linux-kernel - Re: [PATCH v15 19/26] x86/tsc: calibrate tsc only once

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Message-ID: <alpine.DEB.2.21.1809051240560.1416@nanos.tec.linutronix.de>
Date:   Wed, 5 Sep 2018 12:42:38 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Chuan Hua, Lei" <chuanhua.lei@...ux.intel.com>
cc:     Pavel Tatashin <pasha.tatashin@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v15 19/26] x86/tsc: calibrate tsc only once

On Wed, 5 Sep 2018, Chuan Hua, Lei wrote:

> > static unsigned long __init get_loops_per_jiffy(void)
> > {
> > 	unsigned long lpj = tsc_khz * KHZ;
> > 
> > 	do_div(lpj, HZ);
> > 	return lpj;
> > }
> Just tried this with 4.19-rc2 on x86(32bit). lpj return as zero which is not
> expected
> After disassembling the code,
> 	0xc1239a9e <+199>:   imul   $0x3e8,0xc12296e4,%edx
> 	0xc1239aa8 <+209>:   xor    %ecx,%ecx
> 	0xc1239aaa <+211>:   test   %edx,%edx
> 	0xc1239aac <+213>:   mov    %eax,%ebx
> 	0xc1239aae <+215>:   je     0xc1239abd <tsc_init+230>
> 	0xc1239ab0 <+217>:   mov    $0x64,%ecx
> 	0xc1239ab5 <+222>:   mov    %edx,%eax
> 	0xc1239ab7 <+224>:   xor    %edx,%edx
> 	0xc1239ab9 <+226>:   div    %ecx
> 	0xc1239abb <+228>:   mov    %eax,%ecx
> 	0xc1239abd <+230>:   mov    %ebx,%eax
> 	0xc1239abf <+232>:   mov    $0x64,%ebx
> 	0xc1239ac4 <+237>:   div    %ebx
> 	0xc1239ac6 <+239>:   mov    %ecx,%edx
> imul will load the result into %edx, %edx supposed to be high 32 bit which is
> not zero,
> It should be zero in this case. both lpj and tsc_khz should be u64 to work
> properly.

Good catch! Care to send a patch?

Thanks,

	tglx