[uart] Allow for the existence of non-16550 UARTs

Remove the assumption that all platforms use a fixed number of 16550
UARTs identifiable by a simple numeric index.  Create an abstraction
allowing for dynamic instantiation and registration of any number of
arbitrary UART models.

The common case of the serial console on x86 uses a single fixed UART
specified at compile time.  Avoid unnecessarily dragging in the
dynamic instantiation code in this use case by allowing COMCONSOLE to
refer to a single static UART object representing the relevant port.

When selecting a UART by command-line argument (as used in the
"gdbstub serial <port>" command), allow the UART to be specified as
either a numeric index (to retain backwards compatiblity) or a
case-insensitive port name such as "COM2".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/uart/ns16550.c

@@ -0,0 +1,179 @@
+/*
+ * Copyright (C) 2025 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/** @file
+ *
+ * 16550-compatible UART
+ *
+ */
+
+#include <unistd.h>
+#include <errno.h>
+#include <ipxe/uart.h>
+#include <ipxe/ns16550.h>
+
+/** Timeout for transmit holding register to become empty */
+#define NS16550_THRE_TIMEOUT_MS 100
+
+/** Timeout for transmitter to become empty */
+#define NS16550_TEMT_TIMEOUT_MS 1000
+
+/**
+ * Transmit data
+ *
+ * @v uart		UART
+ * @v data		Data
+ */
+static void ns16550_transmit ( struct uart *uart, uint8_t data ) {
+	struct ns16550_uart *ns16550 =
+		container_of ( uart, struct ns16550_uart, uart );
+	unsigned int i;
+	uint8_t lsr;
+
+	/* Wait for transmitter holding register to become empty */
+	for ( i = 0 ; i < NS16550_THRE_TIMEOUT_MS ; i++ ) {
+		lsr = ns16550_read ( ns16550, NS16550_LSR );
+		if ( lsr & NS16550_LSR_THRE )
+			break;
+		mdelay ( 1 );
+	}
+
+	/* Transmit data (even if we timed out) */
+	ns16550_write ( ns16550, NS16550_THR, data );
+}
+
+/**
+ * Check if data is ready
+ *
+ * @v uart		UART
+ * @ret ready		Data is ready
+ */
+static int ns16550_data_ready ( struct uart *uart ) {
+	struct ns16550_uart *ns16550 =
+		container_of ( uart, struct ns16550_uart, uart );
+	uint8_t lsr;
+
+	/* Check for receive data ready */
+	lsr = ns16550_read ( ns16550, NS16550_LSR );
+	return ( lsr & NS16550_LSR_DR );
+}
+
+/**
+ * Receive data
+ *
+ * @v uart		UART
+ * @ret data		Data
+ */
+static uint8_t ns16550_receive ( struct uart *uart ) {
+	struct ns16550_uart *ns16550 =
+		container_of ( uart, struct ns16550_uart, uart );
+	uint8_t rbr;
+
+	/* Receive byte */
+	rbr = ns16550_read ( ns16550, NS16550_RBR );
+	return rbr;
+}
+
+/**
+ * Flush transmitted data
+ *
+ * @v uart		UART
+ */
+static void ns16550_flush ( struct uart *uart ) {
+	struct ns16550_uart *ns16550 =
+		container_of ( uart, struct ns16550_uart, uart );
+	unsigned int i;
+	uint8_t lsr;
+
+	/* Wait for transmitter to become empty */
+	for ( i = 0 ; i < NS16550_TEMT_TIMEOUT_MS ; i++ ) {
+		lsr = ns16550_read ( ns16550, NS16550_LSR );
+		if ( lsr & NS16550_LSR_TEMT )
+			break;
+	}
+}
+
+/**
+ * Initialise UART
+ *
+ * @v uart		UART
+ * @v baud		Baud rate, or zero to leave unchanged
+ * @ret rc		Return status code
+ */
+static int ns16550_init ( struct uart *uart, unsigned int baud ) {
+	struct ns16550_uart *ns16550 =
+		container_of ( uart, struct ns16550_uart, uart );
+	uint8_t dlm;
+	uint8_t dll;
+
+	/* Fail if UART scratch register seems not to be present */
+	ns16550_write ( ns16550, NS16550_SCR, 0x18 );
+	if ( ns16550_read ( ns16550, NS16550_SCR ) != 0x18 )
+		return -ENODEV;
+	ns16550_write ( ns16550, NS16550_SCR, 0xae );
+	if ( ns16550_read ( ns16550, NS16550_SCR ) != 0xae )
+		return -ENODEV;
+
+	/* Configure divisor and line control register, if applicable */
+	ns16550_write ( ns16550, NS16550_LCR,
+			( NS16550_LCR_8N1 | NS16550_LCR_DLAB ) );
+	if ( baud ) {
+		ns16550->divisor = ( NS16550_MAX_BAUD / baud );
+		dlm = ( ( ns16550->divisor >> 8 ) & 0xff );
+		dll = ( ( ns16550->divisor >> 0 ) & 0xff );
+		ns16550_write ( ns16550, NS16550_DLM, dlm );
+		ns16550_write ( ns16550, NS16550_DLL, dll );
+	} else {
+		dlm = ns16550_read ( ns16550, NS16550_DLM );
+		dll = ns16550_read ( ns16550, NS16550_DLL );
+		ns16550->divisor = ( ( dlm << 8 ) | dll );
+	}
+	ns16550_write ( ns16550, NS16550_LCR, NS16550_LCR_8N1 );
+
+	/* Disable interrupts */
+	ns16550_write ( ns16550, NS16550_IER, 0 );
+
+	/* Enable FIFOs */
+	ns16550_write ( ns16550, NS16550_FCR, NS16550_FCR_FE );
+
+	/* Assert DTR and RTS */
+	ns16550_write ( ns16550, NS16550_MCR,
+			( NS16550_MCR_DTR | NS16550_MCR_RTS ) );
+
+	/* Flush any stale received data */
+	while ( ns16550_data_ready ( uart ) )
+		ns16550_receive ( uart );
+
+	return 0;
+}
+
+/** 16550 UART operations */
+struct uart_operations ns16550_operations = {
+	.transmit = ns16550_transmit,
+	.data_ready = ns16550_data_ready,
+	.receive = ns16550_receive,
+	.init = ns16550_init,
+	.flush = ns16550_flush,
+};