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.
  
  dma-buf operations for device dma only
  --------------------------------------
  
  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_named(void *priv, struct dma_buf_ops *ops,
  				     size_t size, int flags,
  				     const char *exp_name)
  
     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.
  
     'exp_name' is the name of exporter - to facilitate information while
     debugging.
  
     Exporting modules which do not wish to provide any specific name may use the
     helper define 'dma_buf_export()', with the same arguments as above, but
     without the last argument; a __FILE__ pre-processor directive will be
     inserted in place of 'exp_name' instead.
  
  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.
  
  Kernel cpu access to a dma-buf buffer object
  --------------------------------------------
  
  The motivation to allow cpu access from the kernel to a dma-buf object from the
  importers side are:
  - fallback operations, e.g. if the devices is connected to a usb bus and the
    kernel needs to shuffle the data around first before sending it away.
  - full transparency for existing users on the importer side, i.e. userspace
    should not notice the difference between a normal object from that subsystem
    and an imported one backed by a dma-buf. This is really important for drm
    opengl drivers that expect to still use all the existing upload/download
    paths.
  
  Access to a dma_buf from the kernel context involves three steps:
  
  1. Prepare access, which invalidate any necessary caches and make the object
     available for cpu access.
  2. Access the object page-by-page with the dma_buf map apis
  3. Finish access, which will flush any necessary cpu caches and free reserved
     resources.
  
  1. Prepare access
  
     Before an importer can access a dma_buf object with the cpu from the kernel
     context, it needs to notify the exporter of the access that is about to
     happen.
  
     Interface:
        int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
  				   size_t start, size_t len,
  				   enum dma_data_direction direction)
  
     This allows the exporter to ensure that the memory is actually available for
     cpu access - the exporter might need to allocate or swap-in and pin the
     backing storage. The exporter also needs to ensure that cpu access is
     coherent for the given range and access direction. The range and access
     direction can be used by the exporter to optimize the cache flushing, i.e.
     access outside of the range or with a different direction (read instead of
     write) might return stale or even bogus data (e.g. when the exporter needs to
     copy the data to temporary storage).
  
     This step might fail, e.g. in oom conditions.
  
  2. Accessing the buffer
  
     To support dma_buf objects residing in highmem cpu access is page-based using
     an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
     PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
     a pointer in kernel virtual address space. Afterwards the chunk needs to be
     unmapped again. There is no limit on how often a given chunk can be mapped
     and unmapped, i.e. the importer does not need to call begin_cpu_access again
     before mapping the same chunk again.
  
     Interfaces:
        void *dma_buf_kmap(struct dma_buf *, unsigned long);
        void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
  
     There are also atomic variants of these interfaces. Like for kmap they
     facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
     the callback) is allowed to block when using these.
  
     Interfaces:
        void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
        void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
  
     For importers all the restrictions of using kmap apply, like the limited
     supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
     atomic dma_buf kmaps at the same time (in any given process context).
  
     dma_buf kmap calls outside of the range specified in begin_cpu_access are
     undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
     the partial chunks at the beginning and end but may return stale or bogus
     data outside of the range (in these partial chunks).
  
     Note that these calls need to always succeed. The exporter needs to complete
     any preparations that might fail in begin_cpu_access.
  
     For some cases the overhead of kmap can be too high, a vmap interface
     is introduced. This interface should be used very carefully, as vmalloc
     space is a limited resources on many architectures.
  
     Interfaces:
        void *dma_buf_vmap(struct dma_buf *dmabuf)
        void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
  
     The vmap call can fail if there is no vmap support in the exporter, or if it
     runs out of vmalloc space. Fallback to kmap should be implemented. Note that
     the dma-buf layer keeps a reference count for all vmap access and calls down
     into the exporter's vmap function only when no vmapping exists, and only
     unmaps it once. Protection against concurrent vmap/vunmap calls is provided
     by taking the dma_buf->lock mutex.
  
  3. Finish access
  
     When the importer is done accessing the range specified in begin_cpu_access,
     it needs to announce this to the exporter (to facilitate cache flushing and
     unpinning of any pinned resources). The result of any dma_buf kmap calls
     after end_cpu_access is undefined.
  
     Interface:
        void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
  				  size_t start, size_t len,
  				  enum dma_data_direction dir);
  
  
  Direct Userspace Access/mmap Support
  ------------------------------------
  
  Being able to mmap an export dma-buf buffer object has 2 main use-cases:
  - CPU fallback processing in a pipeline and
  - supporting existing mmap interfaces in importers.
  
  1. CPU fallback processing in a pipeline
  
     In many processing pipelines it is sometimes required that the cpu can access
     the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
     the need to handle this specially in userspace frameworks for buffer sharing
     it's ideal if the dma_buf fd itself can be used to access the backing storage
     from userspace using mmap.
  
