Re: [PATCH v2 2/2] pwm: Add clock based PWM output driver

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

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Hello,

On Mon, Dec 13, 2021 at 08:03:35PM +0500, Nikita Travkin wrote:
> Some systems have clocks exposed to external devices. If the clock
> controller supports duty-cycle configuration, such clocks can be used as
> pwm outputs. In fact PWM and CLK subsystems are interfaced with in a
> similar way and an "opposite" driver already exists (clk-pwm). Add a
> driver that would enable pwm devices to be used via clk subsystem.
> 
> Signed-off-by: Nikita Travkin <nikita@trvn.ru>
> --
> 
> Changes in v2:
>  - Address Uwe's review comments:
>    - Round set clk rate up
>    - Add a description with limitations of the driver
>    - Disable and unprepare clock before removing pwmchip
> ---
>  drivers/pwm/Kconfig   |  10 +++
>  drivers/pwm/Makefile  |   1 +
>  drivers/pwm/pwm-clk.c | 143 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 154 insertions(+)
>  create mode 100644 drivers/pwm/pwm-clk.c
> 
> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> index 21e3b05a5153..daa2491a4054 100644
> --- a/drivers/pwm/Kconfig
> +++ b/drivers/pwm/Kconfig
> @@ -140,6 +140,16 @@ config PWM_BRCMSTB
>  	  To compile this driver as a module, choose M Here: the module
>  	  will be called pwm-brcmstb.c.
>  
> +config PWM_CLK
> +	tristate "Clock based PWM support"
> +	depends on HAVE_CLK || COMPILE_TEST
> +	help
> +	  Generic PWM framework driver for outputs that can be
> +	  muxed to clocks.
> +
> +	  To compile this driver as a module, choose M here: the module
> +	  will be called pwm-clk.
> +
>  config PWM_CLPS711X
>  	tristate "CLPS711X PWM support"
>  	depends on ARCH_CLPS711X || COMPILE_TEST
> diff --git a/drivers/pwm/Makefile b/drivers/pwm/Makefile
> index 708840b7fba8..4a860103c470 100644
> --- a/drivers/pwm/Makefile
> +++ b/drivers/pwm/Makefile
> @@ -10,6 +10,7 @@ obj-$(CONFIG_PWM_BCM_KONA)	+= pwm-bcm-kona.o
>  obj-$(CONFIG_PWM_BCM2835)	+= pwm-bcm2835.o
>  obj-$(CONFIG_PWM_BERLIN)	+= pwm-berlin.o
>  obj-$(CONFIG_PWM_BRCMSTB)	+= pwm-brcmstb.o
> +obj-$(CONFIG_PWM_CLK)		+= pwm-clk.o
>  obj-$(CONFIG_PWM_CLPS711X)	+= pwm-clps711x.o
>  obj-$(CONFIG_PWM_CRC)		+= pwm-crc.o
>  obj-$(CONFIG_PWM_CROS_EC)	+= pwm-cros-ec.o
> diff --git a/drivers/pwm/pwm-clk.c b/drivers/pwm/pwm-clk.c
> new file mode 100644
> index 000000000000..55fd320b9c19
> --- /dev/null
> +++ b/drivers/pwm/pwm-clk.c
> @@ -0,0 +1,143 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Clock based PWM controller
> + *
> + * Copyright (c) 2021 Nikita Travkin <nikita@trvn.ru>
> + *
> + * This is an "adapter" driver that allows PWM consumers to use
> + * system clocks with duty cycle control as PWM outputs.
> + *
> + * Limitations:
> + * - There is no way to atomically set both clock rate and
> + *   duty-cycle so glitches are possible when new pwm state
> + *   is applied.
> + * - Period depends on the underlying clock driver and,
> + *   in general, not guaranteed.
> + * - Underlying clock may not be able to give 100%
> + *   duty cycle (constant on) and only set the closest
> + *   possible duty cycle. (e.g. 99.9%)

What about 0%?

 - Periods are not completed on changes in general.
 - Behaviour on disable depends on the underlaying clk, don't assume it
   to provide the inactive level.

> + */
> +
> +#include <linux/clk.h>
> +#include <linux/err.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/pwm.h>
> +
> +struct pwm_clk_chip {
> +	struct pwm_chip chip;
> +	struct clk *clk;
> +	bool clk_enabled;
> +};
> +
> +#define to_pwm_clk_chip(_chip) container_of(_chip, struct pwm_clk_chip, chip)
> +
> +static int pwm_clk_apply(struct pwm_chip *pwm_chip, struct pwm_device *pwm,
> +			 const struct pwm_state *state)
> +{
> +	struct pwm_clk_chip *chip = to_pwm_clk_chip(pwm_chip);
> +	int ret;
> +	u32 rate;
> +	u64 period = state->period;
> +	u64 duty_cycle = state->duty_cycle;
> +
> +	if (!state->enabled) {
> +		if (pwm->state.enabled) {
> +			clk_disable(chip->clk);
> +			chip->clk_enabled = false;
> +		}
> +		return 0;
> +	} else if (!pwm->state.enabled) {
> +		ret = clk_enable(chip->clk);
> +		chip->clk_enabled = true;
> +		if (ret)
> +			return ret;

if clk_enable() failed better don't set chip->clk_enabled = true;

> +	}
> +
> +	rate = DIV_ROUND_UP(NSEC_PER_SEC, period);
> +	ret = clk_set_rate(chip->clk, rate);
> +	if (ret)
> +		return ret;
> +
> +	if (state->polarity == PWM_POLARITY_INVERSED)
> +		duty_cycle = period - duty_cycle;
> +
> +	ret = clk_set_duty_cycle(chip->clk, duty_cycle, period);
> +	if (ret)
> +		return ret;
> +
> +	return ret;

This can be simplified to

	return clk_set_duty_cycle(chip->clk, duty_cycle, period);

> +}
> +
> +static const struct pwm_ops pwm_clk_ops = {
> +	.apply = pwm_clk_apply,
> +	.owner = THIS_MODULE,
> +};
> +
> +static int pwm_clk_probe(struct platform_device *pdev)
> +{
> +	struct pwm_clk_chip *chip;
> +	int ret;
> +
> +	chip = devm_kzalloc(&pdev->dev, sizeof(*chip), GFP_KERNEL);
> +	if (!chip)
> +		return -ENOMEM;
> +
> +	chip->clk = devm_clk_get(&pdev->dev, NULL);
> +	if (IS_ERR(chip->clk))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(chip->clk), "Failed to get clock\n");
> +
> +	chip->chip.dev = &pdev->dev;
> +	chip->chip.ops = &pwm_clk_ops;
> +	chip->chip.of_xlate = of_pwm_xlate_with_flags;
> +	chip->chip.of_pwm_n_cells = 2;

I'd just skip those two assignments. These are the default, anyhow.
(Assuming you have #pwm-cells = <2> in the device tree.)

> +	chip->chip.npwm = 1;
> +
> +	ret = clk_prepare(chip->clk);
> +	if (ret < 0)
> +		dev_err_probe(&pdev->dev, ret, "Failed to prepare clock\n");
> +
> +	ret = pwmchip_add(&chip->chip);
> +	if (ret < 0)
> +		dev_err_probe(&pdev->dev, ret, "Failed to add pwm chip\n");
> +
> +	platform_set_drvdata(pdev, chip);
> +	return 0;
> +}

Best regards
Uwe

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

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