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

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Date:	Mon, 7 Sep 2015 11:34:36 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
cc:	linux-arm-kernel@...ts.infradead.org,
	Yongtaek Lee <ytk.lee@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: fix bug which lowmem size is limited to 760MB

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.


Nicolas
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