linux/Documentation/dma-buf-sharing.txt
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   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