Re: [PATCH 1/2] pwm: loongson: Fix low pulse buffer register handling

[Uwe Kleine-König]

On Tue, Jun 16, 2026 at 07:13:17PM +0800, Keguang Zhang via B4 Relay wrote:
> From: Keguang Zhang <keguang.zhang@xxxxxxxxx>
> 
> The Loongson PWM register at offset 0x4 is documented as the Low
> Pulse Buffer Register, which stores the low pulse width rather than
> the duty cycle.
> 
> However, this register was incorrectly defined and treated as a
> duty-cycle register. As a result, the duty cycle and low pulse cycle
> are swapped in the generated PWM waveform.
> 
> Program the low pulse (period - duty) into the register and
> adjust pwm_loongson_get_state() accordingly when reconstructing the
> duty cycle.
> 
> Also return -ERANGE when the requested period exceeds the hardware
> 32-bit limit instead of silently truncating the value.

This is the intended behaviour.

> Signed-off-by: Keguang Zhang <keguang.zhang@xxxxxxxxx>
> ---
>  drivers/pwm/pwm-loongson.c | 29 ++++++++++++++---------------
>  1 file changed, 14 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-loongson.c b/drivers/pwm/pwm-loongson.c
> index 31a57edecfd0..dc77f82fd888 100644
> --- a/drivers/pwm/pwm-loongson.c
> +++ b/drivers/pwm/pwm-loongson.c
> @@ -22,6 +22,7 @@
>   */
>  
>  #include <linux/acpi.h>
> +#include <linux/bitfield.h>
>  #include <linux/clk.h>
>  #include <linux/device.h>
>  #include <linux/init.h>
> @@ -33,10 +34,12 @@
>  #include <linux/units.h>
>  
>  /* Loongson PWM registers */
> -#define LOONGSON_PWM_REG_DUTY		0x4 /* Low Pulse Buffer Register */
> +#define LOONGSON_PWM_REG_LOW		0x4 /* Low Pulse Buffer Register */
>  #define LOONGSON_PWM_REG_PERIOD		0x8 /* Pulse Period Buffer Register */
>  #define LOONGSON_PWM_REG_CTRL		0xc /* Control Register */
>  
> +#define LOONGSON_PWM_MAX_PERIOD		GENMASK(31, 0)
> +
>  /* Control register bits */
>  #define LOONGSON_PWM_CTRL_REG_EN	BIT(0)  /* Counter Enable Bit */
>  #define LOONGSON_PWM_CTRL_REG_OE	BIT(3)  /* Pulse Output Enable Control Bit, Valid Low */
> @@ -118,20 +121,16 @@ static int pwm_loongson_enable(struct pwm_chip *chip, struct pwm_device *pwm)
>  static int pwm_loongson_config(struct pwm_chip *chip, struct pwm_device *pwm,
>  			       u64 duty_ns, u64 period_ns)
>  {
> -	u64 duty, period;
> +	u64 low, period;
>  	struct pwm_loongson_ddata *ddata = to_pwm_loongson_ddata(chip);
>  
> -	/* duty = duty_ns * ddata->clk_rate / NSEC_PER_SEC */
> -	duty = mul_u64_u64_div_u64(duty_ns, ddata->clk_rate, NSEC_PER_SEC);
> -	if (duty > U32_MAX)
> -		duty = U32_MAX;
> -
> -	/* period = period_ns * ddata->clk_rate / NSEC_PER_SEC */
>  	period = mul_u64_u64_div_u64(period_ns, ddata->clk_rate, NSEC_PER_SEC);
> -	if (period > U32_MAX)
> -		period = U32_MAX;
> +	if ((!FIELD_FIT(LOONGSON_PWM_MAX_PERIOD, period)))
> +		return -ERANGE;

As noted above, this is wrong. If period is too big, you're supposed to
pick the biggest possible period and not return an error.

> +
> +	low = mul_u64_u64_div_u64(period_ns - duty_ns, ddata->clk_rate, NSEC_PER_SEC);

this is also wrong. You're supposed to pick a configuration where the
duty is the biggest not bigger than the requested value. However as
mul_u64_u64_div_u64 rounds down, you're rounding in the wrong direction.

The register layout suggests that the period starts with the inactive
part, did you reverify that?

Best regards
Uwe

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