On Sun, Jun 07, 2020 at 08:18:31PM +0200, Hans de Goede wrote: > While looking into adding atomic-pwm support to the pwm-crc driver I > noticed something odd, there is a PWM_BASE_CLK define of 6 MHz and > there is a clock-divider which divides this with a value between 1-128, > and there are 256 duty-cycle steps. > > The pwm-crc code before this commit assumed that a clock-divider > setting of 1 means that the PWM output is running at 6 MHZ, if that > is true, where do these 256 duty-cycle steps come from? > > This would require an internal frequency of 256 * 6 MHz = 1.5 GHz, that > seems unlikely for a PMIC which is using a silicon process optimized for > power-switching transistors. It is way more likely that there is an 8 > bit counter for the duty cycle which acts as an extra fixed divider > wrt the PWM output frequency. > > The main user of the pwm-crc driver is the i915 GPU driver which uses it > for backlight control. Lets compare the PWM register values set by the > video-BIOS (the GOP), assuming the extra fixed divider is present versus > the PWM frequency specified in the Video-BIOS-Tables: > > Device: PWM Hz set by BIOS PWM Hz specified in VBT > Asus T100TA 200 200 > Asus T100HA 200 200 > Lenovo Miix 2 8 23437 20000 > Toshiba WT8-A 23437 20000 > > So as we can see if we assume the extra division by 256 then the register > values set by the GOP are an exact match for the VBT values, where as > otherwise the values would be of by a factor of 256. > > This commit fixes the period / duty_cycle calculations to take the > extra division by 256 into account. ... > +#define NSEC_PER_MHZ 1000 This is against physics. What this cryptic name means actually? Existing NSEC_PER_USEC ? -- With Best Regards, Andy Shevchenko
