[PATCH] drm/i915/bxt: fix incorrect BIOS backlight PWM frequency setting

[Imre Deak <imre.deak@intel.com>]

There seem to be BIOSes with that set an invalid PWM frequency, assuming the
wrong base frequency. This base frequency is 19.2MHz on BXT except for A
stepping where it's the current CD clock frequency. The BIOSes in question
don't take this difference into account and program the frequency
assuming it's based on the CD clock frequency even on B stepping.

Since we don't care about A stepping any more, simply assume the 19.2MHz
everywhere and require a minimum of 100Hz PWM frequency, regard anything
below this as incorrect. In case we correct the frequency we also have to
disable fastboot, that being the simplest way to reprogram the backlight
settings.

Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 5 +++++
 drivers/gpu/drm/i915/intel_drv.h     | 1 +
 drivers/gpu/drm/i915/intel_panel.c   | 7 +++++++
 3 files changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index e9bb860..bdd8506 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -13271,6 +13271,11 @@ static void calc_watermark_data(struct drm_atomic_state *state)
 	}
 }
 
+void intel_disable_fastboot(void)
+{
+	i915.fastboot = 0;
+}
+
 /**
  * intel_atomic_check - validate state object
  * @dev: drm device
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 76dfa28..e957494 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -1057,6 +1057,7 @@ int intel_pch_rawclk(struct drm_device *dev);
 int intel_hrawclk(struct drm_device *dev);
 void intel_mark_busy(struct drm_device *dev);
 void intel_mark_idle(struct drm_device *dev);
+void intel_disable_fastboot(void);
 void intel_crtc_restore_mode(struct drm_crtc *crtc);
 int intel_display_suspend(struct drm_device *dev);
 void intel_encoder_destroy(struct drm_encoder *encoder);
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 72183a0..c598b6b 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1647,6 +1647,13 @@ bxt_setup_backlight(struct intel_connector *connector, enum pipe unused)
 	panel->backlight.active_low_pwm = pwm_ctl & BXT_BLC_PWM_POLARITY;
 	panel->backlight.max =
 		I915_READ(BXT_BLC_PWM_FREQ(panel->backlight.controller));
+	if (panel->backlight.hz_to_pwm &&
+	    panel->backlight.max > panel->backlight.hz_to_pwm(connector, 100)) {
+		DRM_INFO("Correcting innvalid BIOS backlight PWM frequency (%d), disabling fastboot\n",
+			 panel->backlight.max);
+		intel_disable_fastboot();
+		panel->backlight.max = 0;
+	}
 
 	if (!panel->backlight.max)
 		panel->backlight.max = get_backlight_max_vbt(connector);
-- 
2.5.0

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