[PATCH v2] pwm: imx27: Fix variable truncation in .apply()

[Ronaldo Nunez <rnunez@baylibre.com>]

Fix a variable truncation when calculating period in microseconds as
part of the solution for the ERR051198 in .apply() callback.

Example scenario:
 - Period of 3us (PWMPR = 196 and prescaler = 1)
 - Expected value in tmp: 198000000000 (NSEC_PER_SEC * (196 + 2) * 1)
 - Actual value is 431504384 (truncation to u32)

Signed-off-by: Ronaldo Nunez <rnunez@baylibre.com>
---
Changes in v2:
- Added example with actual PWMPR/prescaler values per Frank Li's feedback
- Dropped testing section
---
 drivers/pwm/pwm-imx27.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index 3d34cdc4a3a5..c8b801fcb525 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -200,7 +200,7 @@ static void pwm_imx27_wait_fifo_slot(struct pwm_chip *chip,
 static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 			   const struct pwm_state *state)
 {
-	unsigned long period_cycles, duty_cycles, prescale, period_us, tmp;
+	unsigned long period_cycles, duty_cycles, prescale, period_us;
 	struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
 	unsigned long long c;
 	unsigned long long clkrate;
@@ -208,6 +208,7 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	int val;
 	int ret;
 	u32 cr;
+	u64 tmp;
 
 	clkrate = clk_get_rate(imx->clks[PWM_IMX27_PER].clk);
 	c = clkrate * state->period;
@@ -249,6 +250,11 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	val = readl(imx->mmio_base + MX3_PWMPR);
 	val = val >= MX3_PWMPR_MAX ? MX3_PWMPR_MAX : val;
 	cr = readl(imx->mmio_base + MX3_PWMCR);
+
+	/*
+	 * tmp stores period in nanoseconds. Result fits in u64 since
+	 * val <= 0xfffe and prescaler in [1, 0x1000].
+	 */
 	tmp = NSEC_PER_SEC * (u64)(val + 2) * MX3_PWMCR_PRESCALER_GET(cr);
 	tmp = DIV_ROUND_UP_ULL(tmp, clkrate);
 	period_us = DIV_ROUND_UP_ULL(tmp, 1000);
-- 
2.53.0