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 [off-list ref] 写道：

quoted

On 05/22/2014 08:26 AM, 管雪涛 wrote:

quoted

----- Chen Gang [off-list ref] 写道：

quoted

'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 <redacted>
---
 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.

quoted

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