Re: [Intel-gfx] [PATCH v8 08/17] pwm: crc: Fix period / duty_cycle times being off by a factor of 256

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

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On Sun, Aug 30, 2020 at 02:57:44PM +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.
> 
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v3:
> - Use NSEC_PER_USEC instead of adding a new (non-sensical) NSEC_PER_MHZ define
> ---
>  drivers/pwm/pwm-crc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)

Acked-by: Thierry Reding <treding@nvidia.com>

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