Re: [PATCH v2 4/8] pwm: Add STM32 LPTimer PWM driver

[Thierry Reding <thierry.reding@gmail.com>]

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On Wed, Jun 21, 2017 at 04:30:11PM +0200, Fabrice Gasnier wrote:
> Add support for single PWM channel on Low-Power Timer, that can be
> found on some STM32 platforms.
> 
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
> ---
> Changes in v2:
> - s/Low Power/Low-Power
> - update few comment lines
> ---
>  drivers/pwm/Kconfig        |  10 +++
>  drivers/pwm/Makefile       |   1 +
>  drivers/pwm/pwm-stm32-lp.c | 216 +++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 227 insertions(+)
>  create mode 100644 drivers/pwm/pwm-stm32-lp.c
> 
> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> index 313c107..7cb982b 100644
> --- a/drivers/pwm/Kconfig
> +++ b/drivers/pwm/Kconfig
> @@ -417,6 +417,16 @@ config PWM_STM32
>  	  To compile this driver as a module, choose M here: the module
>  	  will be called pwm-stm32.
>  
> +config PWM_STM32_LP
> +	tristate "STMicroelectronics STM32 PWM LP"
> +	depends on MFD_STM32_LPTIMER || COMPILE_TEST
> +	help
> +	  Generic PWM framework driver for STMicroelectronics STM32 SoCs
> +	  with Low-Power Timer (LPTIM).
> +
> +	  To compile this driver as a module, choose M here: the module
> +	  will be called pwm-stm32-lp.
> +
>  config PWM_STMPE
>  	bool "STMPE expander PWM export"
>  	depends on MFD_STMPE
> diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
> index 93da1f7..a3a4bee 100644
> --- a/drivers/pwm/Makefile
> +++ b/drivers/pwm/Makefile
> @@ -40,6 +40,7 @@ obj-$(CONFIG_PWM_SAMSUNG)	+= pwm-samsung.o
>  obj-$(CONFIG_PWM_SPEAR)		+= pwm-spear.o
>  obj-$(CONFIG_PWM_STI)		+= pwm-sti.o
>  obj-$(CONFIG_PWM_STM32)		+= pwm-stm32.o
> +obj-$(CONFIG_PWM_STM32_LP)	+= pwm-stm32-lp.o
>  obj-$(CONFIG_PWM_STMPE)		+= pwm-stmpe.o
>  obj-$(CONFIG_PWM_SUN4I)		+= pwm-sun4i.o
>  obj-$(CONFIG_PWM_TEGRA)		+= pwm-tegra.o
> diff --git a/drivers/pwm/pwm-stm32-lp.c b/drivers/pwm/pwm-stm32-lp.c
> new file mode 100644
> index 0000000..eb997a8
> --- /dev/null
> +++ b/drivers/pwm/pwm-stm32-lp.c
> @@ -0,0 +1,216 @@
> +/*
> + * STM32 Low-Power Timer PWM driver
> + *
> + * Copyright (C) STMicroelectronics 2017
> + *
> + * Author: Gerald Baeza <gerald.baeza@st.com>
> + *
> + * License terms: GNU General Public License (GPL), version 2
> + *
> + * Inspired by Gerald Baeza's pwm-stm32 driver
> + */
> +
> +#include <linux/bitfield.h>
> +#include <linux/mfd/stm32-lptimer.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/pwm.h>
> +
> +struct stm32_pwm_lp {
> +	struct pwm_chip chip;
> +	struct clk *clk;
> +	struct regmap *regmap;
> +};
> +
> +static inline struct stm32_pwm_lp *to_stm32_pwm_lp(struct pwm_chip *chip)
> +{
> +	return container_of(chip, struct stm32_pwm_lp, chip);
> +}
> +
> +static const u8 prescalers[] = {1, 2, 4, 8, 16, 32, 64, 128};
> +
> +static int stm32_pwm_lp_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> +			      struct pwm_state *state)
> +{
> +	struct stm32_pwm_lp *priv = to_stm32_pwm_lp(chip);
> +	unsigned long long prd, div, dty;
> +	struct pwm_state cstate;
> +	u32 val, mask, cfgr, wavpol, presc = 0;
> +	bool reenable = false;
> +	int ret;
> +
> +	pwm_get_state(pwm, &cstate);
> +
> +	if (!state->enabled) {
> +		if (cstate.enabled) {
> +			/* Disable LP timer */
> +			ret = regmap_write(priv->regmap, STM32_LPTIM_CR, 0);
> +			if (ret)
> +				return ret;
> +			clk_disable(priv->clk);
> +		}
> +		return 0;
> +	}
> +
> +	/* Calculate the period and prescaler value */
> +	div = (unsigned long long)clk_get_rate(priv->clk) * state->period;
> +	do_div(div, NSEC_PER_SEC);
> +	prd = div;
> +	while (div > STM32_LPTIM_MAX_ARR) {
> +		presc++;
> +		if (presc >= ARRAY_SIZE(prescalers)) {
> +			dev_err(priv->chip.dev, "max prescaler exceeded\n");
> +			return -EINVAL;
> +		}
> +		div = prd;
> +		do_div(div, prescalers[presc]);
> +	}
> +	prd = div;
> +
> +	/* Calculate the duty cycle */
> +	dty = prd * state->duty_cycle;
> +	do_div(dty, state->period);
> +
> +	wavpol = FIELD_PREP(STM32_LPTIM_WAVPOL, state->polarity);
> +
> +	if (!cstate.enabled) {
> +		ret = clk_enable(priv->clk);
> +		if (ret)
> +			return ret;
> +	}

