Re: [PATCH 5/8] pwm: Add support for Azoteq IQS620A PWM generator

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hello Jeff,

On Sun, Oct 20, 2019 at 11:11:20PM -0500, Jeff LaBundy wrote:
> This patch adds support for the Azoteq IQS620A, capable of generating
> a 1-kHz PWM output with duty cycle between 0.4% and 100% (inclusive).
> 
> Signed-off-by: Jeff LaBundy <jeff@labundy.com>
> ---
>  drivers/pwm/Kconfig       |  10 +++
>  drivers/pwm/Makefile      |   1 +
>  drivers/pwm/pwm-iqs620a.c | 167 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 178 insertions(+)
>  create mode 100644 drivers/pwm/pwm-iqs620a.c
> 
> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> index e3a2518..712445e 100644
> --- a/drivers/pwm/Kconfig
> +++ b/drivers/pwm/Kconfig
> @@ -222,6 +222,16 @@ config PWM_IMX_TPM
>  	  To compile this driver as a module, choose M here: the module
>  	  will be called pwm-imx-tpm.
>  
> +config PWM_IQS620A
> +	tristate "Azoteq IQS620A PWM support"
> +	depends on MFD_IQS62X

This is only a runtime dependency if I'm not mistaken, so it would be
great to have

	depends on MFD_IQS62X || COMPILE_TEST
	depends on REGMAP

here.

> +	help
> +	  Generic PWM framework driver for the Azoteq IQS620A multi-function
> +	  sensor.
> +
> +	  To compile this driver as a module, choose M here: the module will
> +	  be called pwm-iqs620a.
> +
>  config PWM_JZ4740
>  	tristate "Ingenic JZ47xx PWM support"
>  	depends on MACH_INGENIC
> diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
> index 26326ad..27c9bfa 100644
> --- a/drivers/pwm/Makefile
> +++ b/drivers/pwm/Makefile
> @@ -20,6 +20,7 @@ obj-$(CONFIG_PWM_IMG)		+= pwm-img.o
>  obj-$(CONFIG_PWM_IMX1)		+= pwm-imx1.o
>  obj-$(CONFIG_PWM_IMX27)		+= pwm-imx27.o
>  obj-$(CONFIG_PWM_IMX_TPM)	+= pwm-imx-tpm.o
> +obj-$(CONFIG_PWM_IQS620A)	+= pwm-iqs620a.o
>  obj-$(CONFIG_PWM_JZ4740)	+= pwm-jz4740.o
>  obj-$(CONFIG_PWM_LP3943)	+= pwm-lp3943.o
>  obj-$(CONFIG_PWM_LPC18XX_SCT)	+= pwm-lpc18xx-sct.o
> diff --git a/drivers/pwm/pwm-iqs620a.c b/drivers/pwm/pwm-iqs620a.c
> new file mode 100644
> index 0000000..6451eb1
> --- /dev/null
> +++ b/drivers/pwm/pwm-iqs620a.c
> @@ -0,0 +1,167 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Azoteq IQS620A PWM Generator
> + *
> + * Copyright (C) 2019
> + * Author: Jeff LaBundy <jeff@labundy.com>
> + */
> +
> +#include <linux/device.h>
> +#include <linux/kernel.h>
> +#include <linux/mfd/iqs62x.h>
> +#include <linux/module.h>
> +#include <linux/platform_device.h>
> +#include <linux/pwm.h>
> +#include <linux/regmap.h>
> +#include <linux/slab.h>
> +
> +#define IQS620_PWR_SETTINGS			0xD2
> +#define IQS620_PWR_SETTINGS_PWM_OUT		BIT(7)
> +
> +#define IQS620_PWM_DUTY_CYCLE			0xD8
> +
> +#define IQS620_PWM_PERIOD_NS			1000000
> +
> +struct iqs620_pwm_private {
> +	struct iqs62x_core *iqs62x;
> +	struct pwm_chip chip;
> +	struct notifier_block notifier;
> +	bool ready;

This is always true, so you can drop it.

> +};
> +
> +static int iqs620_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> +			    struct pwm_state *state)

Since

	71523d1812ac ("pwm: Ensure pwm_apply_state() doesn't modify the state argument")

this isn't the right prototype.

> +{
> +	struct iqs620_pwm_private *iqs620_pwm;
> +	struct iqs62x_core *iqs62x;
> +	int error;
> +	int duty_calc = state->duty_cycle * 256 / IQS620_PWM_PERIOD_NS - 1;
> +	u8 duty_clamp = clamp(duty_calc, 0, 0xFF);
> +
> +	iqs620_pwm = container_of(chip, struct iqs620_pwm_private, chip);
> +	iqs62x = iqs620_pwm->iqs62x;
> +
> +	error = regmap_write(iqs62x->map, IQS620_PWM_DUTY_CYCLE, duty_clamp);
> +	if (error)
> +		return error;
> +
> +	state->period = IQS620_PWM_PERIOD_NS;
> +	state->duty_cycle = (duty_clamp + 1) * IQS620_PWM_PERIOD_NS / 256;

This suggests that if the value in the IQS620_PWM_DUTY_CYCLE is 0 the
duty cycle is 1/256 ms with a period of 1 ms and the output cannot be
constant inactive. If this is right please add a paragraph in the
driver's comment at the top:

	* Limitations:
	* - The hardware cannot generate a 0% duty cycle

(Please stick to this format, other drivers use it, too.)

