Re: [PATCH] ARM: fix bug which lowmem size is limited to 760MB

From: Nicolas Pitre
Date: Mon Sep 07 2015 - 11:34:44 EST

On Mon, 7 Sep 2015, Arnd Bergmann wrote:

> Given how much more common 1GB hardware configurations are compared to 768MB
> configuration, we could however think about adding a VMSPLIT_3G_OPT option
> that x86 has (also VMSPLIT_2_75G on ARCH_TILE), to allow using the entire
> 1GB of lowmem without going all the way to VMSPLIT_2G. That option would
> also let us use the entire 768MB on the machines that Yongtaek Lee is
> interested in.

That's easy enough:

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 0d1b717e1e..a63970f211 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1470,6 +1470,8 @@ choice

config VMSPLIT_3G
bool "3G/1G user/kernel split"
+ config VMSPLIT_3G_OPT
+ bool "3G/1G user/kernel split (for full 1G low memory)"
config VMSPLIT_2G
bool "2G/2G user/kernel split"
config VMSPLIT_1G
@@ -1481,6 +1483,7 @@ config PAGE_OFFSET
default PHYS_OFFSET if !MMU
default 0x40000000 if VMSPLIT_1G
default 0x80000000 if VMSPLIT_2G
+ default 0xAF000000 if VMSPLIT_3G_OPT
default 0xC0000000

config NR_CPUS

That shifts the risk to user space though. But if there is a regression
there, it will manifest itself on all systems and not only with some
particular hardware.

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