[PATCH 03/16] pwm: lpss: Add range limit check for the base_unit register value

[Hans de Goede <hdegoede@redhat.com>]

When the user requests a high enough period ns value, then the
calculations in pwm_lpss_prepare() might result in a base_unit value of 0.

But according to the data-sheet the way the PWM controller works is that
each input clock-cycle the base_unit gets added to a N bit counter and
that counter overflowing determines the PWM output frequency. Adding 0
to the counter is a no-op. The data-sheet even explicitly states that
writing 0 to the base_unit bits will result in the PWM outputting a
continuous 0 signal.

base_unit values > (base_unit_range / 256), or iow base_unit values using
the 8 most significant bits, cause loss of resolution of the duty-cycle.
E.g. assuming a base_unit_range of 65536 steps, then a base_unit value of
768 (256 * 3), limits the duty-cycle resolution to 65536 / 768 = 85 steps.
Clamp the max base_unit value to base_unit_range / 32 to ensure a
duty-cycle resolution of at least 32 steps. This limits the maximum
output frequency to 600 KHz / 780 KHz depending on the base clock.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 drivers/pwm/pwm-lpss.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
index 9d965ffe66d1..cae74ce61654 100644
--- a/drivers/pwm/pwm-lpss.c
+++ b/drivers/pwm/pwm-lpss.c
@@ -97,6 +97,14 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
 	freq *= base_unit_range;
 
 	base_unit = DIV_ROUND_CLOSEST_ULL(freq, c);
+	/*
+	 * base_unit must not be 0 and for values > (base_unit_range / 256)
+	 * (values using the 8 most significant bits) the duty-cycle resolution
+	 * degrades. Clamp the maximum value to base_unit_range / 32 which
+	 * leaves a duty-cycle resolution of 32 steps.
+	 */
+	base_unit = clamp_t(unsigned long long, base_unit, 1,
+			    base_unit_range / 32);
 
 	on_time_div = 255ULL * duty_ns;
 	do_div(on_time_div, period_ns);
@@ -105,7 +113,6 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
 	orig_ctrl = ctrl = pwm_lpss_read(pwm);
 	ctrl &= ~PWM_ON_TIME_DIV_MASK;
 	ctrl &= ~(base_unit_range << PWM_BASE_UNIT_SHIFT);
-	base_unit &= base_unit_range;
 	ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
 	ctrl |= on_time_div;
 
-- 
2.26.2