How does the hardware behave on changes? For example you're first
committing the duty cycle and then on/off. Can it happen that between

	pwm_apply_state(pwm, { .duty_cycle = 3900, .period = 1000000, .enabled = true)
	...
	pwm_apply_state(pwm, { .duty_cycle = 1000000, .period = 1000000, .enabled = false)

the output is active for longer than 4 µs because the iqs620_pwm_apply
function is preempted between the two register writes and so we already
have .duty_cycle = 1000000 but still .enabled = true in the hardware?

Does a change complete the currently running period? Does disabling
complete the currently running period? If so, does regmap_update_bits
block until the new setting is active?

The .apply function fails to check for .pwm_polarity. You want something
like:

	if (state->polarity != PWM_POLARITY_NORMAL)
		return -ENOTSUPP;

(That's what pwm-rcar and the core (in the absence of .set_polarity for
old-style drivers) are using. @Thierry: It would be great to fix the
vaule that should be returned in this case. pwm-lpss and sifive use
-EINVAL.)

> +	return regmap_update_bits(iqs62x->map, IQS620_PWR_SETTINGS,
> +				  IQS620_PWR_SETTINGS_PWM_OUT,
> +				  state->enabled ? 0xFF : 0);
> +}
> +
> +static int iqs620_pwm_notifier(struct notifier_block *notifier,
> +			       unsigned long event_flags, void *context)
> +{
> +	struct iqs620_pwm_private *iqs620_pwm;
> +	struct pwm_state state;
> +	int error;
> +
> +	iqs620_pwm = container_of(notifier, struct iqs620_pwm_private,
> +				  notifier);
> +
> +	if (!iqs620_pwm->ready || !(event_flags & BIT(IQS62X_EVENT_SYS_RESET)))
> +		return NOTIFY_DONE;
> +
> +	pwm_get_state(&iqs620_pwm->chip.pwms[0], &state);
> +
> +	error = iqs620_pwm_apply(&iqs620_pwm->chip,
> +				 &iqs620_pwm->chip.pwms[0], &state);
> +	if (error) {
> +		dev_err(iqs620_pwm->chip.dev,
> +			"Failed to re-initialize device: %d\n", error);
> +		return NOTIFY_BAD;
> +	}
> +
> +	return NOTIFY_OK;

So the PWM can loose it's state sometimes? When does that happen?

> +}
> +
> +static void iqs620_pwm_notifier_unregister(void *context)
> +{
> +	struct iqs620_pwm_private *iqs620_pwm = context;
> +	int error;
> +
> +	error = blocking_notifier_chain_unregister(&iqs620_pwm->iqs62x->nh,
> +						   &iqs620_pwm->notifier);
> +	if (error)
> +		dev_err(iqs620_pwm->chip.dev,
> +			"Failed to unregister notifier: %d\n", error);
> +}
> +
> +static const struct pwm_ops iqs620_pwm_ops = {
> +	.apply	= iqs620_pwm_apply,

Please implement a .get_state callback.

> +	.owner	= THIS_MODULE,
> +};
> +
> +static int iqs620_pwm_probe(struct platform_device *pdev)
> +{
> +	struct iqs620_pwm_private *iqs620_pwm;
> +	int error;
> +
> +	iqs620_pwm = devm_kzalloc(&pdev->dev, sizeof(*iqs620_pwm), GFP_KERNEL);
> +	if (!iqs620_pwm)
> +		return -ENOMEM;
> +
> +	platform_set_drvdata(pdev, iqs620_pwm);
> +	iqs620_pwm->iqs62x = dev_get_drvdata(pdev->dev.parent);
> +
> +	iqs620_pwm->chip.dev = &pdev->dev;
> +	iqs620_pwm->chip.ops = &iqs620_pwm_ops;
> +	iqs620_pwm->chip.base = -1;
> +	iqs620_pwm->chip.npwm = 1;
> +
> +	iqs620_pwm->notifier.notifier_call = iqs620_pwm_notifier;
> +	error = blocking_notifier_chain_register(&iqs620_pwm->iqs62x->nh,
> +						 &iqs620_pwm->notifier);
> +	if (error) {
> +		dev_err(&pdev->dev, "Failed to register notifier: %d\n", error);
> +		return error;
> +	}
> +
> +	error = devm_add_action_or_reset(&pdev->dev,
> +					 iqs620_pwm_notifier_unregister,
> +					 iqs620_pwm);

I wonder if this is safe. If in iqs620_pwm_notifier_unregister()
unregistering of the notifier goes wrong (not sure when this can happen)
the memory behind iqs620_pwm goes away. Then later iqs620_pwm_notifier
might be called trying to use *iqs620_pwm ...

> +	if (error) {
> +		dev_err(&pdev->dev, "Failed to add action: %d\n", error);
> +		return error;
> [...]
> 
> +static struct platform_driver iqs620_pwm_platform_driver = {
> +	.driver = {
> +		.name	= IQS620_DRV_NAME_PWM,
> +	},
> +	.probe		= iqs620_pwm_probe,
> +	.remove		= iqs620_pwm_remove,
> +};

I'm not a big fan of aligning the = in struct initializers. The downside
is that if you later add

	.prevent_deferred_probe = true,

you either have to touch all (otherwise unrelated) lines to realign
which adds churn, or the structure is only partially aligned which looks
ugly. That's why I stick to a single space before the =.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |