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

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Message-ID: <b4f947f1-72e3-8dc1-7e9b-591d92306e55@linux.intel.com>
Date:   Wed, 5 Sep 2018 17:08:10 +0800
From:   "Chuan Hua, Lei" <chuanhua.lei@...ux.intel.com>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v15 19/26] x86/tsc: calibrate tsc only once

> 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.