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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
This commit is contained in:
Chen-Yu Tsai 2017-10-14 12:02:49 +08:00 committed by Maxime Ripard
parent cff2192f80
commit 936598d16e
1 changed files with 8 additions and 1 deletions
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9
drivers/gpu/drm/sun4i/sun4i_backend.c
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@ -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);