| Message ID | 537BF116.9000900@gmail.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
----- Chen Gang <gang.chen.5i5j@gmail.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@gmail.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. > > do_div(delta, 10); > return delta; > -- > 1.9.2.459.g68773ac -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 05/22/2014 08:26 AM, 管雪涛 wrote: > > ----- Chen Gang <gang.chen.5i5j@gmail.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@gmail.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 <gang.chen.5i5j@gmail.com> 写道： > On 05/22/2014 08:26 AM, 管雪涛 wrote: > > > > ----- Chen Gang <gang.chen.5i5j@gmail.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@gmail.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. > > > 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 05/22/2014 09:06 AM, 管雪涛 wrote: > > ----- Chen Gang <gang.chen.5i5j@gmail.com> 写道： >> On 05/22/2014 08:26 AM, 管雪涛 wrote: >>> >>> ----- Chen Gang <gang.chen.5i5j@gmail.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@gmail.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
From: Chen Gang <gang.chen.5i5j@gmail.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@gmail.com> Applied to net-next, thanks. 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.
----- David Miller <davem@davemloft.net> 写道： > From: Chen Gang <gang.chen.5i5j@gmail.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@gmail.com> > > Applied to net-next, thanks. > > 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. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 05/23/2014 07:58 AM, 管雪涛 wrote: > > ----- David Miller <davem@davemloft.net> 写道： >> From: Chen Gang <gang.chen.5i5j@gmail.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@gmail.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.
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_div(delta, 10); return delta;
'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@gmail.com> --- net/dccp/timer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)