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

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Message-ID: <567128A0.8060107@linaro.org>
Date:	Wed, 16 Dec 2015 10:02:24 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Jisheng Zhang <jszhang@...vell.com>
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

On 12/16/2015 08:11 AM, Jisheng Zhang wrote:
> 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;
> }

Ok, I reproduced it. I had an issue with the signed/unsigned type.

That's a good catch !


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