linux-kernel - Re: [PATCH 1/2] pwm: pca9685: fix period change with same duty cycle

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Message-ID: <20170118105650.GL18989@ulmo.ba.sec>
Date:   Wed, 18 Jan 2017 11:56:50 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stable@...r.kernel.org,
        Florian Vaussard <florian.vaussard@...g-vd.ch>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH 1/2] pwm: pca9685: fix period change with same duty cycle

On Tue, Dec 13, 2016 at 04:52:50PM +0100, Clemens Gruber wrote:
> When first implementing support for changing the output frequency, an
> optimization was added to continue the PWM after changing the prescaler
> without having to reprogram the ON and OFF registers for the duty cycle,
> in case the duty cycle stayed the same.
> This was flawed, because we compared the absolute value of the duty
> cycle in nanoseconds instead of the ratio to the period.
> 
> Fix the problem by removing the shortcut.
> 
> Fixes: 01ec8472009c9 ("pwm-pca9685: Support changing the output frequency")
> Cc: <stable@...r.kernel.org> # v4.3+
> Signed-off-by: Clemens Gruber <clemens.gruber@...ruber.com>
> ---
>  drivers/pwm/pwm-pca9685.c | 11 -----------
>  1 file changed, 11 deletions(-)

Mika, Andy, can you guys help review this and 2/2?

Thanks,
Thierry

> 
> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 117fccf..01a6a83 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -65,7 +65,6 @@
>  #define PCA9685_MAXCHAN		0x10
>  
>  #define LED_FULL		(1 << 4)
> -#define MODE1_RESTART		(1 << 7)
>  #define MODE1_SLEEP		(1 << 4)
>  #define MODE2_INVRT		(1 << 4)
>  #define MODE2_OUTDRV		(1 << 2)
> @@ -117,16 +116,6 @@ static int pca9685_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  			udelay(500);
>  
>  			pca->period_ns = period_ns;
> -
> -			/*
> -			 * If the duty cycle did not change, restart PWM with
> -			 * the same duty cycle to period ratio and return.
> -			 */
> -			if (duty_ns == pca->duty_ns) {
> -				regmap_update_bits(pca->regmap, PCA9685_MODE1,
> -						   MODE1_RESTART, 0x1);
> -				return 0;
> -			}
>  		} else {
>  			dev_err(chip->dev,
>  				"prescaler not set: period out of bounds!\n");
> -- 
> 2.10.2
> 

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