Why do you need the checks here? Clock enabled are reference counted, so
you could do the clk_enable() unconditionally.

Speaking of which, I don't see a clk_prepare() anywhere. Doesn't the clk
core warn about clk_enable() being called on a clock that's not been
prepared?

> +
> +	ret = regmap_read(priv->regmap, STM32_LPTIM_CFGR, &cfgr);
> +	if (ret)
> +		goto err;
> +
> +	if ((wavpol != FIELD_GET(STM32_LPTIM_WAVPOL, cfgr)) ||

This looks wrong to me. Looking at the macro definitions, FIELD_PREP()
will store the shifted value in wavpol, but FIELD_GET() will shift the
value before returning, so you will compare an in-register value with
a field value. I don't see how those could ever match (unless they're
0 or the field is at position 0, which isn't the case for WAVPOL).

> +	    (presc != FIELD_GET(STM32_LPTIM_PRESC, cfgr))) {
> +		val = FIELD_PREP(STM32_LPTIM_PRESC, presc) | wavpol;
> +		mask = STM32_LPTIM_PRESC | STM32_LPTIM_WAVPOL;
> +
> +		/* Must disable LP timer to modify CFGR */
> +		ret = regmap_write(priv->regmap, STM32_LPTIM_CR, 0);
> +		if (ret)
> +			goto err;
> +		reenable = true;

The placement of this is somewhat odd. It suggests that it is somehow
related to the disabling of the LP timer, whereas it really isn't.

> +		ret = regmap_update_bits(priv->regmap, STM32_LPTIM_CFGR, mask,
> +					 val);
> +		if (ret)
> +			goto err;
> +	}
> +
> +	if (!cstate.enabled || reenable) {

You have this condition in a couple of places and it's rather difficult
to parse. Maybe this could be simplified a little:

	bool reenable = !cstate.enabled;
	...
	if (...) {
		...
		reenable = true;
		...
	}
	...
	if (reenable) {
		...
	}

