Index: dev/fdt/com_fdt.c
===================================================================
RCS file: /cvs/src/sys/dev/fdt/com_fdt.c,v
retrieving revision 1.5
diff -u -p -r1.5 com_fdt.c
--- dev/fdt/com_fdt.c	24 Apr 2021 10:33:09 -0000	1.5
+++ dev/fdt/com_fdt.c	12 Nov 2021 10:47:46 -0000
@@ -33,8 +33,6 @@
 #include <dev/ofw/ofw_clock.h>
 #include <dev/ofw/ofw_pinctrl.h>
 
-#define com_usr 31	/* Synopsys DesignWare UART */
-
 int	com_fdt_match(struct device *, void *, void *);
 void	com_fdt_attach(struct device *, struct device *, void *);
 int	com_fdt_intr_designware(void *);
@@ -144,8 +142,10 @@ com_fdt_attach(struct device *parent, st
 	sc->sc_reg_shift = OF_getpropint(faa->fa_node, "reg-shift", shift);
 
 	if (OF_is_compatible(faa->fa_node, "snps,dw-apb-uart") ||
-	    OF_is_compatible(faa->fa_node, "marvell,armada-38x-uart"))
+	    OF_is_compatible(faa->fa_node, "marvell,armada-38x-uart")) {
+		sc->sc_uarttype = COM_UART_DW_APB;
 		intr = com_fdt_intr_designware;
+	}
 
 	if (OF_is_compatible(faa->fa_node, "ti,omap3-uart") ||
 	    OF_is_compatible(faa->fa_node, "ti,omap4-uart"))
Index: dev/ic/com.c
===================================================================
RCS file: /cvs/src/sys/dev/ic/com.c,v
retrieving revision 1.174
diff -u -p -r1.174 com.c
--- dev/ic/com.c	6 May 2021 20:35:21 -0000	1.174
+++ dev/ic/com.c	12 Nov 2021 10:47:46 -0000
@@ -1300,7 +1300,7 @@ void
 com_attach_subr(struct com_softc *sc)
 {
 	int probe = 0;
-	u_int8_t lcr;
+	u_int8_t lcr, fifo;
 
 	sc->sc_ier = 0;
 	/* disable interrupts */
@@ -1480,6 +1480,25 @@ com_attach_subr(struct com_softc *sc)
 		SET(sc->sc_hwflags, COM_HW_FIFO);
 		sc->sc_fifolen = 256;
 		break;
+	case COM_UART_DW_APB:
+		printf(": DesignWare APB UART, ");
+		SET(sc->sc_hwflags, COM_HW_FIFO);
+		sc->sc_fifolen = CPR_FIFO_MODE(com_read_reg(sc, com_cpr)) * 16;
+		if (sc->sc_fifolen) {
+			printf("%d byte fifo\n", sc->sc_fifolen);
+		} else {
+			printf("no fifo\n");
+			/*
+			 * Allwinner H6's DW-APB configuration does not have
+			 * CPR register and detect as no fifo.
+			 * But this UART has 256 bytes FIFO and disabling FIFO
+			 * makes problem; LSR_RXRDY is still set after
+			 * reading com_data when FIFO is disabled (errata?).
+			 * For workaround, treat as 1 byte FIFO.
+			 */
+			sc->sc_fifolen = 1;
+		}
+		break;
 	default:
 		panic("comattach: bad fifo type");
 	}
@@ -1496,10 +1515,13 @@ com_attach_subr(struct com_softc *sc)
 	}
 
 	/* clear and disable fifo */
-	com_write_reg(sc, com_fifo, FIFO_RCV_RST | FIFO_XMT_RST);
+	/* DW-APB UART cannot turn off FIFO here (ddb will not work) */
+	fifo = (sc->sc_uarttype == COM_UART_DW_APB) ?
+		(FIFO_ENABLE | FIFO_TRIGGER_1) : 0;
+	com_write_reg(sc, com_fifo, fifo | FIFO_RCV_RST | FIFO_XMT_RST);
 	if (ISSET(com_read_reg(sc, com_lsr), LSR_RXRDY))
 		(void)com_read_reg(sc, com_data);
-	com_write_reg(sc, com_fifo, 0);
+	com_write_reg(sc, com_fifo, fifo);
 
 	sc->sc_mcr = 0;
 	com_write_reg(sc, com_mcr, sc->sc_mcr);
Index: dev/ic/comreg.h
===================================================================
RCS file: /cvs/src/sys/dev/ic/comreg.h,v
retrieving revision 1.20
diff -u -p -r1.20 comreg.h
--- dev/ic/comreg.h	14 Aug 2020 18:14:11 -0000	1.20
+++ dev/ic/comreg.h	12 Nov 2021 10:47:46 -0000
@@ -180,6 +180,9 @@
 #define ISR_TXPL	0x08	/* negative transmit data polarity */
 #define ISR_RXPL	0x10	/* negative receive data polarity */
 
+/* component parameter register (Synopsys DesignWare APB UART) */
+#define	CPR_FIFO_MODE(x)	(((x) >> 16) & 0xff)
+
 #define	COM_NPORTS	8
 
 /* Exar XR17V35X */
Index: dev/ic/comvar.h
===================================================================
RCS file: /cvs/src/sys/dev/ic/comvar.h,v
retrieving revision 1.58
diff -u -p -r1.58 comvar.h
--- dev/ic/comvar.h	14 Aug 2020 18:14:11 -0000	1.58
+++ dev/ic/comvar.h	12 Nov 2021 10:47:46 -0000
@@ -104,6 +104,7 @@ struct com_softc {
 #define	COM_UART_XR16850	0x10		/* 128 byte fifo */
 #define	COM_UART_OX16C950	0x11		/* 128 byte fifo */
 #define	COM_UART_XR17V35X	0x12		/* 256 byte fifo */
+#define	COM_UART_DW_APB		0x13		/* configurable */
 
 	u_char sc_hwflags;
 #define	COM_HW_NOIEN	0x01
Index: dev/ic/ns16550reg.h
===================================================================
RCS file: /cvs/src/sys/dev/ic/ns16550reg.h,v
retrieving revision 1.5
diff -u -p -r1.5 ns16550reg.h
--- dev/ic/ns16550reg.h	2 Jun 2003 23:28:02 -0000	1.5
+++ dev/ic/ns16550reg.h	12 Nov 2021 10:47:46 -0000
@@ -50,3 +50,9 @@
 #define	com_lsr		5	/* line status register (R/W) */
 #define	com_msr		6	/* modem status register (R/W) */
 #define com_scratch	7	/* scratch register (R/W) */
+
+/*
+ * Synopsys DesignWare APB UART additional registers
+ */
+#define	com_usr		31	/* UART status register (R) */
+#define	com_cpr		61	/* component parameter register (R) */