From: Chen-Yu Tsai <wens@csie.org>
Date: Sat, 14 Oct 2017 12:02:49 +0800
Subject: drm/sun4i: backend: Add comment explaining why registers are cleared
Git-commit: 936598d16e9111e3802d45a60c028ad36a3a7013
Patch-mainline: v4.15-rc1
References: FATE#326289 FATE#326079 FATE#326049 FATE#322398 FATE#326166

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

Acked-by: Petr Tesarik <ptesarik@suse.com>
---
 drivers/gpu/drm/sun4i/sun4i_backend.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- 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 dev
 
 	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);