Re: [PATCH v10 06/17] pwm: lpss: Make pwm_lpss_apply() not rely on existing hardware state

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, Sep 03, 2020 at 01:23:26PM +0200, Hans de Goede wrote:
> Before this commit pwm_lpss_apply() was assuming 2 pre-conditions
> were met by the existing hardware state:
> 
> 1. That the base-unit and on-time-div read back from the
> control register are those actually in use, so that it
> can skip setting the update bit if the read-back value
> matches the desired values.
> 
> 2. That the controller is enabled when the cached
> pwm_state.enabled says that the controller is enabled.
> 
> As the long history of fixes for subtle (often suspend/resume)
> lpss-pwm issues shows, these assumptions are not necessary
> always true.
> 
> 1. Specifically is not true on some (*) Cherry Trail devices
> with a nasty GFX0._PS3 method which: a. saves the ctrl reg value.
> b. sets the base-unit to 0 and writes the update bit to apply/commit
> c. restores the original ctrl value without setting the update bit,
> so that the 0 base-unit value is still in use.
> 
> 2. Assumption 2. currently is true, but only because of the code which
> saves/restores the state on suspend/resume. By convention restoring the
> PWM state should be done by the PWM consumer and the presence of this
> code in the pmw-lpss driver is a bug. Therefor the save/restore code will
> be dropped in the next patch in this series, after which this assumption
> also is no longer true.
> 
> This commit changes the pwm_lpss_apply() to not make any assumptions about
> the state the hardware is in. Instead it makes pwm_lpss_apply() always
> fully program the PWM controller, making it much less fragile.
> 
> *) Seen on the Acer One 10 S1003, Lenovo Ideapad Miix 310 and 320 models
> and various Medion models.

Good one!
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

> Acked-by: Thierry Reding <thierry.reding@gmail.com>
> Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> ---
> Changes in v10:
> - Fixup some wording / mistakes in the commit message
> 
> Changes in v9:
> - This is a new patch in v9 of this series
> ---
>  drivers/pwm/pwm-lpss.c | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
> index 8a136ba2a583..9c5c7217c9b6 100644
> --- a/drivers/pwm/pwm-lpss.c
> +++ b/drivers/pwm/pwm-lpss.c
> @@ -85,7 +85,7 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
>  	unsigned long long on_time_div;
>  	unsigned long c = lpwm->info->clk_rate, base_unit_range;
>  	unsigned long long base_unit, freq = NSEC_PER_SEC;
> -	u32 orig_ctrl, ctrl;
> +	u32 ctrl;
>  
>  	do_div(freq, period_ns);
>  
> @@ -104,16 +104,14 @@ static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
>  	do_div(on_time_div, period_ns);
>  	on_time_div = 255ULL - on_time_div;
>  
> -	orig_ctrl = ctrl = pwm_lpss_read(pwm);
> +	ctrl = pwm_lpss_read(pwm);
>  	ctrl &= ~PWM_ON_TIME_DIV_MASK;
>  	ctrl &= ~((base_unit_range - 1) << PWM_BASE_UNIT_SHIFT);
>  	ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
>  	ctrl |= on_time_div;
>  
> -	if (orig_ctrl != ctrl) {
> -		pwm_lpss_write(pwm, ctrl);
> -		pwm_lpss_write(pwm, ctrl | PWM_SW_UPDATE);
> -	}
> +	pwm_lpss_write(pwm, ctrl);
> +	pwm_lpss_write(pwm, ctrl | PWM_SW_UPDATE);
>  }
>  
>  static inline void pwm_lpss_cond_enable(struct pwm_device *pwm, bool cond)
> @@ -124,8 +122,7 @@ static inline void pwm_lpss_cond_enable(struct pwm_device *pwm, bool cond)
>  
>  static int pwm_lpss_prepare_enable(struct pwm_lpss_chip *lpwm,
>  				   struct pwm_device *pwm,
> -				   const struct pwm_state *state,
> -				   bool enable)
> +				   const struct pwm_state *state)
>  {
>  	int ret;
>  
> @@ -134,12 +131,12 @@ static int pwm_lpss_prepare_enable(struct pwm_lpss_chip *lpwm,
>  		return ret;
>  
>  	pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
> -	pwm_lpss_cond_enable(pwm, enable && lpwm->info->bypass == false);
> +	pwm_lpss_cond_enable(pwm, lpwm->info->bypass == false);
>  	ret = pwm_lpss_wait_for_update(pwm);
>  	if (ret)
>  		return ret;
>  
> -	pwm_lpss_cond_enable(pwm, enable && lpwm->info->bypass == true);
> +	pwm_lpss_cond_enable(pwm, lpwm->info->bypass == true);
>  	return 0;
>  }
>  
> @@ -152,11 +149,11 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	if (state->enabled) {
>  		if (!pwm_is_enabled(pwm)) {
>  			pm_runtime_get_sync(chip->dev);
> -			ret = pwm_lpss_prepare_enable(lpwm, pwm, state, true);
> +			ret = pwm_lpss_prepare_enable(lpwm, pwm, state);
>  			if (ret)
>  				pm_runtime_put(chip->dev);
>  		} else {
> -			ret = pwm_lpss_prepare_enable(lpwm, pwm, state, false);
> +			ret = pwm_lpss_prepare_enable(lpwm, pwm, state);
>  		}
>  	} else if (pwm_is_enabled(pwm)) {
>  		pwm_lpss_write(pwm, pwm_lpss_read(pwm) & ~PWM_ENABLE);
> -- 
> 2.28.0
> 

-- 
With Best Regards,
Andy Shevchenko