Re: [PATCH v4 2/2] pwm: Add clock based PWM output driver

[Krzysztof Kozlowski]

On 26/01/2022 13:58, 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
> ---
>  drivers/pwm/Kconfig   |  10 +++
>  drivers/pwm/Makefile  |   1 +
>  drivers/pwm/pwm-clk.c | 139 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 150 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..b3bfa12a0e73
> --- /dev/null
> +++ b/drivers/pwm/pwm-clk.c
> @@ -0,0 +1,139 @@
> +// 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:
> + * - Glitches are possible when new pwm state is applied.
> + * - Due to the fact that exact behavior depends on the underlying
> + *   clock driver, various limitations are possible.
> + * - Period depends on the clock and, in general, not guaranteed.
> + * - 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.
> + */
> +
> +#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 *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);
> +		if (ret)
> +			return ret;
> +		chip->clk_enabled = true;
> +	}
> +
> +	rate = DIV64_U64_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;
> +
> +	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.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");
> +

What is the point of probing the driver if pwmchip_add() fails? This
should be rather fatal error.

The same with clock. If preparing clock fails, there is little point in
enabling/disabling it later.

Best regards,
Krzysztof