Merge tag 'drm-misc-next-2016-12-30' of git://anongit.freedesktop.org/git/drm-misc into drm-intel-next-queued

Directly merge drm-misc into drm-intel since Dave is on vacation and
we need the various drm-misc patches (fb format rework, drm mm fixes,
selftest framework and others). Also pulled back -rc2 in first to
resync with drm-intel-fixes and make sure I can reuse the exact rerere
solutions from drm-tip for safety, and because I'm lazy.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

4 files changed, 139 insertions, 483 deletions

diff --git a/Documentation/devicetree/bindings/display/bridge/ti,ths8135.txt b/Documentation/devicetree/bindings/display/bridge/ti,ths8135.txt
new file mode 100644
index 000000000000..6ec1a880ac18
--- /dev/null
+++ b/Documentation/devicetree/bindings/display/bridge/ti,ths8135.txt
@@ -0,0 +1,46 @@
+THS8135 Video DAC
+-----------------
+
+This is the binding for Texas Instruments THS8135 Video DAC bridge.
+
+Required properties:
+
+- compatible: Must be "ti,ths8135"
+
+Required nodes:
+
+This device has two video ports. Their connections are modelled using the OF
+graph bindings specified in Documentation/devicetree/bindings/graph.txt.
+
+- Video port 0 for RGB input
+- Video port 1 for VGA output
+
+Example
+-------
+
+vga-bridge {
+	compatible = "ti,ths8135";
+	#address-cells = <1>;
+	#size-cells = <0>;
+
+	ports {
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		port@0 {
+			reg = <0>;
+
+			vga_bridge_in: endpoint {
+				remote-endpoint = <&lcdc_out_vga>;
+			};
+		};
+
+		port@1 {
+			reg = <1>;
+
+			vga_bridge_out: endpoint {
+				remote-endpoint = <&vga_con_in>;
+			};
+		};
+	};
+};
diff --git a/Documentation/devicetree/bindings/display/hisilicon/hisi-ade.txt b/Documentation/devicetree/bindings/display/hisilicon/hisi-ade.txt
index 38dc9d60eef8..305a0e72a900 100644
--- a/Documentation/devicetree/bindings/display/hisilicon/hisi-ade.txt
+++ b/Documentation/devicetree/bindings/display/hisilicon/hisi-ade.txt
@@ -16,7 +16,7 @@ Required properties:
   "clk_ade_core" for the ADE core clock.
   "clk_codec_jpeg" for the media NOC QoS clock, which use the same clock with
   jpeg codec.
-  "clk_ade_pix" for the ADE pixel clok.
+  "clk_ade_pix" for the ADE pixel clock.
 - assigned-clocks: Should contain "clk_ade_core" and "clk_codec_jpeg" clocks'
   phandle + clock-specifier pairs.
 - assigned-clock-rates: clock rates, one for each entry in assigned-clocks.
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
deleted file mode 100644
index ca44c5820585..000000000000
--- a/Documentation/dma-buf-sharing.txt
+++ /dev/null
@@ -1,482 +0,0 @@
-                    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,
-				   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/
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: