Messages in this thread Patch in this message | | | From | Gregory Fong <> | Subject | [PATCH 1/3] ARM: Add default SPARSEMEM settings | Date | Thu, 4 Jun 2015 18:58:36 -0700 |
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From: Kevin Cernekee <cernekee@gmail.com>
We can still override these settings via mach/memory.h, but let's provide sensible defaults so that SPARSEMEM is available in the multiplatform kernels.
Two platforms currently use SECTION_SIZE_BITS < 28, but are expected to work with 28 (albeit slightly less efficiently if not all banks are populated):
- mach-rpc: uses 26 bits. Based on mach/hardware.h it looks like this platform puts RAM at 0x1000_0000 - 0x1fff_ffff, and I/O below 0x1000_0000.
- mach-sa1100: uses 27 bits. mach/memory.h indicates that RAM occupies the entire range of 0xc000_0000 - 0xdfff_ffff.
But Arnd says in that rpc and sa1100 will never have to use the default since they cannot be part of a multiplatform kernel, and that is unlikely to change.
Several platforms need MAX_PHYSMEM_BITS >= 36 so we'll pick that as the minimum. Anything higher and we'll fail the SECTIONS_WIDTH + NODES_WIDTH + ZONES_WIDTH test in <linux/mm.h>.
Some analysis from Russell King at http://lists.infradead.org/pipermail/linux-arm-kernel/2014-October/298957.html:
I think this is fine in as far as it goes - this means we end up with 256 entries in the mem_section array which means it occupies one page, which I think is acceptable overhead.
The other thing to be aware of here is the obvious:
#if (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS #error Allocator MAX_ORDER exceeds SECTION_SIZE #endif
Which means that with 28 bits of section, that's a maximum allocator order of 16. We appear to allow FORCE_MAX_ZONEORDER to be set up to 64 in the case of shmobile, which doesn't seem like a sensible upper limit - and certainly isn't when sparsemem is enabled.
Given this, I think that FORCE_MAX_ZONEORDER's help, and the dependencies probably could do with some improvement to make the issues more transparent.
Signed-off-by: Kevin Cernekee <cernekee@gmail.com> Acked-by: Arnd Bergmann <arnd@arndb.de> [gregory.0xf0: added notes from Arnd and Russell] Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com> --- arch/arm/include/asm/sparsemem.h | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/arch/arm/include/asm/sparsemem.h b/arch/arm/include/asm/sparsemem.h index 0009861..73e5e85 100644 --- a/arch/arm/include/asm/sparsemem.h +++ b/arch/arm/include/asm/sparsemem.h @@ -15,10 +15,11 @@ * Eg, if you have 2 banks of up to 64MB at 0x80000000, 0x84000000, * then MAX_PHYSMEM_BITS is 32, SECTION_SIZE_BITS is 26. * - * Define these in your mach/memory.h. + * These can be overridden in your mach/memory.h. */ -#if !defined(SECTION_SIZE_BITS) || !defined(MAX_PHYSMEM_BITS) -#error Sparsemem is not supported on this platform +#if !defined(MAX_PHYSMEM_BITS) || !defined(SECTION_SIZE_BITS) +#define MAX_PHYSMEM_BITS 36 +#define SECTION_SIZE_BITS 28 #endif #endif -- 1.9.1
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