linux-kernel - Re: [PATCH] pwm: Add clk enable/disable for pwm_samsung_enable/pwm_samsung

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Date:	Wed, 15 Apr 2015 10:34:07 +0200
From:	Sjoerd Simons <sjoerd.simons@...labora.co.uk>
To:	Anand Moon <linux.amoon@...il.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH] pwm: Add clk enable/disable for
 pwm_samsung_enable/pwm_samsung_disable

On Wed, 2015-04-15 at 03:35 +0930, Anand Moon wrote:
> diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
> index 3e9b583..b579753 100644
> --- a/drivers/pwm/pwm-samsung.c
> +++ b/drivers/pwm/pwm-samsung.c
> @@ -247,6 +247,7 @@ static int pwm_samsung_enable(struct pwm_chip *chip, struct pwm_device *pwm)
>  	tcon &= ~TCON_MANUALUPDATE(tcon_chan);
>  	tcon |= TCON_START(tcon_chan) | TCON_AUTORELOAD(tcon_chan);
>  	writel(tcon, our_chip->base + REG_TCON);
> +	clk_prepare_enable(our_chip->base_clk);
>  
>  	spin_unlock_irqrestore(&samsung_pwm_lock, flags);
>  
> @@ -265,6 +266,7 @@ static void pwm_samsung_disable(struct pwm_chip *chip, struct pwm_device *pwm)
>  	tcon = readl(our_chip->base + REG_TCON);
>  	tcon &= ~TCON_AUTORELOAD(tcon_chan);
>  	writel(tcon, our_chip->base + REG_TCON);
> +	clk_disable_unprepare(our_chip->base_clk);
>  
>  	spin_unlock_irqrestore(&samsung_pwm_lock, flags);
>  }

As far as i can tell this code doesn't have any effect. 

clk_enable is refcounted, so the clock will stay enabled for as long as
the driver is loaded (as it's enabled in _probe). Your code above just
raises and lowers the clocks enabled refcount, but won't actually ever
cause it to be disabled.

With respect to trying to disabling the clocks on pwm_disable, that will
need some more work to ensure the output signal has the expected level
when you turn of the clock. Specifically, when disabling from a non-100%
duty state the driver relies on the PWM turning the output signal low at
the end of a duty cycle. However if you turn off the clock at the start
of a duty cycle while the output signal is still high it will
unexpectedly remain high.

-- 
Sjoerd Simons <sjoerd.simons@...labora.co.uk>
Collabora Ltd.
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