linux-kernel - Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated clocksource read value

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Date:	Wed, 16 Dec 2015 15:11:25 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Daniel Lezcano <daniel.lezcano@...aro.org>
CC:	<tglx@...utronix.de>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clocksource/drivers/pistachio: Fix wrong calculated
 clocksource read value

Dear Daniel,

On Tue, 15 Dec 2015 21:59:30 +0100 Daniel Lezcano wrote:

> On 11/25/2015 04:42 PM, Jisheng Zhang wrote:
> > Let's assume the counter value is 0xf000000, the pistachio clocksource
> > read cycles function would return 0xffffffff0fffffff, but it should
> > return 0xfffffff.
> >
> > We fix this issue by calculating bitwise-not counter, then cast to
> > cycle_t.
> >
> > Signed-off-by: Jisheng Zhang <jszhang@...vell.com>  
> 
> Hi Jisheng,
> 
> I tried to reproduce this behavior on x86_64 but without success.
> 
> On which architecture did you produce this result ? Do you have a simple 
> test program to check with ?

I have no HW platforms with pistachio, just read the code and run the
following test code in x86_64 and x86_32:

#include <stdio.h>
unsigned long long pistachio_clocksource_read_cycles()
{
	unsigned int counter = 0xf000000;
	return ~(unsigned long long)counter;
}
int main()
{
	printf("%llx\n", pistachio_clocksource_read_cycles());
	return 0;
}

Thanks,
Jisheng
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