[ARM] add CONFIG_HIGHMEM option

Here it is... HIGHMEM for the ARM architecture.  :-)

If you don't have enough ram for highmem pages to be allocated and still
want to test this, then the cmdline option "vmalloc=" can be used with
a value large enough to force the highmem threshold down.

Successfully tested on a Marvell DB-78x00-BP Development Board with
2 GB of RAM.

Signed-off-by: Nicolas Pitre <nico@marvell.com>

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index dbfdf87..2b28786 100644 (file)
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -915,6 +915,23 @@ config NODES_SHIFT
        default "2"
        depends on NEED_MULTIPLE_NODES
 
+config HIGHMEM
+       bool "High Memory Support (EXPERIMENTAL)"
+       depends on MMU && EXPERIMENTAL
+       help
+         The address space of ARM processors is only 4 Gigabytes large
+         and it has to accommodate user address space, kernel address
+         space as well as some memory mapped IO. That means that, if you
+         have a large amount of physical memory and/or IO, not all of the
+         memory can be "permanently mapped" by the kernel. The physical
+         memory that is not permanently mapped is called "high memory".
+
+         Depending on the selected kernel/user memory split, minimum
+         vmalloc space and actual amount of RAM, you may not need this
+         option which should result in a slightly faster kernel.
+
+         If unsure, say n.
+
 source "mm/Kconfig"
 
 config LEDS