[PATCH v2 03/15] pwm: lpss: Add range limit check for the base_unit register value

[Andy Shevchenko]

On Fri, Jun 12, 2020 at 12:12:42AM +0200, Uwe Kleine-König wrote:
> On Mon, Jun 08, 2020 at 01:07:12PM +0200, Hans de Goede wrote:
> > On 6/8/20 5:50 AM, Andy Shevchenko wrote:
> > > On Sun, Jun 07, 2020 at 08:18:28PM +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.
> > > 
> > > So, and why it's a problem?
> > 
> > Lets sya the user requests a PWM output frequency of 100Hz on Cherry Trail
> > which has a 19200000 Hz clock this will result in 100 * 65536 / 19200000 =
> > 0.3 -> 0 as base-unit value. So instead of getting 100 Hz the user will
> > now get a pin which is always outputting low.
> 
> I didn't follow the complete discussion but note that the general rule
> is:
> 
> 	round period down to the next possible implementable period
> 	round duty_cycle down to the next possible implementable duty_cycle
> 
> so if a small enough period (and so a small duty_cycle) is requested it
> is expected that duty_cycle will be zero.

...which brings me an idea that PWM framework should expose API to get a
capabilities, like DMA Engine has.

In such capabilities, in particular, caller can get ranges of the correct
frequencies of the underneath hardware.

-- 
With Best Regards,
Andy Shevchenko