linux-kernel - Re: 回复： [PATCH linux-next] net/dccp/timer.c: use 'u64' instead of 's64' to avoid compiler's warning

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Message-ID: <537D4C73.40901@gmail.com>
Date:	Thu, 22 May 2014 09:01:39 +0800
From:	Chen Gang <gang.chen.5i5j@...il.com>
To:	管雪涛 <gxt@....edu.cn>
CC:	gerrit@....abdn.ac.uk, Guan Xuetao <gxt@...c.pku.edu.cn>,
	David Miller <davem@...emloft.net>, dccp@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: 回复： [PATCH linux-next] net/dccp/timer.c: use 'u64' instead of 's64' to avoid compiler's warning

On 05/22/2014 08:26 AM, 管雪涛 wrote:
> 
> ----- Chen Gang <gang.chen.5i5j@...il.com> 写道：
>> 'dccp_timestamp_seed' is initialized once by ktime_get_real() in
>> dccp_timestamping_init(). It is always less than ktime_get_real()
>> in dccp_timestamp().
>>
>> Then, ktime_us_delta() in dccp_timestamp() will always return positive
>> number. So can use manual type cast to let compiler and do_div() know
>> about it to avoid warning.
>>
>> The related warning (with allmodconfig under unicore32):
>>
>>     CC [M]  net/dccp/timer.o
>>   net/dccp/timer.c: In function ‘dccp_timestamp’:
>>   net/dccp/timer.c:285: warning: comparison of distinct pointer types lacks a cast
>>
>>
>> Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
>> ---
>>  net/dccp/timer.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/dccp/timer.c b/net/dccp/timer.c
>> index 16f0b22..1cd46a3 100644
>> --- a/net/dccp/timer.c
>> +++ b/net/dccp/timer.c
>> @@ -280,7 +280,7 @@ static ktime_t dccp_timestamp_seed;
>>   */
>>  u32 dccp_timestamp(void)
>>  {
>> -	s64 delta = ktime_us_delta(ktime_get_real(), dccp_timestamp_seed);
>> +	u64 delta = (u64)ktime_us_delta(ktime_get_real(), dccp_timestamp_seed);
> 
> Do you assume that delta should be very small?
> Otherwise, return value will be different if data type is changed.
> 

'u64' is a very very large number. after calculation, if it is based on
nano second (although I am not quite sure whether it is based on it).

        a hour,            3,600,000,000,000ns
        a day,            90,000,000,000,000ns
        a year,       50,000,000,000,000,000ns
        10 years,    500,000,000,000,000,000ns
        100 years, 5,000,000,000,000,000,000ns
        4G * 4G = 16,000,000,000,000,000,000ns

So we can assume it will never overflow for 'u64'.


Thanks.
-- 
Chen Gang

Open, share, and attitude like air, water, and life which God blessed
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