[U-Boot] [U-Boot, 3/3] pwm: sunxi: choose best prescaler to improve PWM resolution

[Vagrant Cascadian]

On 2018-10-16, Vasily Khoruzhick wrote:
> Choose best prescaler to improve PWM resolution. Without this change
> driver chooses first prescaler that gives us period value within
> range, but it could be not the best one.
>
> Signed-off-by: Vasily Khoruzhick <anarsoul at gmail.com>

Fixes LCD backlight issues with the pinebook series:

  https://patchwork.ozlabs.org/project/uboot/list/?series=71358

Tested-by: Vagrant Cascadian <vagrant at debian.org>

> ---
>  drivers/pwm/sunxi_pwm.c | 32 +++++++++++++++++++-------------
>  1 file changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/pwm/sunxi_pwm.c b/drivers/pwm/sunxi_pwm.c
> index 6284409b4f..8a55e4f461 100644
> --- a/drivers/pwm/sunxi_pwm.c
> +++ b/drivers/pwm/sunxi_pwm.c
> @@ -67,49 +67,55 @@ static int sunxi_pwm_set_config(struct udevice *dev, uint channel,
>  {
>  	struct sunxi_pwm_priv *priv = dev_get_priv(dev);
>  	struct sunxi_pwm *regs = priv->regs;
> -	int prescaler;
> -	u32 v, period = 0, duty;
> -	u64 scaled_freq = 0;
> +	int best_prescaler = 0;
> +	u32 v, best_period = 0, duty;
> +	u64 best_scaled_freq = 0;
>  	const u32 nsecs_per_sec = 1000000000U;
>  
>  	debug("%s: period_ns=%u, duty_ns=%u\n", __func__, period_ns, duty_ns);
>  
> -	for (prescaler = 0; prescaler <= SUNXI_PWM_CTRL_PRESCALE0_MASK;
> +	for (int prescaler = 0; prescaler <= SUNXI_PWM_CTRL_PRESCALE0_MASK;
>  	     prescaler++) {
> +		u32 period = 0;
> +		u64 scaled_freq = 0;
>  		if (!prescaler_table[prescaler])
>  			continue;
>  		scaled_freq = lldiv(OSC_24MHZ, prescaler_table[prescaler]);
>  		period = lldiv(scaled_freq * period_ns, nsecs_per_sec);
> -		if (period - 1 <= SUNXI_PWM_CH0_PERIOD_MAX)
> -			break;
> +		if ((period - 1 <= SUNXI_PWM_CH0_PERIOD_MAX) &&
> +		    best_period < period) {
> +			best_period = period;
> +			best_scaled_freq = scaled_freq;
> +			best_prescaler = prescaler;
> +		}
>  	}
>  
> -	if (period - 1 > SUNXI_PWM_CH0_PERIOD_MAX) {
> +	if (best_period - 1 > SUNXI_PWM_CH0_PERIOD_MAX) {
>  		debug("%s: failed to find prescaler value\n", __func__);
>  		return -EINVAL;
>  	}
>  
> -	duty = lldiv(scaled_freq * duty_ns, nsecs_per_sec);
> +	duty = lldiv(best_scaled_freq * duty_ns, nsecs_per_sec);
>  
> -	if (priv->prescaler != prescaler) {
> +	if (priv->prescaler != best_prescaler) {
>  		/* Mask clock to update prescaler */
>  		v = readl(&regs->ctrl);
>  		v &= ~SUNXI_PWM_CTRL_CLK_GATE;
>  		writel(v, &regs->ctrl);
>  		v &= ~SUNXI_PWM_CTRL_PRESCALE0_MASK;
> -		v |= (prescaler & SUNXI_PWM_CTRL_PRESCALE0_MASK);
> +		v |= (best_prescaler & SUNXI_PWM_CTRL_PRESCALE0_MASK);
>  		writel(v, &regs->ctrl);
>  		v |= SUNXI_PWM_CTRL_CLK_GATE;
>  		writel(v, &regs->ctrl);
> -		priv->prescaler = prescaler;
> +		priv->prescaler = best_prescaler;
>  	}
>  
> -	writel(SUNXI_PWM_CH0_PERIOD_PRD(period) |
> +	writel(SUNXI_PWM_CH0_PERIOD_PRD(best_period) |
>  	       SUNXI_PWM_CH0_PERIOD_DUTY(duty), &regs->ch0_period);
>  
>  	debug("%s: prescaler: %d, period: %d, duty: %d\n",
>  	      __func__, priv->prescaler,
> -	      period, duty);
> +	      best_period, duty);
>  
>  	return 0;
>  }
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