Re: [Intel-gfx] [PATCH v8 10/17] pwm: crc: Fix period changes not having any effect

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

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On Sun, Aug 30, 2020 at 02:57:46PM +0200, Hans de Goede wrote:
> The pwm-crc code is using 2 different enable bits:
> 1. bit 7 of the PWM0_CLK_DIV (PWM_OUTPUT_ENABLE)
> 2. bit 0 of the BACKLIGHT_EN register
> 
> The BACKLIGHT_EN register at address 0x51 really controls a separate
> output-only GPIO which is earmarked to be used as output connected to the
> backlight-enable pin for LCD panels, this GPO is part of the PMIC's
> "Display Panel Control Block." . This pin should probably be moved over
> to a GPIO provider driver (and consumers modified accordingly), but that
> is something for an(other) patch.
> 
> Enabling / disabling the actual PWM output is controlled by the
> PWM_OUTPUT_ENABLE bit of the PWM0_CLK_DIV register.
> 
> As the comment in the old code already indicates we must disable the PWM
> before we can change the clock divider. But the crc_pwm_disable() and
> crc_pwm_enable() calls the old code make for this only change the
> BACKLIGHT_EN register; and the value of that register does not matter for
> changing the period / the divider. What does matter is that the
> PWM_OUTPUT_ENABLE bit must be cleared before a new value can be written.
> 
> This commit modifies crc_pwm_config() to clear PWM_OUTPUT_ENABLE instead
> when changing the period, so that period changes actually work.
> 
> Note this fix will cause a significant behavior change on some devices
> using the CRC PWM output to drive their backlight. Before the PWM would
> always run with the output frequency configured by the BIOS at boot, now
> the period time specified by the i915 driver will actually be honored.
> 
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
>  drivers/pwm/pwm-crc.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)

Acked-by: Thierry Reding <thierry.reding@gmail.com>

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