linux-kernel - Re: [PATCH] pwm: twl: Reliably disable TWL6030 PWMs

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Message-ID: <20160711105516.GD5503@ulmo.ba.sec>
Date:	Mon, 11 Jul 2016 12:55:16 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Paul Kocialkowski <contact@...lk.fr>,
	Axel Lin <axel.lin@...ics.com>,
	Peter Ujfalusi <peter.ujfalusi@...com>
Cc:	linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org,
	Tony Lindgren <tony@...mide.com>, linux-omap@...r.kernel.org
Subject: Re: [PATCH] pwm: twl: Reliably disable TWL6030 PWMs

On Tue, Mar 29, 2016 at 08:55:45PM +0200, Paul Kocialkowski wrote:
> The current TWL6030 code for the TWL PWM driver does not reliably disable the
> PWM output, as tested with LEDs. The previous commit to that driver introduced
> that regression.
> 
> However, it does make sense to disable the PWM clock after resetting the PWM,
> but for some obscure reason, doing it all at once simply doesn't work.
> 
> The TWL6030 datasheet mentions that PWMs have to be disabled in two distinct
> steps. However, clearing the clock enable bit in a second step (after issuing a
> reset first) does not work.
> 
> The only approach that works is the one that was in place before the previous
> commit to the driver. It consists in enabling the PWM clock after issuing a
> reset. This is what TI kernel trees and production code seem to be using.
> 
> However, adding an extra step to disable the PWM clock seems to work reliably,
> despite looking quite odd.
> 
> Signed-off-by: Paul Kocialkowski <contact@...lk.fr>
> ---
>  drivers/pwm/pwm-twl.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)

Axel, Peter,

can you guys comment on this?

Thierry

> 
> diff --git a/drivers/pwm/pwm-twl.c b/drivers/pwm/pwm-twl.c
> index 04f7672..7a993b0 100644
> --- a/drivers/pwm/pwm-twl.c
> +++ b/drivers/pwm/pwm-twl.c
> @@ -269,6 +269,22 @@ static void twl6030_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
>  		goto out;
>  	}
>  
> +	val |= TWL6030_PWM_TOGGLE(pwm->hwpwm, TWL6030_PWMXEN);
> +
> +	ret = twl_i2c_write_u8(TWL6030_MODULE_ID1, val, TWL6030_TOGGLE3_REG);
> +	if (ret < 0) {
> +		dev_err(chip->dev, "%s: Failed to disable PWM\n", pwm->label);
> +		goto out;
> +	}
> +
> +	val &= ~TWL6030_PWM_TOGGLE(pwm->hwpwm, TWL6030_PWMXEN);
> +
> +	ret = twl_i2c_write_u8(TWL6030_MODULE_ID1, val, TWL6030_TOGGLE3_REG);
> +	if (ret < 0) {
> +		dev_err(chip->dev, "%s: Failed to disable PWM\n", pwm->label);
> +		goto out;
> +	}
> +
>  	twl->twl6030_toggle3 = val;
>  out:
>  	mutex_unlock(&twl->mutex);
> -- 
> 2.7.4
> 

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