dma_buf: Add documentation for the new cpu access support

v2: Fix spelling issues noticed by Rob Clark.
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NRob Clark <rob@ti.com>
Signed-off-by: NSumit Semwal <sumit.semwal@linaro.org>

Documentation/dma-buf-sharing.txt

@@ -32,8 +32,12 @@ The buffer-user
 *IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details]
 For this first version, A buffer shared using the dma_buf sharing API:
 - *may* be exported to user space using "mmap" *ONLY* by exporter, outside of
-   this framework.
-- may be used *ONLY* by importers that do not need CPU access to the buffer.
+  this framework.
+- with this new iteration of the dma-buf api cpu access from the kernel has been
+  enable, see below for the details.
+
+dma-buf operations for device dma only
+--------------------------------------
 
 The dma_buf buffer sharing API usage contains the following steps:
 
@@ -219,7 +223,99 @@ NOTES:
    If the exporter chooses not to allow an attach() operation once a
    map_dma_buf() API has been called, it simply returns an error.
 
-Miscellaneous notes:
+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,
+				   size_t start, size_t len,
+				   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 given range and access direction. The range and access
+   direction can be used by the exporter to optimize the cache flushing, i.e.
+   access outside of the range or 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.
+
+3. Finish access
+
+   When the importer is done accessing the range specified in begin_cpu_access,
+   it needs to announce this to the exporter (to facilitate cache flushing and
+   unpinning of any pinned resources). The result of of any dma_buf kmap calls
+   after end_cpu_access is undefined.
+
+   Interface:
+      void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
+				  size_t start, size_t len,
+				  enum dma_data_direction dir);
+
+
+Miscellaneous notes
+-------------------
+
 - Any exporters or users of the dma-buf buffer sharing framework must have
   a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.