'ARM Server Base Boot Requiremets' [1] mentions SPCR (Serial Port
Console Redirection Table) [2] as a mandatory ACPI table that
specifies the configuration of serial console.

Defer initialization of DT earlycon until ACPI/DT decision is made.

If ACPI is enabled, parse the table, setup earlycon
and enable the specified console.

If it is disabled, try to set up earlycon from DT.

Thanks to Peter Hurley for explaining how this should work.

[1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0044a/index.html
[2] https://msdn.microsoft.com/en-us/library/windows/hardware/dn639132(v=vs.85).aspx

Signed-off-by: Aleksey Makarov <redacted>
---
 drivers/acpi/Kconfig          |   3 +
 drivers/acpi/Makefile         |   1 +
 drivers/acpi/spcr.c           | 125 ++++++++++++++++++++++++++++++++++++++++++
 drivers/tty/serial/earlycon.c |  11 +++-
 include/linux/acpi.h          |   8 +++
 5 files changed, 146 insertions(+), 2 deletions(-)
 create mode 100644 drivers/acpi/spcr.c

diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
index 65fb483..5611eb6 100644
--- a/drivers/acpi/Kconfig
+++ b/drivers/acpi/Kconfig

@@ -77,6 +77,9 @@ config ACPI_DEBUGGER_USER
 
 endif
 
+config ACPI_SPCR_TABLE
+	bool
+
 config ACPI_SLEEP
 	bool
 	depends on SUSPEND || HIBERNATION

diff --git a/drivers/acpi/Makefile b/drivers/acpi/Makefile
index 7395928..f70ae14 100644
--- a/drivers/acpi/Makefile
+++ b/drivers/acpi/Makefile

@@ -82,6 +82,7 @@ obj-$(CONFIG_ACPI_EC_DEBUGFS)	+= ec_sys.o
 obj-$(CONFIG_ACPI_CUSTOM_METHOD)+= custom_method.o
 obj-$(CONFIG_ACPI_BGRT)		+= bgrt.o
 obj-$(CONFIG_ACPI_CPPC_LIB)	+= cppc_acpi.o
+obj-$(CONFIG_ACPI_SPCR_TABLE)	+= spcr.o
 obj-$(CONFIG_ACPI_DEBUGGER_USER) += acpi_dbg.o
 
 # processor has its own "processor." module_param namespace

diff --git a/drivers/acpi/spcr.c b/drivers/acpi/spcr.c
new file mode 100644
index 0000000..31e667e
--- /dev/null
+++ b/drivers/acpi/spcr.c

@@ -0,0 +1,125 @@
+/*
+ * Copyright (c) 2012, Intel Corporation
+ * Copyright (c) 2015, Red Hat, Inc.
+ * Copyright (c) 2015, 2016 Linaro Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+
+#define pr_fmt(fmt) "ACPI: SPCR: " fmt
+
+#include <linux/acpi.h>
+#include <linux/console.h>
+#include <linux/kernel.h>
+#include <linux/serial_core.h>
+#include <linux/of_fdt.h>
+
+static bool earlycon_init_is_deferred __initdata;
+
+void __init defer_earlycon_init(void)
+{
+	earlycon_init_is_deferred = true;
+}
+
+/**
+ * parse_spcr() - parse ACPI SPCR table and add preferred console
+ *
+ * For the architectures with support for ACPI, CONFIG_ACPI_SPCR_TABLE may be
+ * defined to parse ACPI SPCR table.  As a result of the parsing preferred
+ * console is registered.
+ *
+ * When CONFIG_ACPI_SPCR_TABLE is defined, this function should should be called
+ * from arch inintialization code as soon as the DT/ACPI decision is made.
+ *
+ * When CONFIG_ACPI_SPCR_TABLE is defined, "earlycon" without parameters in
+ * command line does not start DT earlycon immediately, instead it defers
+ * starting it until DT/ACPI decision is made.  If ACPI is enabled at that time,
+ * parse_spcr() parses the table, adds preferred console and sets up it as an
+ * earlycon.  If ACPI is disabled at that time, it tries to set up earlycon
+ * from DT.
+ */
+int __init parse_spcr(void)
+{
+#define OPTS_LEN 64