Re: [PATCH] pwm: samsung: Fix a bit test

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello Dan,

On Tue, Oct 17, 2023 at 05:04:08PM +0300, Dan Carpenter wrote:
> This code has two problems.  First, it passes the wrong bit parameter to
> test_bit().  Second, it mixes using PWMF_REQUESTED in test_bit() and in
> open coded bit tests.
> 
> The test_bit() function takes a bit number.  In other words,
> "if (test_bit(0, &flags))" is the equivalent of "if (flags & (1 << 0))".
> Passing (1 << 0) to test_bit() is like writing BIT(BIT(0)).  It's a
> double shift bug.
> 
> In pwm_samsung_resume() these issues mean that the flag is never set and
> the function is essentially a no-op.
> 
> Fixes: 4c9548d24c0d ("pwm: samsung: Put per-channel data into driver data")
> Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
> ---
> From static analysis and not tested.
> 
>  drivers/pwm/pwm-samsung.c | 2 +-
>  include/linux/pwm.h       | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
> index 10fe2c13cd80..acf4a0d8d990 100644
> --- a/drivers/pwm/pwm-samsung.c
> +++ b/drivers/pwm/pwm-samsung.c
> @@ -630,7 +630,7 @@ static int pwm_samsung_resume(struct device *dev)
>  		struct pwm_device *pwm = &chip->pwms[i];
>  		struct samsung_pwm_channel *chan = &our_chip->channel[i];
>  
> -		if (!(pwm->flags & PWMF_REQUESTED))
> +		if (!test_bit(PWMF_REQUESTED, &pwm->flags))
>  			continue;
>  
>  		if (our_chip->variant.output_mask & BIT(i))
> diff --git a/include/linux/pwm.h b/include/linux/pwm.h
> index e3b437587b32..3eee5bf367fb 100644
> --- a/include/linux/pwm.h
> +++ b/include/linux/pwm.h
> @@ -41,8 +41,8 @@ struct pwm_args {
>  };
>  
>  enum {
> -	PWMF_REQUESTED = 1 << 0,
> -	PWMF_EXPORTED = 1 << 1,
> +	PWMF_REQUESTED = 0,
> +	PWMF_EXPORTED  = 1,

I'd want s/  / / here. Or even not assign explicit values at all?

>  };
>  
>  /*

I'd say these are two separate issues, with the one in pwm-samsung being
bad and the one in <linux/pwm.h> "only" ugly.

I wonder how I could get the samsung part wrong. All current usages of
PMWF_REQUESTED (and also PWMF_EXPORTED) use test_bit (et al). Grepping
through history pwm-pca9685.c got this wrong in a similar way for some
time, but otherwise it was always used correctly.

The definition of the flags in <linux/pwm.h> is ugly since 
f051c466cf69 ("pwm: Allow chips to support multiple PWMs") from 2011!

@Dan: Would you split the patch in two please?

Thanks for catching that!

Best regards
Uwe

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

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