pwm: lpss: Add range limit check for the base_unit register value

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.

When the user requestes a low enough period ns value, then the
calculations in pwm_lpss_prepare() might result in a base_unit value
which is bigger then base_unit_range - 1. Currently the codes for this
deals with this by applying a mask:

	base_unit &= (base_unit_range - 1);

But this means that we let the value overflow the range, we throw away the
higher bits and store whatever value is left in the lower bits into the
register leading to a random output frequency, rather then clamping the
output frequency to the highest frequency which the hardware can do.

This commit fixes both issues by clamping the base_unit value to be
between 1 and (base_unit_range - 1).

Fixes: 684309e5043e ("pwm: lpss: Avoid potential overflow of base_unit")
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200903112337.4113-5-hdegoede@redhat.com

diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
index 43b1fc634af1a88f3b3517bed20348ada51c90a3..da9bc3d10104a61239cee931bfee7876b0ccd8cd 100644 (file)
--- a/drivers/pwm/pwm-lpss.c
+++ b/drivers/pwm/pwm-lpss.c
@@ -97,6 +97,8 @@ 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 we also want to avoid overflowing it */
+       base_unit = clamp_val(base_unit, 1, base_unit_range - 1);
 
        on_time_div = 255ULL * duty_ns;
        do_div(on_time_div, period_ns);
@@ -105,7 +107,6 @@ 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 - 1) << PWM_BASE_UNIT_SHIFT);
-       base_unit &= (base_unit_range - 1);
        ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
        ctrl |= on_time_div;