ACPI / tables: table override from built-in initrd

In some scenario, we need to build initrd with kernel in a single image.
This can simplify system deployment process by downloading the whole system
once, such as in IC verification.

This patch adds support to override ACPI tables from built-in initrd.

Signed-off-by: Shunyong Yang <[e-mail]>
[ rjw: Minor cleanups ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/acpi/initrd_table_override.txt

···
 via upgrading the ACPI tables provided by the BIOS with an instrumented,
 modified, more recent version one, or installing brand new ACPI tables.
 
+When building initrd with kernel in a single image, option
+ACPI_TABLE_OVERRIDE_VIA_BUILTIN_INITRD should also be true for this
+feature to work.
+
 For a full list of ACPI tables that can be upgraded/installed, take a look
 at the char *table_sigs[MAX_ACPI_SIGNATURE]; definition in
 drivers/acpi/tables.c.

drivers/acpi/Kconfig

···
 	  initrd, therefore it's safe to say Y.
 	  See Documentation/acpi/initrd_table_override.txt for details
 
+config ACPI_TABLE_OVERRIDE_VIA_BUILTIN_INITRD
+	bool "Override ACPI tables from built-in initrd"
+	depends on ACPI_TABLE_UPGRADE
+	depends on INITRAMFS_SOURCE!="" && INITRAMFS_COMPRESSION=""
+	help
+	  This option provides functionality to override arbitrary ACPI tables
+	  from built-in uncompressed initrd.
+
+	  See Documentation/acpi/initrd_table_override.txt for details
+
 config ACPI_DEBUG
 	bool "Debug Statements"
 	help

drivers/acpi/tables.c

···
 
 void __init acpi_table_upgrade(void)
 {
-	void *data = (void *)initrd_start;
-	size_t size = initrd_end - initrd_start;
+	void *data;
+	size_t size;
 	int sig, no, table_nr = 0, total_offset = 0;
 	long offset = 0;
 	struct acpi_table_header *table;
 	char cpio_path[32] = "kernel/firmware/acpi/";
 	struct cpio_data file;
+
+	if (IS_ENABLED(CONFIG_ACPI_TABLE_OVERRIDE_VIA_BUILTIN_INITRD)) {
+		data = __initramfs_start;
+		size = __initramfs_size;
+	} else {
+		data = (void *)initrd_start;
+		size = initrd_end - initrd_start;
+	}
 
 	if (data == NULL || size == 0)
 		return;

include/linux/initrd.h

···
 extern unsigned long phys_initrd_size;
 
 extern unsigned int real_root_dev;
+
+extern char __initramfs_start[];
+extern unsigned long __initramfs_size;