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

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Message-Id: <20190403151404.26542-3-jernej.skrabec@siol.net>
Date:   Wed,  3 Apr 2019 17:14:04 +0200
From:   Jernej Skrabec <jernej.skrabec@...l.net>
To:     maxime.ripard@...tlin.com, wens@...e.org
Cc:     mturquette@...libre.com, sboyd@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, jernej.skrabec@...l.net
Subject: [PATCH v2 2/2] 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@...l.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);
-- 
2.21.0