[PATCH] pwm: tegra: fix doubled output frequency due to divider truncation

["Ola Chr. Vaage" <o.c.vage@gmail.com>]

From: "Ola Chr. Vaage" <ola.christoffer.vage@scoutdi.com>

The PWM_SCALE frequency divider is computed by rounding down

	clk_rate * period_ns / (NSEC_PER_SEC << PWM_DUTY_WIDTH)

With dynamic clock scaling (Tegra186 and later), the driver doubles its
clock-rate request when the provider cannot meet it. The provider then
typically grants slightly less than the doubled request, the exact
divider lands just below 2, and the round-down truncates it to 1: the
output runs at double the requested frequency. On Tegra234 the BPMP
grants PWM clock rates as 408 MHz / N, so nearly every requested period
is affected: requesting 40000 ns produces 20078 ns, 100000 ns produces
50195 ns. The duty ratio is computed independently and stays correct,
which hides the problem from duty-only consumers such as backlights;
frequency-sensitive loads break.

Measured on a Jetson Orin NX board with the 45334 ns fan period used in
NVIDIA device trees:

	required_clk_rate = ceil((1e9 << 8) / 45334) = 5646977
	clk_round_rate() = 5589041 (408 MHz / 73) -> request doubled
	dev_pm_opp_set_rate(11293954) grants 11027028 (408 MHz / 37)
	divider = trunc(1.953) = 1 -> output period 23217 ns, 43.07 kHz

43 kHz is outside the 21-28 kHz band that 4-wire fan PWM inputs are
designed for. The fan on this board cannot start below duty 110/255 and
stalls below 88/255, so closed-loop fan control oscillates between a
stalled fan and full speed.

Round the divider to the closest integer instead. This bounds the
period error to half a divider step rather than a full one, and
restores what this configuration did before commit 8c193f4714df
("pwm: tegra: Optimize period calculation") switched the rounding from
closest to down: divider round(0.99) = 1 on the undoubled 5589041 Hz
grant, period 45802 ns, +1.0%. Between that commit and commit
5eccd0d9fabc ("pwm: tegra: Ensure the clock rate is not less than
needed") the same request failed with -EINVAL instead. Verified on the
Orin NX with this patch applied: PWM_SCALE reads 1 (21.54 kHz, +2.4%
period), the fan starts at duty <= 50/255 and sustains 80/255, and
closed-loop control is stable. There is no round-closest variant of
mul_u64_u64_div_u64(), so compute twice the quotient and round up the
halving.

Fixes: 5eccd0d9fabc ("pwm: tegra: Ensure the clock rate is not less than needed")
Signed-off-by: Ola Chr. Vaage <ola.christoffer.vage@scoutdi.com>
---
 drivers/pwm/pwm-tegra.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/pwm/pwm-tegra.c b/drivers/pwm/pwm-tegra.c
index 172063b51d4..a5adc4f3ce6 100644
--- a/drivers/pwm/pwm-tegra.c
+++ b/drivers/pwm/pwm-tegra.c
@@ -163,9 +163,15 @@ static int tegra_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
 		pc->clk_rate = clk_get_rate(pc->clk);
 	}
 
-	/* Consider precision in PWM_SCALE_WIDTH rate calculation */
-	rate = mul_u64_u64_div_u64(pc->clk_rate, period_ns,
+	/*
+	 * Consider precision in PWM_SCALE_WIDTH rate calculation. Round to
+	 * the closest integer: there is no round-closest variant of
+	 * mul_u64_u64_div_u64(), so compute twice the quotient and round up
+	 * the halving.
+	 */
+	rate = mul_u64_u64_div_u64(pc->clk_rate, 2 * (u64)period_ns,
 				   (u64)NSEC_PER_SEC << PWM_DUTY_WIDTH);
+	rate = DIV_ROUND_UP_ULL(rate, 2);
 
 	/*
 	 * Since the actual PWM divider is the register's frequency divider
-- 
2.54.0