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diff --git a/zephyr/projects/nissa/include/nissa_hdmi.h b/zephyr/projects/nissa/include/nissa_hdmi.h
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+++ b/zephyr/projects/nissa/include/nissa_hdmi.h
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+/* Copyright 2022 The ChromiumOS Authors
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+/* Nissa shared HDMI sub-board functionality */
+
+#ifndef __CROS_EC_NISSA_NISSA_HDMI_H__
+#define __CROS_EC_NISSA_NISSA_HDMI_H__
+
+#include "common.h"
+
+/** True if the board supports an HDMI sub-board. */
+#define NISSA_BOARD_HAS_HDMI_SUPPORT DT_NODE_EXISTS(DT_NODELABEL(gpio_hdmi_sel))
+
+/**
+ * Configure the GPIO that controls core rails on the HDMI sub-board.
+ *
+ * This is the gpio_en_rails_odl pin, which is configured as active-low
+ * open-drain output to enable power to the HDMI sub-board (typically when the
+ * AP is in S5 or above).
+ *
+ * This function must be called if the pin is connected to the HDMI board and
+ * power is not enabled by default.
+ */
+void nissa_configure_hdmi_rails(void);
+
+/**
+ * Configure the GPIO that controls the HDMI VCC pin on the HDMI sub-board.
+ *
+ * This is the gpio_hdmi_en_odl pin, which is configured as active-low
+ * open-drain output to enable the VCC pin on the HDMI connector (typically when
+ * the AP is on, in S0 or S0ix).
+ *
+ * This function must be called if the pin is connected to the HDMI board and
+ * VCC is not enabled by default.
+ */
+void nissa_configure_hdmi_vcc(void);
+
+/**
+ * Configure the GPIOS controlling HDMI sub-board power (core rails and VCC).
+ *
+ * This function is called from shared code while configuring sub-boards, and
+ * used if an HDMI sub-board is present. The default implementation enables the
+ * core rails control pin (nissa_configure_hdmi_rails) but not VCC
+ * (nissa_configure_hdmi_vcc), assuming that the pin for VCC is not connected
+ * connected on most boards (and that VCC will be turned on whenever the core
+ * rails are turned on).
+ *
+ * A board should override this function if it needs to enable more IOs for
+ * HDMI, or if some pins need to be conditionally enabled.
+ */
+__override_proto void nissa_configure_hdmi_power_gpios(void);
+
+#endif /* __CROS_EC_NISSA_NISSA_HDMI_H__ */