drm/i915: Set PWM divider to match desired frequency in vbt

Read desired PWM frequency from panel vbt and calculate the
value for divider in DPCD address 0x724 and 0x728 to have
as many bits as possible for PWM duty cyle for granularity of
brightness adjustment while the frequency divisor is still
within 25% of the desired value.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170622190339.142671-2-puthik@chromium.org

drivers/gpu/drm/i915/intel_dp_aux_backlight.c

@@ -98,13 +98,87 @@ intel_dp_aux_set_backlight(const struct drm_connector_state *conn_state, u32 lev
 	}
 }
 
+/*
+ * Set PWM Frequency divider to match desired frequency in vbt.
+ * The PWM Frequency is calculated as 27Mhz / (F x P).
+ * - Where F = PWM Frequency Pre-Divider value programmed by field 7:0 of the
+ *             EDP_BACKLIGHT_FREQ_SET register (DPCD Address 00728h)
+ * - Where P = 2^Pn, where Pn is the value programmed by field 4:0 of the
+ *             EDP_PWMGEN_BIT_COUNT register (DPCD Address 00724h)
+ */
+static bool intel_dp_aux_set_pwm_freq(struct intel_connector *connector)
+{
+	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
+	int freq, fxp, fxp_min, fxp_max, fxp_actual, f = 1;
+	u8 pn, pn_min, pn_max;
+
+	/* Find desired value of (F x P)
+	 * Note that, if F x P is out of supported range, the maximum value or
+	 * minimum value will applied automatically. So no need to check that.
+	 */
+	freq = dev_priv->vbt.backlight.pwm_freq_hz;
+	DRM_DEBUG_KMS("VBT defined backlight frequency %u Hz\n", freq);
+	if (!freq) {
+		DRM_DEBUG_KMS("Use panel default backlight frequency\n");
+		return false;
+	}
+
+	fxp = DIV_ROUND_CLOSEST(KHz(DP_EDP_BACKLIGHT_FREQ_BASE_KHZ), freq);
+
+	/* Use highest possible value of Pn for more granularity of brightness
+	 * adjustment while satifying the conditions below.
+	 * - Pn is in the range of Pn_min and Pn_max
+	 * - F is in the range of 1 and 255
+	 * - FxP is within 25% of desired value.
+	 *   Note: 25% is arbitrary value and may need some tweak.
+	 */
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			       DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min) != 1) {
+		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap min\n");
+		return false;
+	}
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			       DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max) != 1) {
+		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap max\n");
+		return false;
+	}
+	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
+	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
+	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
+		DRM_DEBUG_KMS("VBT defined backlight frequency out of range\n");
+		return false;
+	}
+
+	for (pn = pn_max; pn >= pn_min; pn--) {
+		f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
+		fxp_actual = f << pn;
+		if (fxp_min <= fxp_actual && fxp_actual <= fxp_max)
+			break;
+	}
+
+	if (drm_dp_dpcd_writeb(&intel_dp->aux,
+			       DP_EDP_PWMGEN_BIT_COUNT, pn) < 0) {
+		DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+		return false;
+	}
+	if (drm_dp_dpcd_writeb(&intel_dp->aux,
+			       DP_EDP_BACKLIGHT_FREQ_SET, (u8) f) < 0) {
+		DRM_DEBUG_KMS("Failed to write aux backlight freq\n");
+		return false;
+	}
+	return true;
+}
+
 static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_state,
 					  const struct drm_connector_state *conn_state)
 {
 	struct intel_connector *connector = to_intel_connector(conn_state->connector);
 	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
-	uint8_t dpcd_buf = 0;
-	uint8_t edp_backlight_mode = 0;
+	uint8_t dpcd_buf, new_dpcd_buf, edp_backlight_mode;
 
 	if (drm_dp_dpcd_readb(&intel_dp->aux,
 			DP_EDP_BACKLIGHT_MODE_SET_REGISTER, &dpcd_buf) != 1) {
@@ -113,18 +187,15 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 		return;
 	}
 
+	new_dpcd_buf = dpcd_buf;
 	edp_backlight_mode = dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
 
 	switch (edp_backlight_mode) {
 	case DP_EDP_BACKLIGHT_CONTROL_MODE_PWM:
 	case DP_EDP_BACKLIGHT_CONTROL_MODE_PRESET:
 	case DP_EDP_BACKLIGHT_CONTROL_MODE_PRODUCT:
-		dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
-		dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
-		if (drm_dp_dpcd_writeb(&intel_dp->aux,
-			DP_EDP_BACKLIGHT_MODE_SET_REGISTER, dpcd_buf) < 0) {
-			DRM_DEBUG_KMS("Failed to write aux backlight mode\n");
-		}
+		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
+		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
 		break;
 
 	/* Do nothing when it is already DPCD mode */
@@ -133,6 +204,17 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 		break;
 	}
 
+	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_FREQ_AUX_SET_CAP)
+		if (intel_dp_aux_set_pwm_freq(connector))
+			new_dpcd_buf |= DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
+
+	if (new_dpcd_buf != dpcd_buf) {
+		if (drm_dp_dpcd_writeb(&intel_dp->aux,
+			DP_EDP_BACKLIGHT_MODE_SET_REGISTER, new_dpcd_buf) < 0) {
+			DRM_DEBUG_KMS("Failed to write aux backlight mode\n");
+		}
+	}
+
 	set_aux_backlight_enable(intel_dp, true);
 	intel_dp_aux_set_backlight(conn_state, connector->panel.backlight.level);
 }