drm/Documentation: Refine TODO for backlight helpers in tinydrm

Add a summary which resulted from discussions on what should
be done to refactor backlight helpers in tinydrm so that
they can be used in other drivers as well.

Signed-off-by: Meghana Madhyastha <meghana.madhyastha@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927105116.GA30391@meghana-HP-Pavilion-Notebook

Documentation/gpu/todo.rst

@@ -351,7 +351,16 @@ those drivers as simple as possible, so lots of room for refactoring:
 - backlight helpers, probably best to put them into a new drm_backlight.c.
   This is because drivers/video is de-facto unmaintained. We could also
   move drivers/video/backlight to drivers/gpu/backlight and take it all
-  over within drm-misc, but that's more work.
+  over within drm-misc, but that's more work. Backlight helpers require a fair
+  bit of reworking and refactoring. A simple example is the enabling of a backlight.
+  Tinydrm has helpers for this. It would be good if other drivers can also use the
+  helper. However, there are various cases we need to consider i.e different
+  drivers seem to have different ways of enabling/disabling a backlight.
+  We also need to consider the backlight drivers (like gpio_backlight). The situation
+  is further complicated by the fact that the backlight is tied to fbdev
+  via fb_notifier_callback() which has complicated logic. For further details, refer
+  to the following discussion thread:
+  https://groups.google.com/forum/#!topic/outreachy-kernel/8rBe30lwtdA
 
 - spi helpers, probably best put into spi core/helper code. Thierry said
   the spi maintainer is fast&reactive, so shouldn't be a big issue.