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

[Uwe Kleine-König]

[Cc: += linux-pwm which I forgot for the initial submission]

Hello,

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@xxxxxxxxx>
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> ---
> 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?

Note, this patch

Fixes: cdbea243f419 ("pwm: Add PWM driver for Intel Keem Bay")

so this isn't critical for v5.10.

@thierry: If this is ok for you and Vijayakannan, you can squash this
into the original commit.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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