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SubjectRe: [PATCH] x86,APIC: Detect lapic_is_integrated() once - use on and on.
On Fri, 22 May 2009, Rakib Mullick wrote:
> Impact: Reduce text space through efficient coding
>
> Determine apic is integrated or not - by calling
> lapic_is_integrated() once from APIC_init_uniprocessor() and keep it
> in a variable integrated_lapic. Thus we can determine apic is
> integrated or not, by checking the variable instead of calling
> lapic_is_integrated() on and on. Marking lapic_is_integrated() as
> __init, which reduces some text space. Also allows us to get away from
> messy #ifdef and inline.
>
> ---
> Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
>
> --- linus/arch/x86/kernel/apic/apic.c 2009-05-21 22:22:15.000000000 +0600
> +++ rakib/arch/x86/kernel/apic/apic.c 2009-05-21 23:27:26.000000000 +0600
> @@ -127,6 +127,9 @@ early_param("nox2apic", setup_nox2apic);
>
> unsigned long mp_lapic_addr;
> int disable_apic;
> +
> +static int integrated_lapic;
> +
> /* Disable local APIC timer from the kernel commandline or via dmi quirk */
> static int disable_apic_timer __cpuinitdata;
> /* Local APIC timer works in C2 */
> @@ -188,13 +191,12 @@ static inline int lapic_get_version(void
> /*
> * Check, if the APIC is integrated or a separate chip
> */
> -static inline int lapic_is_integrated(void)
> +void __init lapic_is_integrated(void)
> {
> -#ifdef CONFIG_X86_64
> - return 1;
> -#else
> - return APIC_INTEGRATED(lapic_get_version());
> -#endif
> + if (APIC_INTEGRATED(lapic_get_version()))
> + integrated_lapic = 1;
> + else
> + integrated_lapic = 0;
> }

I agree in general, but the patch is flawed as it prevents the
compiler to optimize the X86_64 case out.

What you really want is:

#ifdef CONFIG_X86_64
# define integrated_lapic (1)
#else
static int integrated_lapic __read_mostly;
#endif

static inline void lapic_is_integrated(void)
{
#ifdef CONFIG_X86_32
integrate_lapic = APIC_INTEGRATED(lapic_get_version());
#endif
}

So that way the check is only done for 32bit systems and the compiler
will optimize out the code which depends on !integrated_lapic.

Thanks,

tglx


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