ACPI: introduce CONFIG_ACPI_REDUCED_HARDWARE_ONLY

ACPI hardware reduced mode exists to allow newer platforms to use a
simpler form of ACPI that does not require supporting legacy versions
of the specification and their associated hardware.  This mode was
introduced in the ACPI 5.0 specification.

The ACPI hardware reduced mode is supposed to be used on systems
having the HW_REDUCED_ACPI flag set in the FADT.  ACPICA checks
that flag to determine whether or not it should work in the HW
reduced mode and there are pieces of code in it that will never
be used in that case.

Since some architecutres will always use the ACPI HW reduced mode,
it doesn't make sense for them to ever compile support for anything
else.  Thus, they should set the flag ACPI_REDUCED_HARDWARE to TRUE
in the ACPICA source.  To enable them to do that, introduce a new
kernel configuration option, CONFIG_ACPI_REDUCED_HARDWARE_ONLY, that
will cause the ACPICA's ACPI_REDUCED_HARDWARE flag to be TRUE when
set.

Introducing this configuration item is based on suggestions from Lv
Zheng saying that this does not belong in ACPICA, but rather to the
Linux kernel itself.

References: http://www.spinics.net/lists/linux-acpi/msg46369.htmlSigned-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Al Stone <al.stone@linaro.org>
[rjw: Subject and changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/Kconfig

@@ -343,6 +343,19 @@ config ACPI_BGRT
 	  data from the firmware boot splash. It will appear under
 	  /sys/firmware/acpi/bgrt/ .
 
+config ACPI_REDUCED_HARDWARE_ONLY
+	bool "Hardware-reduced ACPI support only" if EXPERT
+	def_bool n
+	depends on ACPI
+	help
+	This config item changes the way the ACPI code is built.  When this
+	option is selected, the kernel will use a specialized version of
+	ACPICA that ONLY supports the ACPI "reduced hardware" mode.  The
+	resulting kernel will be smaller but it will also be restricted to
+	running in ACPI reduced hardware mode ONLY.
+
+	If you are unsure what to do, do not enable this option.
+
 source "drivers/acpi/apei/Kconfig"
 
 config ACPI_EXTLOG

include/acpi/platform/aclinux.h

@@ -52,6 +52,12 @@
 
 #ifdef __KERNEL__
 
+/* Compile for reduced hardware mode only with this kernel config */
+
+#ifdef CONFIG_ACPI_REDUCED_HARDWARE_ONLY
+#define ACPI_REDUCED_HARDWARE 1
+#endif
+
 #include <linux/string.h>
 #include <linux/kernel.h>
 #include <linux/ctype.h>