linux-kernel - Re: [PATCH] pwm: add support for Intel Low Power Subsystem PWM

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Message-ID: <20140120132814.212929bd@alan.etchedpixels.co.uk>
Date:	Mon, 20 Jan 2014 13:28:14 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Chew Chiau Ee <chiau.ee.chew@...el.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Chew Kean Ho <kean.ho.chew@...el.com>,
	Chang Rebecca Swee Fun <rebecca.swee.fun.chang@...el.com>
Subject: Re: [PATCH] pwm: add support for Intel Low Power Subsystem PWM

> +static int pwm_lpss_config(struct pwm_chip *chip, struct pwm_device *pwm,
> +	int duty_ns, int period_ns)
> +{
> +	struct pwm_lpss_chip *lpwm = to_lpwm(chip);
> +	u8 on_time_div;
> +	unsigned long c = clk_get_rate(lpwm->clk);
> +	unsigned long long base_unit, hz = 1000000000UL;
> +	u32 ctrl;
> +
> +	do_div(hz, period_ns);
> +
> +	/* The equation is: base_unit = ((hz / c) * 65536) + correction */
> +	base_unit = hz * 65536;
> +	do_div(base_unit, c);
> +	base_unit += PWM_DIVISION_CORRECTION;
> +	if (base_unit > PWM_LIMIT)
> +		return -EINVAL;
> +
> +	if (duty_ns <= 0)
> +		duty_ns = 1;
> +	on_time_div = 255 - (255 * duty_ns / period_ns);
> +
> +	ctrl = readl(lpwm->regs + PWM);
> +	ctrl &= ~(PWM_BASE_UNIT_MASK | PWM_ON_TIME_DIV_MASK);
> +	ctrl |= (u16) base_unit << PWM_BASE_UNIT_SHIFT;
> +	ctrl |= on_time_div;
> +	/* request PWM to update on next cycle */
> +	ctrl |= PWM_SW_UPDATE;
> +	writel(ctrl, lpwm->regs + PWM);
> +

Who handles the locking on all these functions. The pwm layer doesn't but
simnply exports stuff like pwm_config() directly to other bits of the
kernel so you are not guaranteed to be called via sysfs.

(This btw looks to be a problem with a pile of the other pwm drivers, and
with the pwm core code which doesn't properly lock its own handling of
pwm->duty_cycle and pwm->period in pwm_config(), nor pwm->polarity in
pwm_set_polarity).

I think the core config methods need some kind of locking ?

Alan
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