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 --- drivers/gpu/drm/sun4i/sun4i_backend.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/sun4i/sun4i_backend.c b/drivers/gpu/drm/sun4i/sun4i_backend.c index 243ddfdc9403..4fefd8add714 100644 --- 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);