linux-kernel - [PATCH RFC] pwm: keembay: Fix build failure with -Os

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Date:   Mon, 16 Nov 2020 10:08:04 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Vijayakannan Ayyathurai <vijayakannan.ayyathurai@...el.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     kbuild-all@...ts.01.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        "Lai, Poey Seng" <poey.seng.lai@...el.com>,
        "Vineetha G. Jaya Kumaran" <vineetha.g.jaya.kumaran@...el.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de,
        kernel test robot <lkp@...el.com>
Subject: [PATCH RFC] pwm: keembay: Fix build failure with -Os

The driver used this construct:

	#define KMB_PWM_LEADIN_MASK             GENMASK(30, 0)

	static inline void keembay_pwm_update_bits(struct keembay_pwm *priv, u32 mask,
						   u32 val, u32 offset)
	{
		u32 buff = readl(priv->base + offset);

		buff = u32_replace_bits(buff, val, mask);
		writel(buff, priv->base + offset);
	}

	...
	keembay_pwm_update_bits(priv, KMB_PWM_LEADIN_MASK, 0,
					KMB_PWM_LEADIN_OFFSET(pwm->hwpwm));

With CONFIG_CC_OPTIMIZE_FOR_SIZE the compiler (here: gcc 10.2.0) this
triggers:

	In file included from /home/uwe/gsrc/linux/drivers/pwm/pwm-keembay.c:16:
	In function ‘field_multiplier’,
	    inlined from ‘keembay_pwm_update_bits’ at /home/uwe/gsrc/linux/include/linux/bitfield.h:124:17:
	/home/uwe/gsrc/linux/include/linux/bitfield.h:119:3: error: call to ‘__bad_mask’ declared with attribute error: bad bitfield mask
	  119 |   __bad_mask();
	      |   ^~~~~~~~~~~~
	In function ‘field_multiplier’,
	    inlined from ‘keembay_pwm_update_bits’ at /home/uwe/gsrc/linux/include/linux/bitfield.h:154:1:
	/home/uwe/gsrc/linux/include/linux/bitfield.h:119:3: error: call to ‘__bad_mask’ declared with attribute error: bad bitfield mask
	  119 |   __bad_mask();
	      |   ^~~~~~~~~~~~

The compiler doesn't seem to be able to notice that with field being
0x3ffffff the expression

	if ((field | (field - 1)) & ((field | (field - 1)) + 1))
		__bad_mask();

can be optimized away.

So use __always_inline and document the problem in a comment to fix
this.

Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
---
Hello,

I'm not sure this is the right fix. Maybe the bitfield stuff can be
changed somehow to make this problem go away, too?

Best regards
Uwe

 drivers/pwm/pwm-keembay.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-keembay.c b/drivers/pwm/pwm-keembay.c
index 2b6dd070daa4..cdfdef66ff8e 100644
--- a/drivers/pwm/pwm-keembay.c
+++ b/drivers/pwm/pwm-keembay.c
@@ -63,7 +63,12 @@ static int keembay_clk_enable(struct device *dev, struct clk *clk)
 	return devm_add_action_or_reset(dev, keembay_clk_unprepare, clk);
 }
 
-static inline void keembay_pwm_update_bits(struct keembay_pwm *priv, u32 mask,
+/*
+ * With gcc 10, CONFIG_CC_OPTIMIZE_FOR_SIZE and only "inline" instead of
+ * "__always_inline" this fails to compile because the compiler doesn't notice
+ * for all valid masks (e.g. KMB_PWM_LEADIN_MASK) that they are ok.
+ */
+static __always_inline void keembay_pwm_update_bits(struct keembay_pwm *priv, u32 mask,
 					   u32 val, u32 offset)
 {
 	u32 buff = readl(priv->base + offset);
-- 
2.28.0