1 files changed, 92 insertions, 0 deletions
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index a9b457a4b949..31671b469627 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -17,6 +17,98 @@ 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>`.
+
+Any exporters or users of the dma-buf buffer sharing framework must have a
+'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
+Userspace Interface Notes
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
+and hence the generic interface exposed is very minimal. There's a few things to
+consider though:
+
+- 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.
+
+- 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().
+
+- Memory mapping the contents of the DMA buffer is also supported. See the
+  discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
+
+- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
+  details.
+
+Basic Operation and Device DMA Access
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: dma buf device access
+
+CPU Access to DMA Buffer Objects
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: cpu access
+
+Fence Poll Support
+~~~~~~~~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+   :doc: fence polling
+
+Kernel Functions and Structures Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 .. kernel-doc:: drivers/dma-buf/dma-buf.c
    :export: