PWM was enabled in bios. i915 driver will save max duty to panel->backlight.max.
So *_hz_to_pwm will not be called if backlight.max not zero. 

        pch_ctl2 = I915_READ(BLC_PWM_PCH_CTL2);
        panel->backlight.max = pch_ctl2 >> 16;

        if (!panel->backlight.max)
                panel->backlight.max = get_backlight_max_vbt(connector);

-----Original Message-----
From: Nikula, Jani 
Sent: Monday, September 19, 2016 4:24 PM
To: intel-gfx@lists.freedesktop.org
Cc: Nikula, Jani <jani.nik...@intel.com>; Lee, Shawn C <shawn.c....@intel.com>
Subject: [PATCH v2 1/2] drm/i915/backlight: setup and cache pwm alternate 
increment value

This will also be needed later on when setting up the alternate increment in 
backlight enable.

Cc: Shawn Lee <shawn.c....@intel.com>
Signed-off-by: Jani Nikula <jani.nik...@intel.com>
---
 drivers/gpu/drm/i915/intel_drv.h   |  1 +
 drivers/gpu/drm/i915/intel_panel.c | 14 +++++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 8fd16adf069b..c3e65e515c9d 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -263,6 +263,7 @@ struct intel_panel {
                bool enabled;
                bool combination_mode;  /* gen 2/4 only */
                bool active_low_pwm;
+               bool alternate_pwm_increment;   /* lpt+ */
 
                /* PWM chip */
                bool util_pin_active_low;       /* bxt+ */
diff --git a/drivers/gpu/drm/i915/intel_panel.c 
b/drivers/gpu/drm/i915/intel_panel.c
index c10e9b0405e8..8bc43205d6a0 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1242,10 +1242,10 @@ static u32 bxt_hz_to_pwm(struct intel_connector 
*connector, u32 pwm_freq_hz)
  */
 static u32 spt_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)  {
-       struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+       struct intel_panel *panel = &connector->panel;
        u32 mul;
 
-       if (I915_READ(SOUTH_CHICKEN1) & SPT_PWM_GRANULARITY)
+       if (panel->backlight.alternate_pwm_increment)
                mul = 128;
        else
                mul = 16;
@@ -1261,9 +1261,10 @@ static u32 spt_hz_to_pwm(struct intel_connector 
*connector, u32 pwm_freq_hz)  static u32 lpt_hz_to_pwm(struct intel_connector 
*connector, u32 pwm_freq_hz)  {
        struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+       struct intel_panel *panel = &connector->panel;
        u32 mul, clock;
 
-       if (I915_READ(SOUTH_CHICKEN2) & LPT_PWM_GRANULARITY)
+       if (panel->backlight.alternate_pwm_increment)
                mul = 16;
        else
                mul = 128;
@@ -1414,6 +1415,13 @@ static int lpt_setup_backlight(struct intel_connector 
*connector, enum pipe unus
        struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
        struct intel_panel *panel = &connector->panel;
        u32 pch_ctl1, pch_ctl2, val;
+       bool alt;
+
+       if (HAS_PCH_LPT(dev_priv))
+               alt = I915_READ(SOUTH_CHICKEN2) & LPT_PWM_GRANULARITY;
+       else
+               alt = I915_READ(SOUTH_CHICKEN1) & SPT_PWM_GRANULARITY;
+       panel->backlight.alternate_pwm_increment = alt;
 
        pch_ctl1 = I915_READ(BLC_PWM_PCH_CTL1);
        panel->backlight.active_low_pwm = pch_ctl1 & BLM_PCH_POLARITY;
--
2.1.4

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