[PATCH] drm: add more document for drm_crtc_from_index()

[Shawn Guo <shawnguo@kernel.org>]

From: Shawn Guo <shawn.guo@linaro.org>

Add a bit more document for function drm_crtc_from_index() to cross
link it with drm_crtc_from_index(), and explain that the function is
useful in vblank code.

While at it, add cross link comment for drm_plane_from_index() as well.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
---
This is a follow-up patch to address Daniel's comment [1] on my patch
"[PATCH 1/3] drm: add crtc helper drm_crtc_from_index()".

Shawn

[1] http://www.spinics.net/lists/dri-devel/msg128062.html

 drivers/gpu/drm/drm_crtc.c  | 5 ++++-
 drivers/gpu/drm/drm_plane.c | 2 +-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 5c1bb1f34697..85a7452d0fb4 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -52,7 +52,10 @@
  * @idx: index of registered CRTC to find for
  *
  * Given a CRTC index, return the registered CRTC from DRM device's
- * list of CRTCs with matching index.
+ * list of CRTCs with matching index. This is the inverse of drm_crtc_index().
+ * It's useful in the vblank callbacks (like &drm_driver.enable_vblank or
+ * &drm_driver.disable_vblank), since that still deals with indices instead
+ * of pointers to &struct drm_crtc."
  */
 struct drm_crtc *drm_crtc_from_index(struct drm_device *dev, int idx)
 {
diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 62b98f386fd1..535dec4f508d 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -254,7 +254,7 @@ void drm_plane_cleanup(struct drm_plane *plane)
  * @idx: index of registered plane to find for
  *
  * Given a plane index, return the registered plane from DRM device's
- * list of planes with matching index.
+ * list of planes with matching index. This is the inverse of drm_plane_index().
  */
 struct drm_plane *
 drm_plane_from_index(struct drm_device *dev, int idx)
-- 
1.9.1

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