     Furthermore Android's ION framework already supports this (and is otherwise
     rather similar to dma-buf from a userspace consumer side with using fds as
     handles, too). So it's beneficial to support this in a similar fashion on
     dma-buf to have a good transition path for existing Android userspace.
  
     No special interfaces, userspace simply calls mmap on the dma-buf fd.
  
  2. Supporting existing mmap interfaces in exporters
  
     Similar to the motivation for kernel cpu access it is again important that
     the userspace code of a given importing subsystem can use the same interfaces
     with a imported dma-buf buffer object as with a native buffer object. This is
     especially important for drm where the userspace part of contemporary OpenGL,
     X, and other drivers is huge, and reworking them to use a different way to
     mmap a buffer rather invasive.
  
     The assumption in the current dma-buf interfaces is that redirecting the
     initial mmap is all that's needed. A survey of some of the existing
     subsystems shows that no driver seems to do any nefarious thing like syncing
     up with outstanding asynchronous processing on the device or allocating
     special resources at fault time. So hopefully this is good enough, since
     adding interfaces to intercept pagefaults and allow pte shootdowns would
     increase the complexity quite a bit.
  
     Interface:
        int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
  		       unsigned long);
  
     If the importing subsystem simply provides a special-purpose mmap call to set
     up a mapping in userspace, calling do_mmap with dma_buf->file will equally
     achieve that for a dma-buf object.
  
  3. Implementation notes for exporters
  
     Because dma-buf buffers have invariant size over their lifetime, the dma-buf
     core checks whether a vma is too large and rejects such mappings. The
     exporter hence does not need to duplicate this check.
  
     Because existing importing subsystems might presume coherent mappings for
     userspace, the exporter needs to set up a coherent mapping. If that's not
     possible, it needs to fake coherency by manually shooting down ptes when
     leaving the cpu domain and flushing caches at fault time. Note that all the
     dma_buf files share the same anon inode, hence the exporter needs to replace
     the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
     required. This is because the kernel uses the underlying inode's address_space
     for vma tracking (and hence pte tracking at shootdown time with
     unmap_mapping_range).
  
     If the above shootdown dance turns out to be too expensive in certain
     scenarios, we can extend dma-buf with a more explicit cache tracking scheme
     for userspace mappings. But the current assumption is that using mmap is
     always a slower path, so some inefficiencies should be acceptable.
  
     Exporters that shoot down mappings (for any reasons) shall not do any
     synchronization at fault time with outstanding device operations.
     Synchronization is an orthogonal issue to sharing the backing storage of a
     buffer and hence should not be handled by dma-buf itself. This is explicitly
     mentioned here because many people seem to want something like this, but if
     different exporters handle this differently, buffer sharing can fail in
     interesting ways depending upong the exporter (if userspace starts depending
     upon this implicit synchronization).
  
  Other Interfaces Exposed to Userspace on the dma-buf FD
  ------------------------------------------------------
  
  - Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
    with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
    the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
    llseek operation will report -EINVAL.
  
    If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
    cases. Userspace can use this to detect support for discovering the dma-buf
    size using llseek.
  
  Miscellaneous notes
  -------------------
  
  - Any exporters or users of the dma-buf buffer sharing framework must have
    a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
  
  - In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
    on the file descriptor.  This is not just a resource leak, but a
    potential security hole.  It could give the newly exec'd application
    access to buffers, via the leaked fd, to which it should otherwise
    not be permitted access.
  
    The problem with doing this via a separate fcntl() call, versus doing it
    atomically when the fd is created, is that this is inherently racy in a
    multi-threaded app[3].  The issue is made worse when it is library code
    opening/creating the file descriptor, as the application may not even be
    aware of the fd's.
  
    To avoid this problem, userspace must have a way to request O_CLOEXEC
    flag be set when the dma-buf fd is created.  So any API provided by
    the exporting driver to create a dmabuf fd must provide a way to let
    userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
  
  - If an exporter needs to manually flush caches and hence needs to fake
    coherency for mmap support, it needs to be able to zap all the ptes pointing
    at the backing storage. Now linux mm needs a struct address_space associated
    with the struct file stored in vma->vm_file to do that with the function
    unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
    with the anon_file struct file, i.e. all dma_bufs share the same file.
  
    Hence exporters need to setup their own file (and address_space) association
    by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
    callback. In the specific case of a gem driver the exporter could use the
    shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
    zap ptes by unmapping the corresponding range of the struct address_space
    associated with their own file.
  
  References:
  [1] struct dma_buf_ops in include/linux/dma-buf.h
  [2] All interfaces mentioned above defined in include/linux/dma-buf.h
  [3] https://lwn.net/Articles/236486/