Re: [PATCHv9 Resend 1/4] pwm: Add Freescale FTM PWM driver support

[Thierry Reding]

Hi,

Sorry for taking so long to get back to you. Things have been quite busy
lately. A few more comments below, but we're getting there.

On Wed, Feb 19, 2014 at 04:38:54PM +0800, Xiubo Li wrote:
[...]
> diff --git a/drivers/pwm/pwm-fsl-ftm.c b/drivers/pwm/pwm-fsl-ftm.c
[...]
> +static unsigned long fsl_pwm_calculate_period(struct fsl_pwm_chip *fpc,
> +					      unsigned long period_ns)
> +{
> +	struct clk *cnt_clk[3];
> +	enum fsl_pwm_clk m0, m1;
> +	unsigned long fix_rate, ext_rate, cycles;
> +
> +	fpc->counter_clk = fpc->sys_clk;
> +	cycles = fsl_pwm_calculate_period_cycles(fpc, period_ns,
> +			FSL_PWM_CLK_SYS);
> +	if (cycles)
> +		return cycles;
> +
> +	cnt_clk[FSL_PWM_CLK_FIX] = devm_clk_get(fpc->chip.dev, "ftm_fix");
> +	if (IS_ERR(cnt_clk[FSL_PWM_CLK_FIX]))
> +		return PTR_ERR(cnt_clk[FSL_PWM_CLK_FIX]);
> +
> +	cnt_clk[FSL_PWM_CLK_EXT] = devm_clk_get(fpc->chip.dev, "ftm_ext");
> +	if (IS_ERR(cnt_clk[FSL_PWM_CLK_EXT]))
> +		return PTR_ERR(cnt_clk[FSL_PWM_CLK_EXT]);
> +
> +	fpc->counter_clk_en = devm_clk_get(fpc->chip.dev, "ftm_cnt_clk_en");
> +	if (IS_ERR(fpc->counter_clk_en))
> +		return PTR_ERR(fpc->counter_clk_en);

You shouldn't do this. You're obtaining a reference to each of these
clocks whenever pwm_config() is called. And devres will only clean those
up after the driver is unbound. Can't you simply keep a reference to
these within struct fsl_pwm_chip?

> +static int fsl_counter_clock_enable(struct fsl_pwm_chip *fpc)
> +{
> +	u32 val;
> +	int ret;
> +
> +	if (fpc->counter_clk_enable++)

This function is always called with the fpc->lock held, so you could
make this much easier by incrementing the .counter_clk_enable field only
at the very end of the function. That way...

> +		return 0;
> +
> +	ret = clk_prepare_enable(fpc->counter_clk);
> +	if (ret) {
> +		fpc->counter_clk_enable--;

... this won't be necessary...

> +		return ret;
> +	}
> +
> +	ret = clk_prepare_enable(fpc->counter_clk_en);
> +	if (ret) {
> +		fpc->counter_clk_enable--;

... and neither will this.

> +static int fsl_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm)
> +{
> +	struct fsl_pwm_chip *fpc = to_fsl_chip(chip);
> +	u32 val;
> +	int ret;
> +
> +	val = readl(fpc->base + FTM_OUTMASK);
> +	val &= ~BIT(pwm->hwpwm);
> +	writel(val, fpc->base + FTM_OUTMASK);
> +
> +	mutex_lock(&fpc->lock);

I think you want to extend the lock to cover the FTM_OUTMASK register
access as well because there could be a race between pwm_enable() and
pwm_disable().

> +	ret = fsl_counter_clock_enable(fpc);
> +	mutex_unlock(&fpc->lock);
> +
> +	return ret;
> +}

Can this function be moved somewhere else so fsl_counter_clock_enable()
and fsl_counter_clock_disable() are grouped together?

> +static void fsl_counter_clock_disable(struct fsl_pwm_chip *fpc)
> +{
> +	u32 val;
> +
> +	if (--fpc->counter_clk_enable)
> +		return;

This is going to break. Consider the case where you call pwm_disable()
on a PWM device and fpc->counter_clk_enable == 1. In that case, this
will decrement counter_clk_enable to 0 and proceed with the remainder of
this function.

Now you call pwm_disable() again. The above will decrement again and
cause fpc->counter_clk_enable to wrap around to UINT_MAX.

So I think a more correct implementation would be:

	/*
	 * already disabled, do nothing (perhaps output warning message
	 * to catch unbalanced calls? )
	 */
	if (fpc->counter_clk_enable == 0)
		return;

	/* there are still users, so can't disable yet */
	if (--fpc->counter_clk_enable > 0)
		return;

	/* no users left, disable clock */

> +static void fsl_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
> +{
> +	struct fsl_pwm_chip *fpc = to_fsl_chip(chip);
> +	u32 val;
> +
> +	val = readl(fpc->base + FTM_OUTMASK);
> +	val |= BIT(pwm->hwpwm);
> +	writel(val, fpc->base + FTM_OUTMASK);
> +
> +	mutex_lock(&fpc->lock);

This lock should also include the access to FTM_OUTMASK above.

> +static int fsl_pwm_probe(struct platform_device *pdev)
> +{
[...]
> +	fpc->sys_clk = devm_clk_get(&pdev->dev, "ftm_sys");
> +	if (IS_ERR(fpc->sys_clk)) {
> +		dev_err(&pdev->dev,
> +				"failed to get \"ftm_sys\" clock\n");

The above easily fits on a single line, no need for the wrapping.

> +		return PTR_ERR(fpc->sys_clk);
> +	}
> +
> +	ret = clk_prepare_enable(fpc->sys_clk);
> +	if (ret)
> +		return ret;
> +
> +	writel(0x00, fpc->base + FTM_CNTIN);
> +	writel(0x00, fpc->base + FTM_OUTINIT);
> +	writel(0xFF, fpc->base + FTM_OUTMASK);
> +	clk_disable_unprepare(fpc->sys_clk);

This looks out of place somehow, perhaps it should be moved off into a
separate function? fsl_pwm_init() perhaps.

Thierry

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