[spcr] Add support for the ACPI Serial Port Console Redirection table

The BIOS may provide an ACPI Serial Port Console Redirection (SPCR)
table to describe the serial port to be used for early boot messages.

Add support for parsing the SPCR and instantiating a 16550-based UART.
We do not currently attempt to support other types of UART, since iPXE
does not yet have drivers for other types.

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

src/core/serial.c

@@ -73,7 +73,7 @@ struct uart *serial_console = NULL;
  *
  * @ret uart		Serial console UART, or NULL
  */
-static struct uart * serial_comconsole ( void ) {
+struct uart * fixed_serial_console ( void ) {
 	struct uart *uart = COMCONSOLE;
 	unsigned int baud = COMSPEED;
 
@@ -201,4 +201,4 @@ struct startup_fn serial_startup_fn __startup_fn ( STARTUP_EARLY ) = {
 };
 
 PROVIDE_SERIAL_INLINE ( null, default_serial_console );
-PROVIDE_SERIAL ( fixed, default_serial_console, serial_comconsole );
+PROVIDE_SERIAL ( fixed, default_serial_console, fixed_serial_console );

src/core/spcr.c

@@ -0,0 +1,162 @@
+/*
+ * 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 (at your option) 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 );
+
+#include <errno.h>
+#include <ipxe/serial.h>
+#include <ipxe/pci.h>
+#include <ipxe/ns16550.h>
+#include <ipxe/spcr.h>
+
+/** @file
+ *
+ * ACPI Serial Port Console Redirection (SPCR)
+ *
+ */
+
+#ifdef SERIAL_SPCR
+#define SERIAL_PREFIX_spcr
+#else
+#define SERIAL_PREFIX_spcr __spcr_
+#endif
+
+/** SPCR-defined UART */
+static struct uart spcr_uart = {
+	.refcnt = REF_INIT ( ref_no_free ),
+	.name = "SPCR",
+};
+
+/** SPCR-defined 16550 UART */
+static struct ns16550_uart spcr_ns16550 = {
+	.clock = NS16550_CLK_DEFAULT,
+};
+
+/** Base baud rate for SPCR divisors */
+#define SPCR_BAUD_BASE 115200
+
+/** SPCR baud rate divisors */
+static const uint8_t spcr_baud_divisor[SPCR_BAUD_MAX] = {
+	[SPCR_BAUD_2400] = ( SPCR_BAUD_BASE / 2400 ),
+	[SPCR_BAUD_4800] = ( SPCR_BAUD_BASE / 4800 ),
+	[SPCR_BAUD_9600] = ( SPCR_BAUD_BASE / 9600 ),
+	[SPCR_BAUD_19200] = ( SPCR_BAUD_BASE / 19200 ),
+	[SPCR_BAUD_38400] = ( SPCR_BAUD_BASE / 38400 ),
+	[SPCR_BAUD_57600] = ( SPCR_BAUD_BASE / 57600 ),
+	[SPCR_BAUD_115200] = ( SPCR_BAUD_BASE / 115200 ),
+};
+
+/**
+ * Configure 16550-based serial console
+ *
+ * @v spcr		SPCR table
+ * @v uart		UART to configure
+ * @ret rc		Return status code
+ */
+static int spcr_16550 ( struct spcr_table *spcr, struct uart *uart ) {
+	struct ns16550_uart *ns16550 = &spcr_ns16550;
+
+	/* Set base address */
+	ns16550->base = acpi_ioremap ( &spcr->base, NS16550_LEN );
+	if ( ! ns16550->base ) {
+		DBGC ( uart, "SPCR could not map registers\n" );
+		return -ENODEV;
+	}
+
+	/* Set clock frequency, if specified */
+	if ( spcr->clock )
+		ns16550->clock = le32_to_cpu ( spcr->clock );
+
+	/* Configure UART as a 16550 */
+	uart->op = &ns16550_operations;
+	uart->priv = ns16550;
+
+	return 0;
+}
+
+/**
+ * Identify default serial console
+ *
+ * @ret uart		Default serial console UART, or NULL
+ */
+static struct uart * spcr_console ( void ) {
+	struct uart *uart = &spcr_uart;
+	struct spcr_table *spcr;
+	unsigned int baud;
+	int rc;
+
+	/* Locate SPCR table */
+	spcr = container_of ( acpi_table ( SPCR_SIGNATURE, 0 ),
+			      struct spcr_table, acpi );
+	if ( ! spcr ) {
+		DBGC ( uart, "SPCR found no table\n" );
+		goto err_table;
+	}
+	DBGC2 ( uart, "SPCR found table:\n" );
+	DBGC2_HDA ( uart, 0, spcr, sizeof ( *spcr ) );
+	DBGC ( uart, "SPCR is type %d at %02x:%08llx\n",
+	       spcr->type, spcr->base.type,
+	       ( ( unsigned long long ) le64_to_cpu ( spcr->base.address ) ) );
+	if ( spcr->pci_vendor_id != cpu_to_le16 ( PCI_ANY_ID ) ) {
+		DBGC ( uart, "SPCR is PCI " PCI_FMT " (%04x:%04x)\n",
+		       spcr->pci_segment, spcr->pci_bus, spcr->pci_dev,
+		       spcr->pci_func, le16_to_cpu ( spcr->pci_vendor_id ),
+		       le16_to_cpu ( spcr->pci_device_id ) );
+	}
+
+	/* Get baud rate */
+	baud = 0;
+	if ( le32_to_cpu ( spcr->acpi.length ) >=
+	     ( offsetof ( typeof ( *spcr ), precise ) +
+	       sizeof ( spcr->precise ) ) ) {
+		baud = le32_to_cpu ( spcr->precise );
+		if ( baud )
+			DBGC ( uart, "SPCR has precise baud rate %d\n", baud );
+	}
+	if ( ( ! baud ) && spcr->baud && ( spcr->baud < SPCR_BAUD_MAX ) ) {
+		baud = ( SPCR_BAUD_BASE / spcr_baud_divisor[spcr->baud] );
+		DBGC ( uart, "SPCR has baud rate %d\n", baud );
+	}
+	uart->baud = baud;
+
+	/* Initialise according to type */
+	switch ( spcr->type ) {
+	case SPCR_TYPE_16550:
+	case SPCR_TYPE_16450:
+		if ( ( rc = spcr_16550 ( spcr, uart ) ) != 0 )
+			goto err_type;
+		break;
+	default:
+		DBGC ( uart, "SPCR unsupported type %d\n", spcr->type );
+		goto err_type;
+	}
+
+	return uart;
+
+ err_type:
+ err_table:
+	/* Fall back to using fixed serial console */
+	return fixed_serial_console();
+}
+
+PROVIDE_SERIAL ( spcr, default_serial_console, spcr_console );