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+                    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. 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,
+				   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 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/