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Subject[PATCH v15 06/11] pwm: imx27: Use 64-bit division macro and function
Date
Since the PWM framework is switching struct pwm_state.period's
datatype to u64, prepare for this transition by using
DIV_ROUND_UP_ULL to handle a 64-bit dividend, and div64_u64 to handle a
64-bit divisor.

Also handle a possible overflow in the calculation of period_cycles when
both clk_rate and period are large numbers. This logic was unit-tested
out by calculating period_cycles using both the existing logic and the
proposed one, and the results are as below.

----------------------------------------------------------------------------
clk_rate period existing proposed
----------------------------------------------------------------------------
1000000000 18446744073709551615 18446744072 18446744073000000000
(U64_MAX)
----------------------------------------------------------------------------
1000000000 4294967291 4294967291 4294967291
----------------------------------------------------------------------------

Overflow occurs in the first case with the existing logic, whereas the
proposed logic handles it better, sacrificing some precision in a best-effort
attempt to handle overflow. As for the second case where there are
more typical values of period, the proposed logic handles that correctly
too.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>
---
drivers/pwm/pwm-imx27.c | 53 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 45 insertions(+), 8 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index a6e40d4..164cb65 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -203,7 +203,7 @@ static void pwm_imx27_wait_fifo_slot(struct pwm_chip *chip,
sr = readl(imx->mmio_base + MX3_PWMSR);
fifoav = FIELD_GET(MX3_PWMSR_FIFOAV, sr);
if (fifoav == MX3_PWMSR_FIFOAV_4WORDS) {
- period_ms = DIV_ROUND_UP(pwm_get_period(pwm),
+ period_ms = DIV_ROUND_UP_ULL(pwm_get_period(pwm),
NSEC_PER_MSEC);
msleep(period_ms);

@@ -213,6 +213,45 @@ static void pwm_imx27_wait_fifo_slot(struct pwm_chip *chip,
}
}

+static int pwm_imx27_calc_period_cycles(const struct pwm_state *state,
+ unsigned long clk_rate,
+ unsigned long *period_cycles)
+{
+ u64 c = 0, c1, c2;
+
+ c1 = clk_rate;
+ c2 = state->period;
+ if (c2 > c1) {
+ c2 = c1;
+ c1 = state->period;
+ }
+
+ if (!c1 || !c2) {
+ pr_err("clk rate and period should be nonzero\n");
+ return -EINVAL;
+ }
+
+ if (c2 <= div64_u64(U64_MAX, c1)) {
+ c = c1 * c2;
+ do_div(c, 1000000000);
+ } else if (c2 <= div64_u64(U64_MAX, div64_u64(c1, 1000))) {
+ do_div(c1, 1000);
+ c = c1 * c2;
+ do_div(c, 1000000);
+ } else if (c2 <= div64_u64(U64_MAX, div64_u64(c1, 1000000))) {
+ do_div(c1, 1000000);
+ c = c1 * c2;
+ do_div(c, 1000);
+ } else if (c2 <= div64_u64(U64_MAX, div64_u64(c1, 1000000000))) {
+ do_div(c1, 1000000000);
+ c = c1 * c2;
+ }
+
+ *period_cycles = c;
+
+ return 0;
+}
+
static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
const struct pwm_state *state)
{
@@ -225,18 +264,16 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,

pwm_get_state(pwm, &cstate);

- c = clk_get_rate(imx->clk_per);
- c *= state->period;
-
- do_div(c, 1000000000);
- period_cycles = c;
+ ret = pwm_imx27_calc_period_cycles(state, clk_get_rate(imx->clk_per),
+ &period_cycles);
+ if (ret)
+ return ret;

prescale = period_cycles / 0x10000 + 1;

period_cycles /= prescale;
c = (unsigned long long)period_cycles * state->duty_cycle;
- do_div(c, state->period);
- duty_cycles = c;
+ duty_cycles = div64_u64(c, state->period);

/*
* according to imx pwm RM, the real period value should be PERIOD
--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
a Linux Foundation Collaborative Project
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