x86: Allow locating the UART from ACPI tables

When coreboot does not pass a UART in its sysinfo struct, there is no
easy way to find it out.

Since coreboot does not actually init the serial device when serial is
disabled, it is not possible to make it add this information to the
sysinfo table.

Add a way to obtain this information from the DBG2 ACPI table, which is
normally set up by coreboot.

For now this only supports a memory-mapped 16550-style UART.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

2 files changed, 116 insertions, 8 deletions

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index 7faf678444..f4767c838f 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -669,6 +669,16 @@ config COREBOOT_SERIAL
 	  a serial console on any platform without needing to change the
 	  device tree, etc.
 
+config COREBOOT_SERIAL_FROM_DBG2
+	bool "Obtain UART from ACPI tables"
+	depends on COREBOOT_SERIAL
+	default y if !SPL
+	help
+	  Select this to try to find a DBG2 record in the ACPI tables, in the
+	  event that coreboot does not provide information about the UART in the
+	  normal sysinfo tables. This provides a useful fallback when serial
+	  is not enabled in coreboot.
+
 config CORTINA_UART
 	bool "Cortina UART support"
 	depends on DM_SERIAL
diff --git a/drivers/serial/serial_coreboot.c b/drivers/serial/serial_coreboot.c
index de09c8681f..23066e4d05 100644
--- a/drivers/serial/serial_coreboot.c
+++ b/drivers/serial/serial_coreboot.c
@@ -5,25 +5,123 @@
  * Copyright 2019 Google LLC
  */
 
+#define LOG_CATGEGORY	UCLASS_SERIAL
+
 #include <common.h>
 #include <dm.h>
+#include <log.h>
 #include <ns16550.h>
 #include <serial.h>
+#include <acpi/acpi_table.h>
 #include <asm/cb_sysinfo.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
+static int read_dbg2(struct ns16550_plat *plat)
+{
+	struct acpi_table_header *tab;
+	struct acpi_dbg2_header *hdr;
+	struct acpi_dbg2_device *dbg;
+	struct acpi_gen_regaddr *addr;
+	u32 *addr_size;
+
+	log_debug("Looking for DBG2 in ACPI tables\n");
+	if (!gd->acpi_start) {
+		log_debug("No ACPI tables\n");
+		return -ENOENT;
+	}
+
+	tab = acpi_find_table("DBG2");
+	if (!tab) {
+		log_debug("No DBG2 table\n");
+		return -ENOENT;
+	}
+	hdr = container_of(tab, struct acpi_dbg2_header, header);
+
+	/* We only use the first device, but check that there is at least one */
+	if (!hdr->devices_count) {
+		log_debug("No devices\n");
+		return -ENOENT;
+	}
+	if (hdr->devices_offset >= tab->length) {
+		log_debug("Invalid offset\n");
+		return -EINVAL;
+	}
+	dbg = (void *)hdr + hdr->devices_offset;
+	if (dbg->revision) {
+		log_debug("Invalid revision %d\n", dbg->revision);
+		return -EINVAL;
+	}
+	if (!dbg->address_count) {
+		log_debug("No addresses\n");
+		return -EINVAL;
+	}
+	if (dbg->port_type != ACPI_DBG2_SERIAL_PORT) {
+		log_debug("Not a serial port\n");
+		return -EPROTOTYPE;
+	}
+	if (dbg->port_subtype != ACPI_DBG2_16550_COMPATIBLE) {
+		log_debug("Incompatible serial port\n");
+		return -EPROTOTYPE;
+	}
+	if (dbg->base_address_offset >= dbg->length ||
+	    dbg->address_size_offset >= dbg->length) {
+		log_debug("Invalid base address/size offsets %d, %d\n",
+			  dbg->base_address_offset, dbg->address_size_offset);
+		return -EINVAL;
+	}
+	addr_size = (void *)dbg + dbg->address_size_offset;
+	if (!*addr_size) {
+		log_debug("Zero address size\n");
+		return -EINVAL;
+	}
+	addr = (void *)dbg + dbg->base_address_offset;
+	if (addr->space_id != ACPI_ADDRESS_SPACE_MEMORY) {
+		log_debug("Incompatible space %d\n", addr->space_id);
+		return -EPROTOTYPE;
+	}
+
+	plat->base = addr->addrl;
+
+	/* ACPI_ACCESS_SIZE_DWORD_ACCESS is 3; we want 2 */
+	plat->reg_shift = addr->access_size - 1;
+	plat->reg_width = 4; /* coreboot sets bit_width to 0 */
+	plat->clock = 1843200;
+	plat->fcr = UART_FCR_DEFVAL;
+	plat->flags = 0;
+	log_debug("Collected UART from ACPI DBG2 table\n");
+
+	return 0;
+}
+
 static int coreboot_of_to_plat(struct udevice *dev)
 {
 	struct ns16550_plat *plat = dev_get_plat(dev);
 	struct cb_serial *cb_info = lib_sysinfo.serial;
+	int ret = -ENOENT;
 
-	plat->base = cb_info->baseaddr;
-	plat->reg_shift = cb_info->regwidth == 4 ? 2 : 0;
-	plat->reg_width = cb_info->regwidth;
-	plat->clock = cb_info->input_hertz;
-	plat->fcr = UART_FCR_DEFVAL;
-	plat->flags = 0;
-	if (cb_info->type == CB_SERIAL_TYPE_IO_MAPPED)
-		plat->flags |= NS16550_FLAG_IO;
+	if (cb_info) {
+		plat->base = cb_info->baseaddr;
+		plat->reg_shift = cb_info->regwidth == 4 ? 2 : 0;
+		plat->reg_width = cb_info->regwidth;
+		plat->clock = cb_info->input_hertz;
+		plat->fcr = UART_FCR_DEFVAL;
+		plat->flags = 0;
+		if (cb_info->type == CB_SERIAL_TYPE_IO_MAPPED)
+			plat->flags |= NS16550_FLAG_IO;
+		ret = 0;
+	} else if (IS_ENABLED(CONFIG_COREBOOT_SERIAL_FROM_DBG2)) {
+		ret = read_dbg2(plat);
+	}
+
+	if (ret) {
+		/*
+		 * Returning an error will cause U-Boot to complain that
+		 * there is no UART, which may panic. So stay silent and
+		 * pray that the video console will work.
+		 */
+		log_debug("Cannot detect UART\n");
+	}
 
 	return 0;
 }