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SubjectRe: [patch 1/4] x86: vSMP: Fix is_vsmp_box()
On Fri, Mar 21, 2008 at 10:11:39AM +0100, Ingo Molnar wrote:
>
>* Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
>
>> >> /* Check if we are running on a ScaleMP vSMP box */
>> >> - if (read_pci_config(0, 0x1f, 0, PCI_VENDOR_ID) ==
>> >> - (PCI_VENDOR_ID_SCALEMP || (PCI_DEVICE_ID_SCALEMP_VSMP_CTL << 16)))
>> >> + if ((read_pci_config_16(0, 0x1f, 0, PCI_VENDOR_ID) ==
>> >> + PCI_VENDOR_ID_SCALEMP) &&
>> >> + (read_pci_config_16(0, 0x1f, 0, PCI_DEVICE_ID) ==
>> >> + PCI_DEVICE_ID_SCALEMP_VSMP_CTL))
>> >> vsmp = 1;
>> >>
>> >> return vsmp;
>> >
>> >why read two times
>> >
>>
>> Well, the pci cfg space read happens just _once_ during the boot, as
>> the result is cached in a static flag. The above code is better
>> readable. So readability is better than micro-optimization here.
>
>i think the patch below results in even more readable and a bit smaller
>
>code got a bit smaller:
>
>arch/x86/kernel/vsmp_64.o:
>
> text data bss dec hex filename
> 205 4 0 209 d1 vsmp_64.o.before
> 181 4 0 185 b9 vsmp_64.o.after

Wow! good to know that avoiding one call shaved so many bytes.
Agreed, the below patch is better.

Thanks,
Kiran

>
>Signed-off-by: Ingo Molnar <mingo@elte.hu>
>---
> arch/x86/kernel/vsmp_64.c | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
>Index: linux-x86.q/arch/x86/kernel/vsmp_64.c
>===================================================================
>--- linux-x86.q.orig/arch/x86/kernel/vsmp_64.c
>+++ linux-x86.q/arch/x86/kernel/vsmp_64.c
>@@ -120,10 +120,8 @@ int is_vsmp_box(void)
> return vsmp;
>
> /* Check if we are running on a ScaleMP vSMP box */
>- if ((read_pci_config_16(0, 0x1f, 0, PCI_VENDOR_ID) ==
>- PCI_VENDOR_ID_SCALEMP) &&
>- (read_pci_config_16(0, 0x1f, 0, PCI_DEVICE_ID) ==
>- PCI_DEVICE_ID_SCALEMP_VSMP_CTL))
>+ if (read_pci_config(0, 0x1f, 0, PCI_VENDOR_ID) ==
>+ (PCI_VENDOR_ID_SCALEMP | (PCI_DEVICE_ID_SCALEMP_VSMP_CTL << 16)))
> vsmp = 1;
>
> return vsmp;


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