linux-kernel - Re: 回复： 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: <537D4F69.6050705@gmail.com>
Date:	Thu, 22 May 2014 09:14:17 +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： Re: 回复： [PATCH linux-next] net/dccp/timer.c: use 'u64' instead of 's64' to avoid compiler's warning



On 05/22/2014 09:06 AM, 管雪涛 wrote:
> 
> ----- Chen Gang <gang.chen.5i5j@...il.com> 写道：
>> 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'.
> 
> However, return value of dccp_timestamp function is u32.
> 

After check the function comments.

/**
 * dccp_timestamp  -  10s of microseconds time source
 * Returns the number of 10s of microseconds since loading DCCP. This is native
 * DCCP time difference format (RFC 4340, sec. 13).
 * Please note: This will wrap around about circa every 11.9 hours.
 */

So, it is still acceptable, although it is truncated into 'u32' before return.

>>
>>
>> Thanks.
>> -- 
>> Chen Gang
>>
>> Open, share, and attitude like air, water, and life which God blessed

-- 
Chen Gang

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