Re: [PATCH v3 2/2] pwm: loongson: Reload PWM configuration through counter reset

[Uwe Kleine-König]

On Wed, Jul 15, 2026 at 07:05:24PM +0800, Keguang Zhang via B4 Relay wrote:
> From: Keguang Zhang <keguang.zhang@xxxxxxxxx>
> 
> The Loongson PWM controller latches the LOW and PERIOD registers only at
> the start of each PWM period. After disabling and re-enabling the PWM,
> the controller resumes from the previous counter value and completes the
> current period before re-latching the updated LOW and PERIOD values.
> 
> Reset the PWM counter when disabling the PWM and release it when
> enabling the PWM so that the updated LOW and PERIOD values are latched
> before the PWM starts running again.
> 
> Fixes: 2b62c89448dd ("pwm: Add Loongson PWM controller support")
> Signed-off-by: Keguang Zhang <keguang.zhang@xxxxxxxxx>
> ---
>  drivers/pwm/pwm-loongson.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-loongson.c b/drivers/pwm/pwm-loongson.c
> index e703217a6d5e..c317d62fe813 100644
> --- a/drivers/pwm/pwm-loongson.c
> +++ b/drivers/pwm/pwm-loongson.c
> @@ -103,6 +103,7 @@ static void pwm_loongson_disable(struct pwm_chip *chip, struct pwm_device *pwm)
>  	struct pwm_loongson_ddata *ddata = to_pwm_loongson_ddata(chip);
>  
>  	val = pwm_loongson_readl(ddata, LOONGSON_PWM_REG_CTRL);
> +	val |= LOONGSON_PWM_CTRL_REG_RST;
>  	val &= ~LOONGSON_PWM_CTRL_REG_EN;
>  	pwm_loongson_writel(ddata, val, LOONGSON_PWM_REG_CTRL);
>  }
> @@ -113,6 +114,7 @@ static int pwm_loongson_enable(struct pwm_chip *chip, struct pwm_device *pwm)
>  	struct pwm_loongson_ddata *ddata = to_pwm_loongson_ddata(chip);
>  
>  	val = pwm_loongson_readl(ddata, LOONGSON_PWM_REG_CTRL);
> +	val &= ~LOONGSON_PWM_CTRL_REG_RST;
>  	val |= LOONGSON_PWM_CTRL_REG_EN;
>  	pwm_loongson_writel(ddata, val, LOONGSON_PWM_REG_CTRL);

I applied this patch to

https://git.kernel.org/pub/scm/linux/kernel/git/ukleinek/linux.git pwm/for-next

, but I wonder if the fix is incomplete. If at driver bind time
LOONGSON_PWM_CTRL_REG_RST isn't set and there are still wrong values in
the shadowed LOW and PERIOD registers the first period will be wrong.

This would need:

 	val = pwm_loongson_readl(ddata, LOONGSON_PWM_REG_CTRL);
+
+	/*
+	 * Ensure that the values of LOW and PERIOD are sampled into
+	 * the shadow register when the hardware starts running.
+	 */
+	if (!(val & LOONGSON_PWM_CTRL_REG_RST))
+		pwm_loongson_writel(ddata, val | LOONGSON_PWM_CTRL_REG_RST, LOONGSON_PWM_REG_CTRL);
+
 	val &= ~LOONGSON_PWM_CTRL_REG_RST;
 	val |= LOONGSON_PWM_CTRL_REG_EN;
 	pwm_loongson_writel(ddata, val, LOONGSON_PWM_REG_CTRL);

I think.

Best regards
Uwe

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