linux-kernel - Re: [PATCH 2/2] pwm: pca9685: fix prescaler initialization

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Date:   Wed, 18 Jan 2017 11:57:35 +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 2/2] pwm: pca9685: fix prescaler initialization

On Tue, Dec 13, 2016 at 04:52:51PM +0100, Clemens Gruber wrote:
> Until now, we assumed that the period is the hardware default of 1/200Hz
> at probe time, but if the period was changed and the user reboots, this
> assumption is wrong.
> 
> Solution: Check if the prescaler is set to the hardware default. If not,
> reprogram the prescaler at first configuration.
> 
> Cc: <stable@...r.kernel.org> # v4.3+
> Signed-off-by: Clemens Gruber <clemens.gruber@...ruber.com>
> ---
>  drivers/pwm/pwm-pca9685.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)

Cc'ing Mika and Andy and quoting verbatim for review.

Thierry

> diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
> index 01a6a83..efc657e 100644
> --- a/drivers/pwm/pwm-pca9685.c
> +++ b/drivers/pwm/pwm-pca9685.c
> @@ -55,6 +55,7 @@
>  #define PCA9685_PRESCALE	0xFE
>  
>  #define PCA9685_PRESCALE_MIN	0x03	/* => max. frequency of 1526 Hz */
> +#define PCA9685_PRESCALE_DEF	0x1E	/* => default frequency of 200 Hz */
>  #define PCA9685_PRESCALE_MAX	0xFF	/* => min. frequency of 24 Hz */
>  
>  #define PCA9685_COUNTER_RANGE	4096
> @@ -289,8 +290,8 @@ static int pca9685_pwm_probe(struct i2c_client *client,
>  				const struct i2c_device_id *id)
>  {
>  	struct pca9685 *pca;
> +	int prescale, mode2;
>  	int ret;
> -	int mode2;
>  
>  	pca = devm_kzalloc(&client->dev, sizeof(*pca), GFP_KERNEL);
>  	if (!pca)
> @@ -304,10 +305,15 @@ static int pca9685_pwm_probe(struct i2c_client *client,
>  		return ret;
>  	}
>  	pca->duty_ns = 0;
> -	pca->period_ns = PCA9685_DEFAULT_PERIOD;
>  
>  	i2c_set_clientdata(client, pca);
>  
> +	regmap_read(pca->regmap, PCA9685_PRESCALE, &prescale);
> +	if (prescale == PCA9685_PRESCALE_DEF)
> +		pca->period_ns = PCA9685_DEFAULT_PERIOD;
> +	else
> +		pca->period_ns = 0;
> +
>  	regmap_read(pca->regmap, PCA9685_MODE2, &mode2);
>  
>  	if (device_property_read_bool(&client->dev, "invert"))
> -- 
> 2.10.2
> 

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