DMA Buffer Sharing API Guide
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
                              Sumit Semwal
                  <sumit dot semwal at linaro dot org>
                   <sumit dot semwal at ti dot com>
  
  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' 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.
  
  The exporter
  - implements and manages operations[1] for the buffer
  - allows other users to share the buffer by using dma_buf sharing APIs,
  - manages the details of buffer allocation,
  - decides about the actual backing storage where this allocation happens,
  - 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.
  - 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.
  
  *IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
  For this first version, A buffer shared using the dma_buf sharing API:
  - *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
     this framework.
  - may be used *ONLY* by importers that do not need CPU access to the buffer.
  
  The dma_buf buffer sharing API usage contains the following steps:
  
  1. Exporter announces that it wishes to export a buffer
  2. Userspace gets the file descriptor associated with the exported buffer, and
     passes it around to potential buffer-users based on use case
  3. Each buffer-user 'connects' itself to the buffer
  4. When needed, buffer-user requests access to the buffer from exporter
  5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
  6. when buffer-user is done using this buffer completely, it 'disconnects'
     itself from the buffer.
  
  
  1. Exporter's announcement of buffer export
  
     The buffer exporter announces its wish to export a buffer. In this, it
     connects its own private buffer data, provides implementation for operations
     that can be performed on the exported dma_buf, and flags for the file
     associated with this buffer.
  
     Interface:
        struct dma_buf *dma_buf_export(void *priv, struct dma_buf_ops *ops,
  				     size_t size, int flags)
  
     If this succeeds, dma_buf_export allocates a dma_buf structure, and returns a
     pointer to the same. It also associates an anonymous file with this buffer,
     so it can be exported. On failure to allocate the dma_buf object, it returns
     NULL.
  
  2. Userspace gets a handle to pass around to potential buffer-users
  
     Userspace entity requests for a file-descriptor (fd) which is a handle to the
     anonymous file associated with the buffer. It can then share the fd with other
     drivers and/or processes.
  
     Interface:
        int dma_buf_fd(struct dma_buf *dmabuf)
  
     This API installs an fd for the anonymous file associated with this buffer;
     returns either 'fd', or error.
  
  3. Each buffer-user 'connects' itself to the buffer
  
     Each buffer-user now gets a reference to the buffer, using the fd passed to
     it.
  
     Interface:
        struct dma_buf *dma_buf_get(int fd)
  
     This API will return a reference to the dma_buf, and increment refcount for
     it.
  
     After this, the buffer-user needs to attach its device with the buffer, which
     helps the exporter to know of device buffer constraints.
  
     Interface:
        struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
                                                  struct device *dev)
  
     This API returns reference to an attachment structure, which is then used
     for scatterlist operations. It will optionally call the 'attach' dma_buf
     operation, if provided by the exporter.
  
     The dma-buf sharing framework does the bookkeeping bits related to managing
     the list of all attachments to a buffer.
  
  Until this stage, the buffer-exporter has the option to choose not to actually
  allocate the backing storage for this buffer, but wait for the first buffer-user
  to request use of buffer for allocation.
  
  
  4. When needed, buffer-user requests access to the buffer
  
     Whenever a buffer-user wants to use the buffer for any DMA, it asks for
     access to the buffer using dma_buf_map_attachment API. At least one attach to
     the buffer must have happened before map_dma_buf can be called.
  
     Interface:
        struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
                                           enum dma_data_direction);
  
     This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
     "dma_buf->ops->" indirection from the users of this interface.
  
     In struct dma_buf_ops, map_dma_buf is defined as
        struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
                                                  enum dma_data_direction);
  
     It is one of the buffer operations that must be implemented by the exporter.
     It should return the sg_table containing scatterlist for this buffer, mapped
     into caller's address space.
  
     If this is being called for the first time, the exporter can now choose to
     scan through the list of attachments for this buffer, collate the requirements
     of the attached devices, and choose an appropriate backing storage for the
     buffer.
  
     Based on enum dma_data_direction, it might be possible to have multiple users
     accessing at the same time (for reading, maybe), or any other kind of sharing
     that the exporter might wish to make available to buffer-users.
  
     map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
  
  
  5. When finished, the buffer-user notifies end-of-DMA to exporter
  
     Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
     the exporter using the dma_buf_unmap_attachment API.
  
     Interface:
        void dma_buf_unmap_attachment(struct dma_buf_attachment *,
                                      struct sg_table *);
  
     This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
     "dma_buf->ops->" indirection from the users of this interface.
  
     In struct dma_buf_ops, unmap_dma_buf is defined as
        void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *);
  
     unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
     map_dma_buf, this API also must be implemented by the exporter.
  
  
  6. when buffer-user is done using this buffer, it 'disconnects' itself from the
     buffer.
  
     After the buffer-user has no more interest in using this buffer, it should
     disconnect itself from the buffer:
  
     - it first detaches itself from the buffer.
  
     Interface:
        void dma_buf_detach(struct dma_buf *dmabuf,
                            struct dma_buf_attachment *dmabuf_attach);
  
     This API removes the attachment from the list in dmabuf, and optionally calls
     dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
  
     - Then, the buffer-user returns the buffer reference to exporter.
  
     Interface:
       void dma_buf_put(struct dma_buf *dmabuf);
  
     This API then reduces the refcount for this buffer.
  
     If, as a result of this call, the refcount becomes 0, the 'release' file
     operation related to this fd is called. It calls the dmabuf->ops->release()
     operation in turn, and frees the memory allocated for dmabuf when exported.
  
  NOTES:
  - Importance of attach-detach and {map,unmap}_dma_buf operation pairs
     The attach-detach calls allow the exporter to figure out backing-storage
     constraints for the currently-interested devices. This allows preferential
     allocation, and/or migration of pages across different types of storage
     available, if possible.
  
     Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
     to allow just-in-time backing of storage, and migration mid-way through a
     use-case.
  
  - Migration of backing storage if needed
     If after
     - at least one map_dma_buf has happened,
     - and the backing storage has been allocated for this buffer,
     another new buffer-user intends to attach itself to this buffer, it might
     be allowed, if possible for the exporter.
  
     In case it is allowed by the exporter:
      if the new buffer-user has stricter 'backing-storage constraints', and the
      exporter can handle these constraints, the exporter can just stall on the
      map_dma_buf until all outstanding access is completed (as signalled by
      unmap_dma_buf).
      Once all users have finished accessing and have unmapped this buffer, the
      exporter could potentially move the buffer to the stricter backing-storage,
      and then allow further {map,unmap}_dma_buf operations from any buffer-user
      from the migrated backing-storage.
  
     If the exporter cannot fulfil the backing-storage constraints of the new
     buffer-user device as requested, dma_buf_attach() would return an error to
     denote non-compatibility of the new buffer-sharing request with the current
     buffer.
  
     If the exporter chooses not to allow an attach() operation once a
     map_dma_buf() API has been called, it simply returns an error.

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

  References:
  [1] struct dma_buf_ops in include/linux/dma-buf.h
  [2] All interfaces mentioned above defined in include/linux/dma-buf.h