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Date: Fri, 23 May 2014 09:43:35 +0800 From: Chen Gang <gang.chen.5i5j@...il.com> To: 管雪涛 <gxt@....edu.cn>, David Miller <davem@...emloft.net> CC: gerrit@....abdn.ac.uk, gxt@...c.pku.edu.cn, dccp@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: 回复: Re: [PATCH linux-next] net/dccp/timer.c: use 'u64' instead of 's64' to avoid compiler's warning On 05/23/2014 07:58 AM, 管雪涛 wrote: > > ----- David Miller <davem@...emloft.net> 写道: >> From: Chen Gang <gang.chen.5i5j@...il.com> >> Date: Wed, 21 May 2014 08:19:34 +0800 >> >>> '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> >> >> Applied to net-next, thanks. >> Thank you for your work. >> But that type check in include/asm-generic/div64.h is bogus, it should >> be checking sizeof(X) == 8 rather than the type thing, it just wants to >> make sure that the value is 64-bit regardless of it's signedness. >> >> The arch local implementations do not do this, and that's why very few >> other people notice this warning. > > Arch-dependent codes implement it with unsigned long long type. > And, every warning should not be ignored. > Yeah, we have to let do_div() no touch (especially for 32-bit machine, which the highest bit is checked). The related code in "include/asm-generic/div64.h": 23 #if BITS_PER_LONG == 64 24 25 # define do_div(n,base) ({ \ 26 uint32_t __base = (base); \ 27 uint32_t __rem; \ 28 __rem = ((uint64_t)(n)) % __base; \ 29 (n) = ((uint64_t)(n)) / __base; \ 30 __rem; \ 31 }) 32 33 #elif BITS_PER_LONG == 32 34 35 extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor); 36 37 /* The unnecessary pointer compare is there 38 * to check for type safety (n must be 64bit) 39 */ 40 # define do_div(n,base) ({ \ 41 uint32_t __base = (base); \ 42 uint32_t __rem; \ 43 (void)(((typeof((n)) *)0) == ((uint64_t *)0)); \ 44 if (likely(((n) >> 32) == 0)) { \ 45 __rem = (uint32_t)(n) % __base; \ 46 (n) = (uint32_t)(n) / __base; \ 47 } else \ 48 __rem = __div64_32(&(n), __base); \ 49 __rem; \ 50 }) And for division operation, architectures are signed/unsigned sensitive, e.g. div_u64() and div_s64(), they are different. Thanks. -- Chen Gang Open, share, and attitude like air, water, and life which God blessed -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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