linux-kernel - Re: [patch 1/4] x86: vSMP: Fix is_vsmp

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Date:	Fri, 21 Mar 2008 10:59:27 -0700
From:	Ravikiran G Thirumalai <kiran@...lex86.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Yinghai Lu <yhlu.kernel@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Andi Kleen <ak@...e.de>, shai@...lex86.org
Subject: Re: [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@...lex86.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@...e.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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