Re: [PATCH v8 04/17] pwm: lpss: Add range limit check for the base_unit register value

[Thierry Reding <thierry.reding@gmail.com>]

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On Sun, Aug 30, 2020 at 02:57:40PM +0200, Hans de Goede wrote:
> 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>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v5:
> - Use clamp_val(... instead of clam_t(unsigned long long, ...
> 
> Changes in v3:
> - Change upper limit of clamp to (base_unit_range - 1)
> - Add Fixes tag
> ---
>  drivers/pwm/pwm-lpss.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Acked-by: Thierry Reding <thierry.reding@gmail.com>

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