dma-buf: Reorganize device dma access docs

- Put the initial overview for dma-buf into dma-buf.rst.
- Put all the comments about detailed semantics into the right
  kernel-doc comment for functions or ops structure member.
- To allow that detail, switch the reworked kerneldoc to inline style
  for dma_buf_ops.
- Tie everything together into a much more streamlined overview
  comment, relying on the hyperlinks for all the details.
- Also sprinkle some links into the kerneldoc for dma_buf and
  dma_buf_attachment to tie it all together.

Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161209185309.1682-4-daniel.vetter@ffwll.ch

2 files changed, 38 insertions, 222 deletions

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index ca44c5820585..dca2fb7ac3b4 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -5,228 +5,6 @@
                 <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
 --------------------------------------------
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index a9b457a4b949..906d1532efad 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -17,6 +17,44 @@ shared or exclusive fence(s) associated with the buffer.
 Shared DMA Buffers
 ------------------
 
+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' or 'importer' 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/importer.
+
+The exporter
+
+ - implements and manages operations in :c:type:`struct dma_buf_ops
+   <dma_buf_ops>` for the buffer,
+ - allows other users to share the buffer by using dma_buf sharing APIs,
+ - manages the details of buffer allocation, wrapped int a :c:type:`struct
+   dma_buf <dma_buf>`,
+ - decides about the actual backing storage where this allocation happens,
+ - and 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.
+ - and 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. This interface is provided by :c:type:`struct
+   dma_buf_attachment <dma_buf_attachment>`.
+
+Basic Operation and Device DMA Access
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: dma buf device access
+
+Kernel Functions and Structures Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :export: