diff --git a/src/mainboard/amd/onyx/mainboard.c b/src/mainboard/amd/onyx/mainboard.c
new file mode 100644
index 0000000..af89e14
--- /dev/null
+++ b/src/mainboard/amd/onyx/mainboard.c
@@ -0,0 +1,51 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <soc/acpi.h>
+#include <soc/amd_pci_int_defs.h>
+#include <amdblocks/amd_pci_util.h>
+#include <types.h>
+
+/* The IRQ mapping in fch_irq_map ends up getting written to the indirect address space that is
+   accessed via I/O ports 0xc00/0xc01. */
+
+/*
+ * This controls the device -> IRQ routing.
+ *
+ * Hardcoded IRQs:
+ *  0: timer < soc/amd/common/acpi/lpc.asl
+ *  1: i8042 - Keyboard
+ *  2: cascade
+ *  8: rtc0 <- soc/amd/common/acpi/lpc.asl
+ *  9: acpi <- soc/amd/common/acpi/lpc.asl
+ */
+static const struct fch_irq_routing fch_irq_map[] = {
+	{ PIRQ_A,	12,		PIRQ_NC },
+	{ PIRQ_B,	14,		PIRQ_NC },
+	{ PIRQ_C,	15,		PIRQ_NC },
+	{ PIRQ_D,	12,		PIRQ_NC },
+	{ PIRQ_E,	14,		PIRQ_NC },
+	{ PIRQ_F,	15,		PIRQ_NC },
+	{ PIRQ_G,	12,		PIRQ_NC },
+	{ PIRQ_H,	14,		PIRQ_NC },
+
+	{ PIRQ_SCI,	ACPI_SCI_IRQ,	ACPI_SCI_IRQ },
+	{ PIRQ_GPIO,	11,		11 },
+	{ PIRQ_I2C0,	10,		10 },
+	{ PIRQ_I2C1,	 7,		 7 },
+	{ PIRQ_I2C2,	 6,		 6 },
+	{ PIRQ_I2C3,	 5,		 5 },
+	{ PIRQ_UART0,	 4,		 4 },
+	{ PIRQ_UART1,	 3,		 3 },
+
+	/* The MISC registers are not interrupt numbers */
+	{ PIRQ_MISC,	0xfa,		0x00 },
+	{ PIRQ_MISC0,	0x91,		0x00 },
+	{ PIRQ_HPET_L,	0x00,		0x00 },
+	{ PIRQ_HPET_H,	0x00,		0x00 },
+};
+
+const struct fch_irq_routing *mb_get_fch_irq_mapping(size_t *length)
+{
+	*length = ARRAY_SIZE(fch_irq_map);
+	return fch_irq_map;
+}