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 8This document serves as a guide to device-driver writers on what is the dma-buf 9buffer sharing API, how to use it for exporting and using shared buffers. 10 11Any device driver which wishes to be a part of DMA buffer sharing, can do so as 12either the 'exporter' of buffers, or the 'user' of buffers. 13 14Say a driver A wants to use buffers created by driver B, then we call B as the 15exporter, and A as buffer-user. 16 17The 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 25The 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] 33For 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 38The dma_buf buffer sharing API usage contains the following steps: 39 401. Exporter announces that it wishes to export a buffer 412. Userspace gets the file descriptor associated with the exported buffer, and 42 passes it around to potential buffer-users based on use case 433. Each buffer-user 'connects' itself to the buffer 444. When needed, buffer-user requests access to the buffer from exporter 455. When finished with its use, the buffer-user notifies end-of-DMA to exporter 466. when buffer-user is done using this buffer completely, it 'disconnects' 47 itself from the buffer. 48 49 501. 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. 65 662. 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. 77 783. 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 103Until this stage, the buffer-exporter has the option to choose not to actually 104allocate the backing storage for this buffer, but wait for the first buffer-user 105to request use of buffer for allocation. 106 107 1084. 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 1415. 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 1606. 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 186NOTES: 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 222Miscellaneous 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 226References: 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