[PATCH v2 2/2] clk: sunxi-ng: nkmp: Explain why zero width check is needed

Jernej Skrabec
2019-04-03T15:14:47Z

Add an explanation why zero width check is needed when generating factor
mask using GENMASK() macro.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
---
drivers/clk/sunxi-ng/ccu_nkmp.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/drivers/clk/sunxi-ng/ccu_nkmp.c b/drivers/clk/sunxi-ng/ccu_nkmp.c
index 69dfc6de1c4e..cbcdf664f336 100644
--- a/drivers/clk/sunxi-ng/ccu_nkmp.c
+++ b/drivers/clk/sunxi-ng/ccu_nkmp.c
@@ -186,6 +186,12 @@ static int ccu_nkmp_set_rate(struct clk_hw *hw, unsigned long rate,

ccu_nkmp_find_best(parent_rate, rate, &_nkmp);

+ /*
+ * If width is 0, GENMASK() macro may not generate expected mask (0)
+ * as it falls under undefined behaviour by C standard due to shifts
+ * which are equal or greater than width of left operand. This can
+ * be easily avoided by explicitly checking if width is 0.
+ */
if (nkmp->n.width)
n_mask = GENMASK(nkmp->n.width + nkmp->n.shift - 1,
nkmp->n.shift);