From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752493AbaEVBOZ (ORCPT ); Wed, 21 May 2014 21:14:25 -0400 Received: from mail-pa0-f45.google.com ([209.85.220.45]:39842 "EHLO mail-pa0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751218AbaEVBOX (ORCPT ); Wed, 21 May 2014 21:14:23 -0400 Message-ID: <537D4F69.6050705@gmail.com> Date: Thu, 22 May 2014 09:14:17 +0800 From: Chen Gang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: =?UTF-8?B?566h6Zuq5rab?= CC: gerrit@erg.abdn.ac.uk, Guan Xuetao , David Miller , dccp@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: =?UTF-8?B?5Zue5aSN77yaIFJlOiDlm57lpI3vvJogW1BBVENIIGxpbnV4LW4=?= =?UTF-8?B?ZXh0XSBuZXQvZGNjcC90aW1lci5jOiB1c2UgJ3U2NCcgaW5zdGVhZCBvZiAnczY=?= =?UTF-8?B?NCcgdG8gYXZvaWQgY29tcGlsZXIncyB3YXJuaW5n?= References: <579164883.46799.1400720806850.JavaMail.root@bj-mail03.pku.edu.cn> In-Reply-To: <579164883.46799.1400720806850.JavaMail.root@bj-mail03.pku.edu.cn> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/22/2014 09:06 AM, 管雪涛 wrote: > > ----- Chen Gang 写道： >> On 05/22/2014 08:26 AM, 管雪涛 wrote: >>> >>> ----- Chen Gang 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 >>>> --- >>>> 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