[RFC PATCH] ARM: enable highmem by default on ARMv6/ARMv7 CPUs

[RFC PATCH] ARM: enable highmem by default on ARMv6/ARMv7 CPUs

[Russell King]

ARMv6 and ARMv7 normally have more than 1G of memory, so it makes sense
to have highmem enabled by default.  Users can still turn highmem off if
they wish by enabling the EXPERT option, which then allows the option to
be visible.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
---
 arch/arm/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 02eb51b6d324..49250dc9856b 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1694,8 +1694,9 @@ config HAVE_GENERIC_RCU_GUP
 	depends on ARM_LPAE
 
 config HIGHMEM
-	bool "High Memory Support"
+	bool "High Memory Support" if !(CPU_32v6 || CPU_32v7) || EXPERT
 	depends on MMU
+	default y if CPU_32v6 || CPU_32v7
 	help
 	  The address space of ARM processors is only 4 Gigabytes large
 	  and it has to accommodate user address space, kernel address
-- 
2.1.0

[RFC PATCH] ARM: enable highmem by default on ARMv6/ARMv7 CPUs

[Arnd Bergmann]

On Tuesday 15 September 2015 21:38:52 Russell King wrote:
> ARMv6 and ARMv7 normally have more than 1G of memory, so it makes sense
> to have highmem enabled by default.  Users can still turn highmem off if
> they wish by enabling the EXPERT option, which then allows the option to
> be visible.
> 
> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

I would have guessed that the cost of kmap_atomic (and the
preempt_disable() when CONFIG_PREEMPT is set) is high enough
even for lowmem-only systems that we want this disabled wherever
we can.

Looking through the memory properties of the dts files in the kernel,
my impression is that all ARMv6 and a majority of the currently supported
ARMv7 machines actually have less than 1GB. This is of course changing if you
take into account only the ARMv7 machines that are getting added these
days, but I would drop at least the ARMv6 dependency.

	Arnd

[RFC PATCH] ARM: enable highmem by default on ARMv6/ARMv7 CPUs

[Enrico Weigelt, metux IT consult]

Am 15.09.2015 um 22:38 schrieb Russell King:
> ARMv6 and ARMv7 normally have more than 1G of memory, so it makes sense
> to have highmem enabled by default.  Users can still turn highmem off if
> they wish by enabling the EXPERT option, which then allows the option to
> be visible.

I just got a new TX6 (Karo) machine - only a few of their SoM's
even have 1G - most of them just 1/4 or 1/2.

Most embedded boxes I know (not counting smartphones) are way
beyond 1G, so I have a bad feeling w/ your proposal.


--mtx

--
Enrico Weigelt, metux IT consult
+49-151-27565287
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