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Re: BogoMIPS

To: Greg Weeks <greg.weeks@timesys.com>
Subject: Re: BogoMIPS
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 13 Apr 2005 14:21:18 +0100
Cc: Stuart Longland <stuartl@longlandclan.hopto.org>, linux-mips@linux-mips.org
In-reply-to: <425D0448.6010700@timesys.com>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <425BDCE4.6070708@timesys.com> <425C9DBF.6090807@longlandclan.hopto.org> <425D0448.6010700@timesys.com>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.4.1i
On Wed, Apr 13, 2005 at 07:36:40AM -0400, Greg Weeks wrote:

> >So honestly, I don't know what's happening with BogoMIPS. :-)  What I do
> >know however, it isn't worth a cracker in terms of benchmarking value. 
> >;-)
> 
> I could care less about it as a benchmarking value, but it's used to do 
> calibrated udelays.  If mips doesn't need it because it calibrates some 
> other way then fine. I suspect not though and we're probably using an 
> initial works everywhere value. I don't know though.

The bugs is the result of the somewhat messy way the calibration code is
working.  calibrate_delay() puts it's result into a global variable,
loops_per_jiffy.  The value is CPU-specific not global, so we need to
copy it to a per-processor data structure which we do on SMP but forget
to do on uniprocessor.  At the same time the actual delay loop code in
delay.h knew to use loops_per_jiffy on uniprocessor, so it was actually
working ok.

   Ralf

Index: arch/mips/kernel/cpu-probe.c
===================================================================
RCS file: /home/cvs/linux/arch/mips/kernel/cpu-probe.c,v
retrieving revision 1.43
diff -u -r1.43 cpu-probe.c
--- arch/mips/kernel/cpu-probe.c        8 Apr 2005 20:36:05 -0000       1.43
+++ arch/mips/kernel/cpu-probe.c        13 Apr 2005 13:19:11 -0000
@@ -17,7 +17,6 @@
 #include <linux/ptrace.h>
 #include <linux/stddef.h>
 
-#include <asm/bugs.h>
 #include <asm/cpu.h>
 #include <asm/fpu.h>
 #include <asm/mipsregs.h>
Index: arch/mips/kernel/smp.c
===================================================================
RCS file: /home/cvs/linux/arch/mips/kernel/smp.c,v
retrieving revision 1.77
diff -u -r1.77 smp.c
--- arch/mips/kernel/smp.c      18 Mar 2005 17:36:53 -0000      1.77
+++ arch/mips/kernel/smp.c      13 Apr 2005 13:19:11 -0000
@@ -226,7 +226,6 @@
 /* called from main before smp_init() */
 void __init smp_prepare_cpus(unsigned int max_cpus)
 {
-       cpu_data[0].udelay_val = loops_per_jiffy;
        init_new_context(current, &init_mm);
        current_thread_info()->cpu = 0;
        smp_tune_scheduling();
Index: include/asm-mips/bugs.h
===================================================================
RCS file: /home/cvs/linux/include/asm-mips/bugs.h,v
retrieving revision 1.10
diff -u -r1.10 bugs.h
--- include/asm-mips/bugs.h     25 Jul 2003 22:49:24 -0000      1.10
+++ include/asm-mips/bugs.h     13 Apr 2005 13:19:12 -0000
@@ -8,12 +8,17 @@
 #define _ASM_BUGS_H
 
 #include <linux/config.h>
+#include <asm/cpu.h>
+#include <asm/cpu-info.h>
 
 extern void check_bugs32(void);
 extern void check_bugs64(void);
 
 static inline void check_bugs(void)
 {
+       unsigned int cpu = smp_processor_id();
+
+       cpu_data[cpu].udelay_val = loops_per_jiffy;
        check_bugs32();
 #ifdef CONFIG_MIPS64
        check_bugs64();
Index: include/asm-mips/delay.h
===================================================================
RCS file: /home/cvs/linux/include/asm-mips/delay.h,v
retrieving revision 1.16
diff -u -r1.16 delay.h
--- include/asm-mips/delay.h    8 Oct 2004 02:41:17 -0000       1.16
+++ include/asm-mips/delay.h    13 Apr 2005 13:19:12 -0000
@@ -15,8 +15,6 @@
 
 #include <asm/compiler.h>
 
-extern unsigned long loops_per_jiffy;
-
 static inline void __delay(unsigned long loops)
 {
        if (sizeof(long) == 4)
@@ -82,11 +80,7 @@
        __delay(usecs);
 }
 
-#ifdef CONFIG_SMP
 #define __udelay_val cpu_data[smp_processor_id()].udelay_val
-#else
-#define __udelay_val loops_per_jiffy
-#endif
 
 #define udelay(usecs) __udelay((usecs),__udelay_val)
 

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