Root/
Source at commit b13e7eb172b6f08e5fc22da162bdde5fcde201b5 created 11 years 11 months ago. By Maarten ter Huurne, fbcon: Add 6x10 font | |
---|---|
1 | DMA Buffer Sharing API Guide |
2 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
3 | |
4 | Sumit Semwal |
5 | <sumit dot semwal at linaro dot org> |
6 | <sumit dot semwal at ti dot com> |
7 | |
8 | This document serves as a guide to device-driver writers on what is the dma-buf |
9 | buffer sharing API, how to use it for exporting and using shared buffers. |
10 | |
11 | Any device driver which wishes to be a part of DMA buffer sharing, can do so as |
12 | either the 'exporter' of buffers, or the 'user' of buffers. |
13 | |
14 | Say a driver A wants to use buffers created by driver B, then we call B as the |
15 | exporter, and A as buffer-user. |
16 | |
17 | The exporter |
18 | - implements and manages operations[1] for the buffer |
19 | - allows other users to share the buffer by using dma_buf sharing APIs, |
20 | - manages the details of buffer allocation, |
21 | - decides about the actual backing storage where this allocation happens, |
22 | - takes care of any migration of scatterlist - for all (shared) users of this |
23 | buffer, |
24 | |
25 | The buffer-user |
26 | - is one of (many) sharing users of the buffer. |
27 | - doesn't need to worry about how the buffer is allocated, or where. |
28 | - needs a mechanism to get access to the scatterlist that makes up this buffer |
29 | in memory, mapped into its own address space, so it can access the same area |
30 | of memory. |
31 | |
32 | *IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] |
33 | For this first version, A buffer shared using the dma_buf sharing API: |
34 | - *may* be exported to user space using "mmap" *ONLY* by exporter, outside of |
35 | this framework. |
36 | - may be used *ONLY* by importers that do not need CPU access to the buffer. |
37 | |
38 | The dma_buf buffer sharing API usage contains the following steps: |
39 | |
40 | 1. Exporter announces that it wishes to export a buffer |
41 | 2. Userspace gets the file descriptor associated with the exported buffer, and |
42 | passes it around to potential buffer-users based on use case |
43 | 3. Each buffer-user 'connects' itself to the buffer |
44 | 4. When needed, buffer-user requests access to the buffer from exporter |
45 | 5. When finished with its use, the buffer-user notifies end-of-DMA to exporter |
46 | 6. when buffer-user is done using this buffer completely, it 'disconnects' |
47 | itself from the buffer. |
48 | |
49 | |
50 | 1. Exporter's announcement of buffer export |
51 | |
52 | The buffer exporter announces its wish to export a buffer. In this, it |
53 | connects its own private buffer data, provides implementation for operations |
54 | that can be performed on the exported dma_buf, and flags for the file |
55 | associated with this buffer. |
56 | |
57 | Interface: |
58 | struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops, |
59 | size_t size, int flags) |
60 | |
61 | If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a |
62 | pointer to the same. It also associates an anonymous file with this buffer, |
63 | so it can be exported. On failure to allocate the dma_buf object, it returns |
64 | NULL. |
65 | |
66 | 2. Userspace gets a handle to pass around to potential buffer-users |
67 | |
68 | Userspace entity requests for a file-descriptor (fd) which is a handle to the |
69 | anonymous file associated with the buffer. It can then share the fd with other |
70 | drivers and/or processes. |
71 | |
72 | Interface: |
73 | int dma_buf_fd(struct dma_buf *dmabuf) |
74 | |
75 | This API installs an fd for the anonymous file associated with this buffer; |
76 | returns either 'fd', or error. |
77 | |
78 | 3. Each buffer-user 'connects' itself to the buffer |
79 | |
80 | Each buffer-user now gets a reference to the buffer, using the fd passed to |
81 | it. |
82 | |
83 | Interface: |
84 | struct dma_buf *dma_buf_get(int fd) |
85 | |
86 | This API will return a reference to the dma_buf, and increment refcount for |
87 | it. |
88 | |
89 | After this, the buffer-user needs to attach its device with the buffer, which |
90 | helps the exporter to know of device buffer constraints. |
91 | |
92 | Interface: |
93 | struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, |
94 | struct device *dev) |
95 | |
96 | This API returns reference to an attachment structure, which is then used |
97 | for scatterlist operations. It will optionally call the 'attach' dma_buf |
98 | operation, if provided by the exporter. |
99 | |
100 | The dma-buf sharing framework does the bookkeeping bits related to managing |
101 | the list of all attachments to a buffer. |
102 | |
103 | Until this stage, the buffer-exporter has the option to choose not to actually |
104 | allocate the backing storage for this buffer, but wait for the first buffer-user |
105 | to request use of buffer for allocation. |
106 | |
107 | |
108 | 4. When needed, buffer-user requests access to the buffer |
109 | |
110 | Whenever a buffer-user wants to use the buffer for any DMA, it asks for |
111 | access to the buffer using dma_buf_map_attachment API. At least one attach to |
112 | the buffer must have happened before map_dma_buf can be called. |
113 | |
114 | Interface: |
115 | struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, |
116 | enum dma_data_direction); |
117 | |
118 | This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the |
119 | "dma_buf->ops->" indirection from the users of this interface. |
