drm/sun4i: backend: Add comment explaining why registers are cleared

Many of the backend's layer configuration registers have undefined
default values. This poses a risk as we use regmap_update_bits in
some places, and don't overwrite the whole register.

At probe/bind time we explicitly clear all the control registers
by writing 0 to them. This patch adds a more detailed explanation
on why we're doing this.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171014040252.9621-5-wens@csie.org

drivers/gpu/drm/sun4i/sun4i_backend.c

@@ -412,7 +412,14 @@ static int sun4i_backend_bind(struct device *dev, struct device *master,
 
 	list_add_tail(&backend->engine.list, &drv->engine_list);
 
-	/* Reset the registers */
+	/*
+	 * Many of the backend's layer configuration registers have
+	 * undefined default values. This poses a risk as we use
+	 * regmap_update_bits in some places, and don't overwrite
+	 * the whole register.
+	 *
+	 * Clear the registers here to have something predictable.
+	 */
 	for (i = 0x800; i < 0x1000; i += 4)
 		regmap_write(backend->engine.regs, i, 0);