Re: [PATCH] ARM: use default ioremap alignment for SMP or LPAE

[Arnd Bergmann <arnd@arndb.de>]

On Friday 23 January 2015 14:52:36 Catalin Marinas wrote:
> On Thu, Jan 22, 2015 at 11:03:00AM +0000, Arnd Bergmann wrote:
> > Unrelated to this question however is whether we want to keep
> > supersection mappings as a performance optimization to save TLBs.
> > It seems useful to me, but not critical.
> 
> Currently in Linux we allow 16MB mappings only if the phys address is
> over 32-bit and !LPAE which makes it unlikely for normal RAM with
> pre-LPAE hardware.

Ah, I missed this part when looking at the code.

> IIRC a bigger problem was that supersections are optional in the
> architecture but there was no CPUID bit field in ARMv6 (and early ARMv7)
> to check for their presence. The ID_MMFR3 contains this information but
> for example on early Cortex-A8 that bitfield was reserved and the TRM
> states "unpredictable" on read (so probably zero in practice).
> 
> On newer ARMv7 (not necessarily with LPAE), we could indeed revisit the
> 16MB section mapping but it won't go well with single zImage if you want
> to support earlier ARMv7 or ARMv6.

I see. If there is desire to have it as an optimization, we could do
it for armv7ve-only kernels. We don't currently have an build-time
option for those, but we should introduce one anyway, in order to
better make use of the idiv instructions and to prevent one from
enabling LPAE on a multiplatform kernel that contains pre-lpae armv7
machines (Cortex a8/a9/a5, and some others I'm not sure about).

	Arnd

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