[PATCH] i915: Introduce quirk for shifting eDP brightness.

[Kevin Chowski <chowski@chromium.org>]

We have observed that Google Pixelbook's backlight hardware is
interpretting these backlight bits from the most-significant side of the
16 bit word (if DP_EDP_PWMGEN_BIT_COUNT < 16), whereas the driver code
assumes the peripheral cares about the least-significant bits.

Testing was done from within Chrome OS's build environment when the
patch is backported to 5.4 (the version we are newly targeting for the
Pixelbook); for the record:
   $ emerge-eve-kernelnext sys-kernel/chromeos-kernel-5_4 && \
      ./update_kernel.sh --remote=$IP

I used `/sys/kernel/debug/dri/0/eDP-1/i915_dpcd` on my laptop to verify
that the registers were being set according to what the actual hardware
expects; I also observe that the backlight is noticeably brighter with
this patch.

Signed-off-by: Kevin Chowski <chowski@chromium.org>
---

 .../drm/i915/display/intel_dp_aux_backlight.c | 34 +++++++++++++++++++
 drivers/gpu/drm/i915/display/intel_quirks.c   | 13 +++++++
 drivers/gpu/drm/i915/i915_drv.h               |  1 +
 3 files changed, 48 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
index acbd7eb66cbe3..99c98f217356d 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -91,6 +91,23 @@ static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
 	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
 		level = (read_val[0] << 8 | read_val[1]);
 
+	if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
+		if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
+						&read_val[0])) {
+			DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
+					DP_EDP_PWMGEN_BIT_COUNT);
+			return 0;
+		}
+		// Only bits 4:0 are used, 7:5 are reserved.
+		read_val[0] = read_val[0] & 0x1F;
+		if (read_val[0] > 16) {
+			DRM_DEBUG_KMS("Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
+						read_val[0]);
+			return 0;
+		}
+		level >>= 16 - read_val[0];
+	}
+
 	return level;
 }
 
@@ -106,6 +123,23 @@ intel_dp_aux_set_backlight(const struct drm_connector_state *conn_state, u32 lev
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 	u8 vals[2] = { 0x0 };
 
+	if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
+		if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
+						&vals[0])) {
+			DRM_DEBUG_KMS("Failed to write aux backlight level: Failed to read DPCD register 0x%x\n",
+					  DP_EDP_PWMGEN_BIT_COUNT);
+			return;
+		}
+		// Only bits 4:0 are used, 7:5 are reserved.
+		vals[0] = vals[0] & 0x1F;
+		if (vals[0] > 16) {
+			DRM_DEBUG_KMS("Failed to write aux backlight level: Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
+						vals[0]);
+			return;
+		}
+		level <<= (16 - vals[0]) & 0xFFFF;
+	}
+
 	vals[0] = level;
 
 	/* Write the MSB and/or LSB */
diff --git a/drivers/gpu/drm/i915/display/intel_quirks.c b/drivers/gpu/drm/i915/display/intel_quirks.c
index 46beb155d835f..63b27d49b2864 100644
--- a/drivers/gpu/drm/i915/display/intel_quirks.c
+++ b/drivers/gpu/drm/i915/display/intel_quirks.c
@@ -53,6 +53,16 @@ static void quirk_increase_ddi_disabled_time(struct drm_i915_private *i915)
 	drm_info(&i915->drm, "Applying Increase DDI Disabled quirk\n");
 }
 
+/*
+ * Some eDP backlight hardware uses the most-significant bits of the brightness
+ * register, so brightness values must be shifted first.
+ */
+static void quirk_shift_edp_backlight_brightness(struct drm_i915_private *i915)
+{
+	i915->quirks |= QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS;
+	DRM_INFO("Applying shift eDP backlight brightness quirk\n");
+}
+
 struct intel_quirk {
 	int device;
 	int subsystem_vendor;
@@ -156,6 +166,9 @@ static struct intel_quirk intel_quirks[] = {
 	/* ASRock ITX*/
 	{ 0x3185, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
 	{ 0x3184, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
+
+	/* Google Pixelbook */
+	{ 0x591E, 0x8086, 0x2212, quirk_shift_edp_backlight_brightness },
 };
 
 void intel_init_quirks(struct drm_i915_private *i915)
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index e4f7f6518945b..cc93bede4fab8 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -525,6 +525,7 @@ struct i915_psr {
 #define QUIRK_PIN_SWIZZLED_PAGES (1<<5)
 #define QUIRK_INCREASE_T12_DELAY (1<<6)
 #define QUIRK_INCREASE_DDI_DISABLED_TIME (1<<7)
+#define QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS (1<<8)
 
 struct intel_fbdev;
 struct intel_fbc_work;
-- 
2.28.0.618.gf4bc123cb7-goog