DMA Buffer Sharing API Guide

                    DMA Buffer Sharing API Guide
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

                            Sumit Semwal
               
                 

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. All these fields are filled in struct
   dma_buf_export_info, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.

   Interface:
      DEFINE_DMA_BUF_EXPORT_INFO(exp_info)
      struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)

   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' in struct dma_buf_export_info is the name of exporter - to
   facilitate information while debugging. It is set to KBUILD_MODNAME by
   default, so exporters don't have to provide a specific name, if they don't
   wish to.

   DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
   zeroes it out and pre-populates exp_name in it.

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, int flags)

   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 *,
                            enum dma_data_direction);

   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 fulfill 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,
                   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 access direction. The direction can be used by the exporter
   to optimize the cache flushing, i.e. access 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 CPU, 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,
                  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, making
   sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
   -EAGAIN or -EINTR, in which case it must be restarted.

   Some systems might need some sort of cache coherency management e.g. when
   CPU and GPU domains are being accessed through dma-buf at the same time. To
   circumvent this problem there are begin/end coherency markers, that forward
   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
   would be used like following:
     - mmap dma-buf fd
     - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
       to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
       want (with the new data being consumed by the GPU or say scanout device)
     - munmap once you don't need the buffer any more

    For correctness and optimal performance, it is always required to use
    SYNC_START and SYNC_END before and after, respectively, when accessing the
    mapped address. Userspace cannot rely on coherent access, even when there
    are systems where it just works without calling these ioctls.

2. Supporting existing mmap interfaces in importers

   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/