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
