drm/i915: Revive combination mode for backlight control

This reverts commit 951f3512dba5bd44cda3e5ee22b4b522e4bb09fb

    drm/i915: Do not handle backlight combination mode specially

since this commit introduced other regressions due to untouched LBPC
register, e.g. the backlight dimmed after resume.

In addition to the revert, this patch includes a fix for the original
issue (weird backlight levels) by removing the wrong bit shift for
computing the current backlight level.
Also, including typo fixes (lpbc -> lbpc).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34524
Acked-by: Indan Zupancic <indan@nul.nu>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/i915_reg.h

@@ -1553,7 +1553,17 @@
 
 /* Backlight control */
 #define BLC_PWM_CTL		0x61254
+#define   BACKLIGHT_MODULATION_FREQ_SHIFT		(17)
 #define BLC_PWM_CTL2		0x61250 /* 965+ only */
+#define   BLM_COMBINATION_MODE (1 << 30)
+/*
+ * This is the most significant 15 bits of the number of backlight cycles in a
+ * complete cycle of the modulated backlight control.
+ *
+ * The actual value is this field multiplied by two.
+ */
+#define   BACKLIGHT_MODULATION_FREQ_MASK		(0x7fff << 17)
+#define   BLM_LEGACY_MODE				(1 << 16)
 /*
  * This is the number of cycles out of the backlight modulation cycle for which
  * the backlight is on.

drivers/gpu/drm/i915/intel_panel.c

@@ -30,6 +30,8 @@
 
 #include "intel_drv.h"
 
+#define PCI_LBPC 0xf4 /* legacy/combination backlight modes */
+
 void
 intel_fixed_panel_mode(struct drm_display_mode *fixed_mode,
 		       struct drm_display_mode *adjusted_mode)
@@ -110,6 +112,19 @@ done:
 	dev_priv->pch_pf_size = (width << 16) | height;
 }
 
+static int is_backlight_combination_mode(struct drm_device *dev)
+{
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	if (INTEL_INFO(dev)->gen >= 4)
+		return I915_READ(BLC_PWM_CTL2) & BLM_COMBINATION_MODE;
+
+	if (IS_GEN2(dev))
+		return I915_READ(BLC_PWM_CTL) & BLM_LEGACY_MODE;
+
+	return 0;
+}
+
 static u32 i915_read_blc_pwm_ctl(struct drm_i915_private *dev_priv)
 {
 	u32 val;
@@ -166,6 +181,9 @@ u32 intel_panel_get_max_backlight(struct drm_device *dev)
 			if (INTEL_INFO(dev)->gen < 4)
 				max &= ~1;
 		}
+
+		if (is_backlight_combination_mode(dev))
+			max *= 0xff;
 	}
 
 	DRM_DEBUG_DRIVER("max backlight PWM = %d\n", max);
@@ -183,6 +201,14 @@ u32 intel_panel_get_backlight(struct drm_device *dev)
 		val = I915_READ(BLC_PWM_CTL) & BACKLIGHT_DUTY_CYCLE_MASK;
 		if (IS_PINEVIEW(dev))
 			val >>= 1;
+
+		if (is_backlight_combination_mode(dev)){
+			u8 lbpc;
+
+			val &= ~1;
+			pci_read_config_byte(dev->pdev, PCI_LBPC, &lbpc);
+			val *= lbpc;
+		}
 	}
 
 	DRM_DEBUG_DRIVER("get backlight PWM = %d\n", val);
@@ -205,6 +231,16 @@ void intel_panel_set_backlight(struct drm_device *dev, u32 level)
 
 	if (HAS_PCH_SPLIT(dev))
 		return intel_pch_panel_set_backlight(dev, level);
+
+	if (is_backlight_combination_mode(dev)){
+		u32 max = intel_panel_get_max_backlight(dev);
+		u8 lbpc;
+
+		lbpc = level * 0xfe / max + 1;
+		level /= lbpc;
+		pci_write_config_byte(dev->pdev, PCI_LBPC, lbpc);
+	}
+
 	tmp = I915_READ(BLC_PWM_CTL);
 	if (IS_PINEVIEW(dev)) {
 		tmp &= ~(BACKLIGHT_DUTY_CYCLE_MASK - 1);