pwm: mediatek: Improve precision in rate calculation

Add a way that turning resolution from in nanosecond into in picosecond
to improve noticeably almost 4.5% precision.

It's necessary to hold the new resolution with type u64 and thus related
operations on u64 are applied instead in those rate calculations.

And the patch has a dependency on [1].

[1] http://lists.infradead.org/pipermail/linux-mediatek/2018-March/012225.html

Cc: stable@vger.kernel.org
Fixes: caf065f8fd58 ("pwm: Add MediaTek PWM support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
index 502c366c7d7c89d1182d560c8a534b54c524f78f..328c124773b23e6501a70f8320d02f4900e6d71a 100644 (file)
--- a/drivers/pwm/pwm-mediatek.c
+++ b/drivers/pwm/pwm-mediatek.c
@@ -135,19 +135,25 @@ static int mtk_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 {
        struct mtk_pwm_chip *pc = to_mtk_pwm_chip(chip);
        struct clk *clk = pc->clks[MTK_CLK_PWM1 + pwm->hwpwm];
-       u32 resolution, clkdiv = 0, reg_width = PWMDWIDTH,
+       u32 clkdiv = 0, cnt_period, cnt_duty, reg_width = PWMDWIDTH,
            reg_thres = PWMTHRES;
+       u64 resolution;
        int ret;
 
        ret = mtk_pwm_clk_enable(chip, pwm);
        if (ret < 0)
                return ret;
 
-       resolution = NSEC_PER_SEC / clk_get_rate(clk);
+       /* Using resolution in picosecond gets accuracy higher */
+       resolution = (u64)NSEC_PER_SEC * 1000;
+       do_div(resolution, clk_get_rate(clk));
 
-       while (period_ns / resolution > 8191) {
+       cnt_period = DIV_ROUND_CLOSEST_ULL((u64)period_ns * 1000, resolution);
+       while (cnt_period > 8191) {
                resolution *= 2;
                clkdiv++;
+               cnt_period = DIV_ROUND_CLOSEST_ULL((u64)period_ns * 1000,
+                                                  resolution);
        }
 
        if (clkdiv > PWM_CLK_DIV_MAX) {
@@ -165,9 +171,10 @@ static int mtk_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
                reg_thres = PWM45THRES_FIXUP;
        }
 
+       cnt_duty = DIV_ROUND_CLOSEST_ULL((u64)duty_ns * 1000, resolution);
        mtk_pwm_writel(pc, pwm->hwpwm, PWMCON, BIT(15) | clkdiv);
-       mtk_pwm_writel(pc, pwm->hwpwm, reg_width, period_ns / resolution);
-       mtk_pwm_writel(pc, pwm->hwpwm, reg_thres, duty_ns / resolution);
+       mtk_pwm_writel(pc, pwm->hwpwm, reg_width, cnt_period);
+       mtk_pwm_writel(pc, pwm->hwpwm, reg_thres, cnt_duty);
 
        mtk_pwm_clk_disable(chip, pwm);