FreeBSD/Linux Kernel Cross Reference
sys/Documentation/dma-buf-sharing.txt

                         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(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.
   
   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.
   
   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 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).
   
   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/

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