> +		/* Must enable LP timer to modify CMP & ARR */
> +		ret = regmap_write(priv->regmap, STM32_LPTIM_CR,
> +				   STM32_LPTIM_ENABLE);
> +		if (ret)
> +			goto err;
> +	}
> +
> +	ret = regmap_write(priv->regmap, STM32_LPTIM_ARR, prd - 1);
> +	if (ret)
> +		goto err;
> +
> +	ret = regmap_write(priv->regmap, STM32_LPTIM_CMP, prd - (1 + dty));
> +	if (ret)
> +		goto err;
> +
> +	/* ensure CMP & ARR registers are properly written */
> +	ret = regmap_read_poll_timeout(priv->regmap, STM32_LPTIM_ISR, val,
> +				       (val & STM32_LPTIM_CMPOK_ARROK),
> +				       100, 1000);
> +	if (ret) {
> +		dev_err(priv->chip.dev, "ARR/CMP registers write issue\n");
> +		goto err;
> +	}
> +	ret = regmap_write(priv->regmap, STM32_LPTIM_ICR,
> +			   STM32_LPTIM_CMPOKCF_ARROKCF);
> +	if (ret)
> +		goto err;
> +
> +	if (!cstate.enabled || reenable) {
> +		/* Start LP timer in continuous mode */
> +		ret = regmap_update_bits(priv->regmap, STM32_LPTIM_CR,
> +					 STM32_LPTIM_CNTSTRT,
> +					 STM32_LPTIM_CNTSTRT);
> +		if (ret) {
> +			regmap_write(priv->regmap, STM32_LPTIM_CR, 0);
> +			goto err;
> +		}
> +	}
> +
> +	return 0;
> +err:
> +	if (!cstate.enabled)
> +		clk_disable(priv->clk);

I think you can drop the clk_disable() here as well.

> +
> +	return ret;
> +}
> +
> +static const struct pwm_ops stm32_pwm_lp_ops = {
> +	.owner = THIS_MODULE,
> +	.apply = stm32_pwm_lp_apply,
> +};

You should implement the .get_state() callback as well, otherwise the
atomic PWM support will be somewhat handicapped.

> +
> +static int stm32_pwm_lp_probe(struct platform_device *pdev)
> +{
> +	struct stm32_lptimer *ddata = dev_get_drvdata(pdev->dev.parent);
> +	struct stm32_pwm_lp *priv;
> +	int ret;
> +
> +	if (IS_ERR_OR_NULL(ddata))
> +		return -EINVAL;

It seems to me like this can never happen. How would you trigger this
condition?

> +
> +	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
> +	if (!priv)
> +		return -ENOMEM;
> +
> +	priv->regmap = ddata->regmap;
> +	priv->clk = ddata->clk;
> +	if (!priv->regmap || !priv->clk)
> +		return -EINVAL;

Likewise for these. the stm32-lptimer driver already checks that these
are valid, which do you need to do it again?

Well, technically you check for !NULL here, whereas stm32-lptimer does
check for IS_ERR(), but neither regmap nor clk looks as though they're
optional, and you won't ever get here if they can't be requested by
stm32-lptimer in the first place.

> +
> +	priv->chip.base = -1;
> +	priv->chip.dev = &pdev->dev;
> +	priv->chip.ops = &stm32_pwm_lp_ops;
> +	priv->chip.npwm = 1;
> +
> +	ret = pwmchip_add(&priv->chip);
> +	if (ret < 0)
> +		return ret;
> +
> +	platform_set_drvdata(pdev, priv);
> +
> +	return 0;
> +}
> +
> +static int stm32_pwm_lp_remove(struct platform_device *pdev)
> +{
> +	struct stm32_pwm_lp *priv = platform_get_drvdata(pdev);
> +
> +	if (pwm_is_enabled(priv->chip.pwms))
> +		pwm_disable(priv->chip.pwms);

It'd be better to use the more idiomatic variant for this:

	for (i = 0; i < priv->chip.npwm; i++)
		if (pwm_is_enabled(priv->chip.npwm))
			pwm_disable(&priv->chip.pwms[i]);

That makes it easier to discern the common pattern and extract a helper,
or move this to the core.

Thierry

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