On Wed, Oct 14, 2015 at 12:20:26PM +0100, Suzuki K. Poulose wrote:
> Introduce helpers for finding the number of page table
> levels required for a given VA width, shift for a particular
> page table level.
>
> Convert the existing users to the new helpers. More users
> to follow.
>
> Cc: Ard Biesheuvel <ard.biesheu...@linaro.org>
> Cc: Mark Rutland <mark.rutl...@arm.com>
> Cc: Catalin Marinas <catalin.mari...@arm.com>
> Cc: Will Deacon <will.dea...@arm.com>
> Cc: Marc Zyngier <marc.zyng...@arm.com>
> Signed-off-by: Suzuki K. Poulose <suzuki.poul...@arm.com>
>
> ---
> Changes since V2:
> - Add comments around the macros
> - Change ARM64_HW_PGTABLE_LEVEL_SHIFT to accept pagetable level as
> described by ARM ARM
> ---
> arch/arm64/include/asm/pgtable-hwdef.h | 25 ++++++++++++++++++++++---
> 1 file changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable-hwdef.h
> b/arch/arm64/include/asm/pgtable-hwdef.h
> index 95c1ec0..c6194ab 100644
> --- a/arch/arm64/include/asm/pgtable-hwdef.h
> +++ b/arch/arm64/include/asm/pgtable-hwdef.h
> @@ -16,13 +16,31 @@
> #ifndef __ASM_PGTABLE_HWDEF_H
> #define __ASM_PGTABLE_HWDEF_H
>
> +/*
> + * Number of page-table levels required to address 'va_bits' wide
> + * address, without section mapping. We resolve the top (va_bits -
> PAGE_SHIFT)
> + * bits with (PAGE_SHIFT - 3) bits at each page table level. Hence:
> + *
> + * levels = DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
> + *
> + * We cannot include linux/kernel.h which defines DIV_ROUND_UP here
> + * due to build issues. So we use the following magic formula.
> + */
> +#define ARM64_HW_PGTABLE_LEVELS(va_bits) (((va_bits) - 4) / (PAGE_SHIFT - 3))
I think I failed the interview question [1]. :(
I read the comment to mean this was a brand-new piece of magic, as
opposed to a constant-folded copy of DIV_ROUND_UP. So it seems there's
still some scope for confusion, even if that only includes me.
I think we should bite the bullet and have a full copy of DIV_ROUND_UP,
e.g.
/*
* We cannot include linux/kernel.h for DIV_ROUND_UP due to build
* issues. Instead, copy & paste its magic formula here for now.
*/
#define __DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
/*
* Number of page-table levels required to address 'va_bits' wide
* address, without section mapping. We resolve the top (va_bits - PAGE_SHIFT)
* bits with (PAGE_SHIFT - 3) bits at each page table level.
*/
#define ARM64_HW_PGTABLE_LEVELS(va_bits) \
__DIV_ROUND_UP((va_bits - PAGE_SHIFT), (PAGE_SHIFT - 3))
It's a little verbose, but it keeps this legible and ensures I get the
job. We can leave the constant-folding to the compiler.
Other than that, this looks good to me. With the above applied:
Acked-by: Mark Rutland <mark.rutl...@arm.com>
Thanks,
Mark.
[1]
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-October/375609.html
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