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

diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.c b/drivers/gpu/drm/sun4i/sun4i_backend.c
index 243ddfdc940344a1118ad3b69b8ad45cae13aeff..4fefd8add714b5b3abf6116938867d0dd25596ed 100644 (file)
--- a/drivers/gpu/drm/sun4i/sun4i_backend.c
+++ b/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);