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SubjectRe: [PATCH 5/7] Alter get_order() so that it can make use of ilog2() on a constant [try #3]
On Wed, 13 Sep 2006 19:35:31 +0100
David Howells <dhowells@redhat.com> wrote:

> 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

This breaks ia64:

In file included from include/asm/ptrace.h:234,
from include/asm/processor.h:19,
from include/linux/prefetch.h:14,
from include/linux/list.h:8,
from include/linux/wait.h:22,
from include/linux/fs.h:266,
from init/do_mounts_initrd.c:3:
include/asm/page.h:165: error: syntax error before '?' token
include/asm/page.h:170: warning: type defaults to `int' in declaration of `order'
include/asm/page.h:170: error: conflicting types for 'order'
include/asm/page.h:168: error: previous declaration of 'order' was here
include/asm/page.h:170: error: braced-group within expression allowed only inside a function
include/asm/page.h:170: warning: type defaults to `int' in declaration of `ia64_intri_res'

But for some reason putting ARCH_HAS_GET_ORDER into
include/asm-ia64/bitops.h and including linux/log2.h in
include/asm-ia64/page.h doesn't fix it.

I didn't pursue it further, because sprinkling ARCH_HAS_FOO things into
bitops.h(!) is all rather hacky. Better to use CONFIG_* so they're always
visible and one knows where to go to find things.

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