pwm: lpss: Only set update bit if we are actually changing the settings

According to the datasheet the update bit must be set if the on-time-div
or the base-unit changes.

Now that we properly order device resume on Cherry Trail so that the GFX0
_PS0 method no longer exits with an error, we end up with a sequence of
events where we are writing the same values twice in a row.

First the _PS0 method restores the duty cycle of 0% the GPU driver set
on suspend and then the GPU driver first updates just the enabled bit in
the pwm_state from 0 to 1, causing us to write the same values again,
before restoring the pre-suspend duty-cycle in a separate pwm_apply call.

When writing the update bit the second time, without changing any of
the values the update bit clears immediately / instantly, instead of
staying 1 for a while as usual. After this the next setting of the update
bit seems to be ignored, causing the restoring of the pre-suspend
duty-cycle to not get applied. This makes the backlight come up with
a 0% dutycycle after suspend/resume.

Any further brightness changes after this do work.

This commit moves the setting of the update bit into pwm_lpss_prepare()
and only sets the bit if we have actually changed any of the values.

This avoids the setting of the update bit the second time we configure
the PWM to 0% dutycycle, this fixes the backlight coming up with 0%
duty-cycle after a suspend/resume.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
index ea93ef9f3672..2ac3a2aa9e53 100644
--- a/drivers/pwm/pwm-lpss.c
+++ b/drivers/pwm/pwm-lpss.c
@@ -88,7 +88,7 @@ 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 ctrl;
+        u32 orig_ctrl, ctrl;
 
         do_div(freq, period_ns);
 
@@ -105,13 +105,17 @@ 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;
 
-        ctrl = pwm_lpss_read(pwm);
+        orig_ctrl = ctrl = pwm_lpss_read(pwm);
         ctrl &= ~PWM_ON_TIME_DIV_MASK;
         ctrl &= ~(base_unit_range << PWM_BASE_UNIT_SHIFT);
         base_unit &= base_unit_range;
         ctrl |= (u32) base_unit << PWM_BASE_UNIT_SHIFT;
         ctrl |= on_time_div;
-        pwm_lpss_write(pwm, ctrl);
+
+        if (orig_ctrl != ctrl) {
+                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)
@@ -135,7 +139,6 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
                                 return ret;
                         }
                         pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
-                        pwm_lpss_write(pwm, pwm_lpss_read(pwm) | PWM_SW_UPDATE);
                         pwm_lpss_cond_enable(pwm, lpwm->info->bypass == false);
                         ret = pwm_lpss_wait_for_update(pwm);
                         if (ret) {
@@ -148,7 +151,6 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
                         if (ret)
                                 return ret;
                         pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
-                        pwm_lpss_write(pwm, pwm_lpss_read(pwm) | PWM_SW_UPDATE);
                         return pwm_lpss_wait_for_update(pwm);
                 }
         } else if (pwm_is_enabled(pwm)) {