clk: sunxi-ng: nkmp: Explain why zero width check is needed

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>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>

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);