[PATCH v2 06/15] pwm: crc: Fix period / duty_cycle times being off by a factor of 256

[Andy Shevchenko]

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