drm: Document drm_cache interface

Notice that this uncovers an issue with the kernel-doc handling of array
arguments, causing the first parameter of drm_clflush_pages() to not
show up in the rst-generated page.  A proposed fix is under review in
linux-doc: <http://www.spinics.net/lists/linux-doc/msg42544.html>

Changes since v1:
 - Add section to drm-mm.rst.
 - Fix kernel-doc style issues.
 - s/memory/kernel memory/.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170109215649.6860-1-krisman@collabora.co.uk

Documentation/gpu/drm-mm.rst

@@ -470,3 +470,9 @@ DRM MM Range Allocator Function References
 
 .. kernel-doc:: include/drm/drm_mm.h
    :internal:
+
+DRM Cache Handling
+==================
+
+.. kernel-doc:: drivers/gpu/drm/drm_cache.c
+   :export:

drivers/gpu/drm/drm_cache.c

@@ -67,6 +67,14 @@ static void drm_cache_flush_clflush(struct page *pages[],
 }
 #endif
 
+/**
+ * drm_clflush_pages - Flush dcache lines of a set of pages.
+ * @pages: List of pages to be flushed.
+ * @num_pages: Number of pages in the array.
+ *
+ * Flush every data cache line entry that points to an address belonging
+ * to a page in the array.
+ */
 void
 drm_clflush_pages(struct page *pages[], unsigned long num_pages)
 {
@@ -101,6 +109,13 @@ drm_clflush_pages(struct page *pages[], unsigned long num_pages)
 }
 EXPORT_SYMBOL(drm_clflush_pages);
 
+/**
+ * drm_clflush_sg - Flush dcache lines pointing to a scather-gather.
+ * @st: struct sg_table.
+ *
+ * Flush every data cache line entry that points to an address in the
+ * sg.
+ */
 void
 drm_clflush_sg(struct sg_table *st)
 {
@@ -125,6 +140,14 @@ drm_clflush_sg(struct sg_table *st)
 }
 EXPORT_SYMBOL(drm_clflush_sg);
 
+/**
+ * drm_clflush_virt_range - Flush dcache lines of a region
+ * @addr: Initial kernel memory address.
+ * @length: Region size.
+ *
+ * Flush every data cache line entry that points to an address in the
+ * region requested.
+ */
 void
 drm_clflush_virt_range(void *addr, unsigned long length)
 {