Re: [PATCH v4 05/16] pwm: lpss: Use pwm_lpss_apply() when restoring state on resume

Andy Shevchenko
2020-07-09T13:36:13Z

On Wed, Jul 08, 2020 at 11:14:21PM +0200, Hans de Goede wrote:
> Before this commit a suspend + resume of the LPSS PWM controller
> would result in the controller being reset to its defaults of
> output-freq = clock/256, duty-cycle=100%, until someone changes
> to the output-freq and/or duty-cycle are made.
>
> This problem has been masked so far because the main consumer
> (the i915 driver) was always making duty-cycle changes on resume.
> With the conversion of the i915 driver to the atomic PWM API the
> driver now only disables/enables the PWM on suspend/resume leaving
> the output-freq and duty as is, triggering this problem.
>
> The LPSS PWM controller has a mechanism where the ctrl register value
> and the actual base-unit and on-time-div values used are latched. When
> software sets the SW_UPDATE bit then at the end of the current PWM cycle,
> the new values from the ctrl-register will be latched into the actual
> registers, and the SW_UPDATE bit will be cleared.
>
> The problem is that before this commit our suspend/resume handling
> consisted of simply saving the PWM ctrl register on suspend and
> restoring it on resume, without setting the PWM_SW_UPDATE bit.
> When the controller has lost its state over a suspend/resume and thus
> has been reset to the defaults, just restoring the register is not
> enough. We must also set the SW_UPDATE bit to tell the controller to
> latch the restored values into the actual registers.
>
> Fixing this problem is not as simple as just or-ing in the value which
> is being restored with SW_UPDATE. If the PWM was enabled before we must
> write the new settings + PWM_SW_UPDATE before setting PWM_ENABLE.
> We must also wait for PWM_SW_UPDATE to become 0 again and depending on the
> model we must do this either before or after the setting of PWM_ENABLE.
>
> All the necessary logic for doing this is already present inside
> pwm_lpss_apply(), so instead of duplicating this inside the resume
> handler, this commit makes the resume handler use pwm_lpss_apply() to
> restore the settings when necessary. This fixes the output-freq and
> duty-cycle being reset to their defaults on resume.

...

> +static int __pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> + const struct pwm_state *state, bool from_resume)
> {
> struct pwm_lpss_chip *lpwm = to_lpwm(chip);
> int ret;
>
> if (state->enabled) {
> if (!pwm_is_enabled(pwm)) {
> - pm_runtime_get_sync(chip->dev);
> + if (!from_resume)
> + pm_runtime_get_sync(chip->dev);
> +
> ret = pwm_lpss_is_updating(pwm);
> if (ret) {
> - pm_runtime_put(chip->dev);
> + if (!from_resume)
> + pm_runtime_put(chip->dev);
> +
> return ret;
> }
> pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
> pwm_lpss_cond_enable(pwm, lpwm->info->bypass == false);
> ret = pwm_lpss_wait_for_update(pwm);
> if (ret) {
> - pm_runtime_put(chip->dev);
> + if (!from_resume)
> + pm_runtime_put(chip->dev);
> +
> return ret;
> }
> pwm_lpss_cond_enable(pwm, lpwm->info->bypass == true);

> }
> } else if (pwm_is_enabled(pwm)) {
> pwm_lpss_write(pwm, pwm_lpss_read(pwm) & ~PWM_ENABLE);
> - pm_runtime_put(chip->dev);
> +
> + if (!from_resume)
> + pm_runtime_put(chip->dev);
> }

I'm wondering if splitting more will make this look better, like:

...
if (from_resume) {
ret = pwm_lpss_prepare_enable(...); // whatever name you think suits better
} else {
pm_runtime_get_sync(...);
ret = pwm_lpss_prepare_enable(...);
if (ret)
pm_runtime_put(...);
}
...

--
With Best Regards,
Andy Shevchenko