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>

1 files changed, 12 insertions, 5 deletions

diff --git a/drivers/pwm/pwm-mediatek.c b/drivers/pwm/pwm-mediatek.c
index 502c366c7d7c..328c124773b2 100644
--- 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);