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    Subject[PATCH 5/7] Alter get_order() so that it can make use of ilog2() on a constant [try #2]
    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 | 14 --------------
    include/linux/log2.h | 45 ++++++++++++++++++++++++++++++++++++++++++++
    2 files changed, 45 insertions(+), 14 deletions(-)

    diff --git a/include/asm-generic/page.h b/include/asm-generic/page.h
    index a96b5d9..a7e374e 100644
    --- a/include/asm-generic/page.h
    +++ b/include/asm-generic/page.h
    @@ -6,20 +6,6 @@ #ifndef __ASSEMBLY__

    #include <linux/compiler.h>

    -/* Pure 2^n version of get_order */
    -static __inline__ __attribute_const__ int get_order(unsigned long size)
    -{
    - int order;
    -
    - size = (size - 1) >> (PAGE_SHIFT - 1);
    - order = -1;
    - do {
    - size >>= 1;
    - order++;
    - } while (size);
    - return order;
    -}
    -
    #endif /* __ASSEMBLY__ */
    #endif /* __KERNEL__ */

    diff --git a/include/linux/log2.h b/include/linux/log2.h
    index 6ce81b7..88b7b0e 100644
    --- a/include/linux/log2.h
    +++ b/include/linux/log2.h
    @@ -25,6 +25,7 @@ int ____ilog2_NaN(void);
    * non-constant log of base 2 calculators
    * - the arch may override these in asm/bitops.h if they can be implemented
    * more efficiently than using fls() and fls64()
    + * - the arch is not required to handle n==0 if implementing the fallback
    */
    #ifndef ARCH_HAS_ILOG2_U32
    static inline __attribute__((const))
    @@ -42,6 +43,36 @@ int __ilog2_u64(u64 n)
    }
    #endif

    +/*
    + * 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);
    + order = -1;
    + do {
    + size >>= 1;
    + order++;
    + } while (size);
    + return order;
    +#endif
    +}
    +#endif
    +
    /**
    * ilog2_u32 - log of base 2 of 32-bit unsigned value
    * @n - parameter
    @@ -185,4 +216,18 @@ #else
    #define ilog2(n) ilog2_u64(n)
    #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 /* _LINUX_LOG2_H */
    -
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