doc: update/fixup dma-buf related DocBook

Split out dma-buf related parts into their own section, add missing
files, and write a bit of overview about how it all fits together.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

Documentation/DocBook/device-drivers.tmpl

@@ -128,16 +128,44 @@ X!Edrivers/base/interface.c
 !Edrivers/base/platform.c
 !Edrivers/base/bus.c
      </sect1>
-     <sect1><title>Device Drivers DMA Management</title>
+     <sect1>
+       <title>Buffer Sharing and Synchronization</title>
+       <para>
+         The dma-buf subsystem provides the framework for sharing buffers
+         for hardware (DMA) access across multiple device drivers and
+         subsystems, and for synchronizing asynchronous hardware access.
+       </para>
+       <para>
+         This is used, for example, by drm "prime" multi-GPU support, but
+         is of course not limited to GPU use cases.
+       </para>
+       <para>
+         The three main components of this are: (1) dma-buf, representing
+         a sg_table and exposed to userspace as a file descriptor to allow
+         passing between devices, (2) fence, which provides a mechanism
+         to signal when one device as finished access, and (3) reservation,
+         which manages the shared or exclusive fence(s) associated with
+         the buffer.
+       </para>
+       <sect2><title>dma-buf</title>
 !Edrivers/dma-buf/dma-buf.c
+!Iinclude/linux/dma-buf.h
+       </sect2>
+       <sect2><title>reservation</title>
+!Pdrivers/dma-buf/reservation.c Reservation Object Overview
+!Edrivers/dma-buf/reservation.c
+!Iinclude/linux/reservation.h
+       </sect2>
+       <sect2><title>fence</title>
 !Edrivers/dma-buf/fence.c
-!Edrivers/dma-buf/seqno-fence.c
 !Iinclude/linux/fence.h
+!Edrivers/dma-buf/seqno-fence.c
 !Iinclude/linux/seqno-fence.h
-!Edrivers/dma-buf/reservation.c
-!Iinclude/linux/reservation.h
 !Edrivers/dma-buf/sync_file.c
 !Iinclude/linux/sync_file.h
+       </sect2>
+     </sect1>
+     <sect1><title>Device Drivers DMA Management</title>
 !Edrivers/base/dma-coherent.c
 !Edrivers/base/dma-mapping.c
      </sect1>