On 05/22/2014 09:06 AM, 管雪涛 wrote:
>
> ----- Chen Gang <gang.chen.5i5j@xxxxxxxxx> 写道：
>> On 05/22/2014 08:26 AM, 管雪涛 wrote:
>>>
>>> ----- Chen Gang <gang.chen.5i5j@xxxxxxxxx> 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@xxxxxxxxx>
>>>> ---
>>>> 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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