120 | |
121 | In struct dma_buf_ops, map_dma_buf is defined as |
122 | struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, |
123 | enum dma_data_direction); |
124 | |
125 | It is one of the buffer operations that must be implemented by the exporter. |
126 | It should return the sg_table containing scatterlist for this buffer, mapped |
127 | into caller's address space. |
128 | |
129 | If this is being called for the first time, the exporter can now choose to |
130 | scan through the list of attachments for this buffer, collate the requirements |
131 | of the attached devices, and choose an appropriate backing storage for the |
132 | buffer. |
133 | |
134 | Based on enum dma_data_direction, it might be possible to have multiple users |
135 | accessing at the same time (for reading, maybe), or any other kind of sharing |
136 | that the exporter might wish to make available to buffer-users. |
137 | |
138 | map_dma_buf() operation can return -EINTR if it is interrupted by a signal. |
139 | |
140 | |
141 | 5. When finished, the buffer-user notifies end-of-DMA to exporter |
142 | |
143 | Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to |
144 | the exporter using the dma_buf_unmap_attachment API. |
145 | |
146 | Interface: |
147 | void dma_buf_unmap_attachment(struct dma_buf_attachment *, |
148 | struct sg_table *); |
149 | |
150 | This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the |
151 | "dma_buf->ops->" indirection from the users of this interface. |
152 | |
153 | In struct dma_buf_ops, unmap_dma_buf is defined as |
154 | void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *); |
155 | |
156 | unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like |
157 | map_dma_buf, this API also must be implemented by the exporter. |
158 | |
159 | |
160 | 6. when buffer-user is done using this buffer, it 'disconnects' itself from the |
161 | buffer. |
162 | |
163 | After the buffer-user has no more interest in using this buffer, it should |
164 | disconnect itself from the buffer: |
165 | |
166 | - it first detaches itself from the buffer. |
167 | |
168 | Interface: |
169 | void dma_buf_detach(struct dma_buf *dmabuf, |
170 | struct dma_buf_attachment *dmabuf_attach); |
171 | |
172 | This API removes the attachment from the list in dmabuf, and optionally calls |
173 | dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits. |
174 | |
175 | - Then, the buffer-user returns the buffer reference to exporter. |
176 | |
177 | Interface: |
178 | void dma_buf_put(struct dma_buf *dmabuf); |
179 | |
180 | This API then reduces the refcount for this buffer. |
181 | |
182 | If, as a result of this call, the refcount becomes 0, the 'release' file |
183 | operation related to this fd is called. It calls the dmabuf->ops->release() |
184 | operation in turn, and frees the memory allocated for dmabuf when exported. |
185 | |
186 | NOTES: |
187 | - Importance of attach-detach and {map,unmap}_dma_buf operation pairs |
188 | The attach-detach calls allow the exporter to figure out backing-storage |
189 | constraints for the currently-interested devices. This allows preferential |
190 | allocation, and/or migration of pages across different types of storage |
191 | available, if possible. |
192 | |
193 | Bracketing of DMA access with {map,unmap}_dma_buf operations is essential |
194 | to allow just-in-time backing of storage, and migration mid-way through a |
195 | use-case. |
196 | |
197 | - Migration of backing storage if needed |
198 | If after |
199 | - at least one map_dma_buf has happened, |
200 | - and the backing storage has been allocated for this buffer, |
201 | another new buffer-user intends to attach itself to this buffer, it might |
202 | be allowed, if possible for the exporter. |
203 | |
204 | In case it is allowed by the exporter: |
205 | if the new buffer-user has stricter 'backing-storage constraints', and the |
206 | exporter can handle these constraints, the exporter can just stall on the |
207 | map_dma_buf until all outstanding access is completed (as signalled by |
208 | unmap_dma_buf). |
209 | Once all users have finished accessing and have unmapped this buffer, the |
210 | exporter could potentially move the buffer to the stricter backing-storage, |
211 | and then allow further {map,unmap}_dma_buf operations from any buffer-user |
212 | from the migrated backing-storage. |
213 | |
214 | If the exporter cannot fulfil the backing-storage constraints of the new |
215 | buffer-user device as requested, dma_buf_attach() would return an error to |
216 | denote non-compatibility of the new buffer-sharing request with the current |
217 | buffer. |
218 | |
219 | If the exporter chooses not to allow an attach() operation once a |
220 | map_dma_buf() API has been called, it simply returns an error. |
221 | |
222 | Miscellaneous notes: |
223 | - Any exporters or users of the dma-buf buffer sharing framework must have |
224 | a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. |
225 | |
226 | References: |
227 | [1] struct dma_buf_ops in include/linux/dma-buf.h |
228 | [2] All interfaces mentioned above defined in include/linux/dma-buf.h |
229 |
Branches:
ben-wpan
ben-wpan-stefan
javiroman/ks7010
jz-2.6.34
jz-2.6.34-rc5
jz-2.6.34-rc6
jz-2.6.34-rc7
jz-2.6.35
jz-2.6.36
jz-2.6.37
jz-2.6.38
jz-2.6.39
jz-3.0
jz-3.1
jz-3.11
jz-3.12
jz-3.13
jz-3.15
jz-3.16
jz-3.18-dt
jz-3.2
jz-3.3
jz-3.4
jz-3.5
jz-3.6
jz-3.6-rc2-pwm
jz-3.9
jz-3.9-clk
jz-3.9-rc8
jz47xx
jz47xx-2.6.38
master
Tags:
od-2011-09-04
od-2011-09-18
v2.6.34-rc5
v2.6.34-rc6
v2.6.34-rc7
v3.9