dma-buf: Final bits of doc polish

- Put all the remaing bits of the old doc into suitable places in the
  new sphinx world.
- Also document the poll support, we forgot to do that.
- Delete dma-buf-sharing.txt.

v2: Don't forget to update MAINTAINERS.

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/20161209215055.3492-1-daniel.vetter@ffwll.ch

1 files changed, 48 insertions, 0 deletions

diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index 92e4170..31671b4 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -46,6 +46,48 @@ The buffer-user
    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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -58,6 +100,12 @@ 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~