1 files changed, 61 insertions, 0 deletions
diff --git a/common/usb_pd_alt_mode_dfp.c b/common/usb_pd_alt_mode_dfp.c
new file mode 100644
index 0000000000..c581b0077d
--- /dev/null
+++ b/common/usb_pd_alt_mode_dfp.c
@@ -0,0 +1,61 @@
+/* Copyright 2020 The Chromium OS Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ *
+ * Alternate Mode Downstream Facing Port (DFP) USB-PD module.
+ */
+
+#include "usb_pd.h"
+#include "util.h"
+
+/*
+ * This algorithm defaults to choosing higher pin config over lower ones in
+ * order to prefer multi-function if desired.
+ *
+ *  NAME | SIGNALING | OUTPUT TYPE | MULTI-FUNCTION | PIN CONFIG
+ * -------------------------------------------------------------
+ *  A    |  USB G2   |  ?          | no             | 00_0001
+ *  B    |  USB G2   |  ?          | yes            | 00_0010
+ *  C    |  DP       |  CONVERTED  | no             | 00_0100
+ *  D    |  PD       |  CONVERTED  | yes            | 00_1000
+ *  E    |  DP       |  DP         | no             | 01_0000
+ *  F    |  PD       |  DP         | yes            | 10_0000
+ *
+ * if UFP has NOT asserted multi-function preferred code masks away B/D/F
+ * leaving only A/C/E.  For single-output dongles that should leave only one
+ * possible pin config depending on whether its a converter DP->(VGA|HDMI) or DP
+ * output.  If UFP is a USB-C receptacle it may assert C/D/E/F.  The DFP USB-C
+ * receptacle must always choose C/D in those cases.
+ */
+int pd_dfp_dp_get_pin_mode(int port, uint32_t status)
+{
+	struct svdm_amode_data *modep =
+				pd_get_amode_data(port, USB_SID_DISPLAYPORT);
+	uint32_t mode_caps;
+	uint32_t pin_caps;
+
+	if (!modep)
+		return 0;
+
+	mode_caps = modep->data->mode_vdo[modep->opos - 1];
+
+	/* TODO(crosbug.com/p/39656) revisit with DFP that can be a sink */
+	pin_caps = PD_DP_PIN_CAPS(mode_caps);
+
+	/* if don't want multi-function then ignore those pin configs */
+	if (!PD_VDO_DPSTS_MF_PREF(status))
+		pin_caps &= ~MODE_DP_PIN_MF_MASK;
+
+	/* TODO(crosbug.com/p/39656) revisit if DFP drives USB Gen 2 signals */
+	pin_caps &= ~MODE_DP_PIN_BR2_MASK;
+
+	/* if C/D present they have precedence over E/F for USB-C->USB-C */
+	if (pin_caps & (MODE_DP_PIN_C | MODE_DP_PIN_D))
+		pin_caps &= ~(MODE_DP_PIN_E | MODE_DP_PIN_F);
+
+	/* get_next_bit returns undefined for zero */
+	if (!pin_caps)
+		return 0;
+
+	return 1 << get_next_bit(&pin_caps);
+}