~~~~~~~~~~~~
153
154.. kernel-doc:: drivers/dma-buf/dma-fence-array.c
155   :export:
156
157.. kernel-doc:: include/linux/dma-fence-array.h
158   :internal:
159
160DMA Fence uABI/Sync File
161~~~~~~~~~~~~~~~~~~~~~~~~
162
163.. kernel-doc:: drivers/dma-buf/sync_file.c
164   :export:
165
166.. kernel-doc:: include/linux/sync_file.h
167   :internal:
168
169