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    Subject[PATCH 3/4] LOG2: Alter get_order() so that it can make use of ilog2() on a constant [try #4]
    Date
    From: David Howells <dhowells@redhat.com>

    Alter get_order() so that it can make use of ilog2() on a constant to produce a
    constant value, retaining the ability for an arch to override it in the
    non-const case.

    Signed-Off-By: David Howells <dhowells@redhat.com>
    ---

    include/asm-generic/page.h | 38 ++++++++++++++++++++++++++++++++++----
    1 files changed, 34 insertions(+), 4 deletions(-)

    diff --git a/include/asm-generic/page.h b/include/asm-generic/page.h
    index a96b5d9..b55052c 100644
    --- a/include/asm-generic/page.h
    +++ b/include/asm-generic/page.h
    @@ -4,21 +4,51 @@ #define _ASM_GENERIC_PAGE_H
    #ifdef __KERNEL__
    #ifndef __ASSEMBLY__

    -#include <linux/compiler.h>
    +#include <linux/log2.h>

    -/* Pure 2^n version of get_order */
    -static __inline__ __attribute_const__ int get_order(unsigned long size)
    +/*
    + * non-const pure 2^n version of get_order
    + * - the arch may override these in asm/bitops.h if they can be implemented
    + * more efficiently than using the arch log2 routines
    + * - we use the non-const log2() instead if the arch has defined one suitable
    + */
    +#ifndef ARCH_HAS_GET_ORDER
    +static inline __attribute__((const))
    +int __get_order(unsigned long size, int page_shift)
    {
    +#if BITS_PER_LONG == 32 && defined(ARCH_HAS_ILOG2_U32)
    + int order = __ilog2_u32(size) - page_shift;
    + return order >= 0 ? order : 0;
    +#elif BITS_PER_LONG == 64 && defined(ARCH_HAS_ILOG2_U64)
    + int order = __ilog2_u64(size) - page_shift;
    + return order >= 0 ? order : 0;
    +#else
    int order;

    - size = (size - 1) >> (PAGE_SHIFT - 1);
    + size = (size - 1) >> (page_shift - 1);
    order = -1;
    do {
    size >>= 1;
    order++;
    } while (size);
    return order;
    +#endif
    }
    +#endif
    +
    +/**
    + * get_order - calculate log2(pages) to hold a block of the specified size
    + * @n - size
    + *
    + * calculate allocation order based on the current page size
    + * - this can be used to initialise global variables from constant data
    + */
    +#define get_order(n) \
    +( \
    + __builtin_constant_p(n) ? \
    + ((n < (1UL << PAGE_SHIFT)) ? 0 : ilog2(n) - PAGE_SHIFT) : \
    + __get_order(n, PAGE_SHIFT) \
    + )

    #endif /* __ASSEMBLY__ */
    #endif /* __KERNEL__ */
    -
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