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

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Message-ID: <20201118180014.GC3552669@ulmo>
Date:   Wed, 18 Nov 2020 19:00:14 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Vijayakannan Ayyathurai <vijayakannan.ayyathurai@...el.com>,
        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: Re: [PATCH RFC] pwm: keembay: Fix build failure with -Os

On Mon, Nov 16, 2020 at 10:08:04AM +0100, Uwe Kleine-König wrote:
> 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(-)

Applied, thanks.

Thierry

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