Re: [PATCH v7 2/2] pwm: Add clock based PWM output driver

[Uwe Kleine-König]

Hello,

On Sun, Jun 12, 2022 at 06:22:03PM +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@xxxxxxx>
> --
> 
> 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
> Changes in v3:
>  - Use 64bit version of div round up
>  - Address Uwe's review comments:
>    - Reword the limitations to avoid incorrect claims
>    - Move the clk_enabled flag assignment
>    - Drop unnecessary statements
> Changes in v5:
>  - add missed returns
> Changes in v6:
>  - Unprepare the clock on error
>  - Drop redundant limitations points
> Changes in v7:
>  - Rename some variables to be in line with common naming
> 
> --
> It seems like my mailserver wasn't able to send the last review
> response to Uwe's so I'll repeat here that afaict clk.h has all the
> methods stubbed out so compiling without HAVE_CLK is possible.
> Sorry for a long delay with sending this since v6.
> 
> ---
>  drivers/pwm/Kconfig   |  10 +++
>  drivers/pwm/Makefile  |   1 +
>  drivers/pwm/pwm-clk.c | 141 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 152 insertions(+)
>  create mode 100644 drivers/pwm/pwm-clk.c
> 
> diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> index 904de8d61828..60d13a949bc5 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 5c08bdb817b4..7bf1a29f02b8 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..357d0c50dedd
> --- /dev/null
> +++ b/drivers/pwm/pwm-clk.c
> @@ -0,0 +1,141 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Clock based PWM controller
> + *
> + * Copyright (c) 2021 Nikita Travkin <nikita@xxxxxxx>
> + *
> + * This is an "adapter" driver that allows PWM consumers to use
> + * system clocks with duty cycle control as PWM outputs.
> + *
> + * Limitations:
> + * - Due to the fact that exact behavior depends on the underlying
> + *   clock driver, various limitations are possible.
> + * - Underlying clock may not be able to give 0% or 100% duty cycle
> + *   (constant off or on), exact behavior will depend on the clock.
> + * - When the PWM is disabled, the clock will be disabled as well,
> + *   line state will depend on the clock.
> + * - The clk API doesn't expose the necessary calls to implement
> + *   .get_state().
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/math64.h>
> +#include <linux/err.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/clk.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 *chip, struct pwm_device *pwm,
> +			 const struct pwm_state *state)
> +{
> +	struct pwm_clk_chip *pcchip = to_pwm_clk_chip(chip);
> +	int ret;
> +	u32 rate;
> +	u64 period = state->period;
> +	u64 duty_cycle = state->duty_cycle;
> +
> +	if (!state->enabled) {
> +		if (pwm->state.enabled) {
> +			clk_disable(pcchip->clk);
> +			pcchip->clk_enabled = false;
> +		}
> +		return 0;
> +	} else if (!pwm->state.enabled) {
> +		ret = clk_enable(pcchip->clk);
> +		if (ret)
> +			return ret;
> +		pcchip->clk_enabled = true;
> +	}

Maybe point out here that this introduces a glitch that cannot be
prevented. Something like:

	/*
	 * We have to enable the clk before setting the rate and
	 * duty_cycle, that however results in a window where the clk is
	 * on with a (potentially) different setting. Also setting
	 * period and duty_cycle are two separate calls, so that
	 * probably isn't atomic either.
	 */

> +	rate = DIV64_U64_ROUND_UP(NSEC_PER_SEC, period);
> +	ret = clk_set_rate(pcchip->clk, rate);
> +	if (ret)
> +		return ret;
> +
> +	if (state->polarity == PWM_POLARITY_INVERSED)
> +		duty_cycle = period - duty_cycle;
> +
> +	return clk_set_duty_cycle(pcchip->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 *pcchip;
> +	int ret;
> +
> +	pcchip = devm_kzalloc(&pdev->dev, sizeof(*pcchip), GFP_KERNEL);
> +	if (!pcchip)
> +		return -ENOMEM;
> +
> +	pcchip->clk = devm_clk_get(&pdev->dev, NULL);

You can use devm_clk_get_prepared() here and drop the clk_prepare()
below and the clk_unprepare in .remove().

> +	if (IS_ERR(pcchip->clk))
> +		return dev_err_probe(&pdev->dev, PTR_ERR(pcchip->clk),
> +				     "Failed to get clock\n");
> +
> +	pcchip->chip.dev = &pdev->dev;
> +	pcchip->chip.ops = &pwm_clk_ops;
> +	pcchip->chip.npwm = 1;
> +
> +	ret = clk_prepare(pcchip->clk);
> +	if (ret < 0)
> +		return dev_err_probe(&pdev->dev, ret, "Failed to prepare clock\n");
> +
> +	ret = pwmchip_add(&pcchip->chip);
> +	if (ret < 0) {
> +		clk_unprepare(pcchip->clk);
> +		return dev_err_probe(&pdev->dev, ret, "Failed to add pwm chip\n");
> +	}
> +
> +	platform_set_drvdata(pdev, pcchip);
> +	return 0;
> +}
> +
> +static int pwm_clk_remove(struct platform_device *pdev)
> +{
> +	struct pwm_clk_chip *pcchip = platform_get_drvdata(pdev);
> +
> +	pwmchip_remove(&pcchip->chip);
> +
> +	if (pcchip->clk_enabled)
> +		clk_disable(pcchip->clk);
> +
> +	clk_unprepare(pcchip->clk);
> +
> +	return 0;
> +}
> +
> +static const struct of_device_id pwm_clk_dt_ids[] = {
> +	{ .compatible = "clk-pwm", },
> +	{ /* sentinel */ }
> +};
> +MODULE_DEVICE_TABLE(of, pwm_clk_dt_ids);
> +
> +static struct platform_driver pwm_clk_driver = {
> +	.driver = {
> +		.name = "pwm-clk",
> +		.of_match_table = pwm_clk_dt_ids,
> +	},
> +	.probe = pwm_clk_probe,
> +	.remove = pwm_clk_remove,
> +};
> +module_platform_driver(pwm_clk_driver);
> +
> +MODULE_ALIAS("platform:pwm-clk");
> +MODULE_AUTHOR("Nikita Travkin <nikita@xxxxxxx>");
> +MODULE_DESCRIPTION("Clock based PWM driver");
> +MODULE_LICENSE("GPL");

Best regards
Uwe

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

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