On Tuesday 12 May 2015 11:44:21 Arnd Bergmann wrote:
There are of course multiple ways to do this. One way would be to change the code to work on 32-bit nanoseconds instead of 32-bit microseconds. This requires proving that the we cannot exceed 4.29 seconds of round-trip time in calc_rttavg(). Is that a valid assumption or not?
If not, we could replace do_gettimeofday() with ktime_get_ts64(). This will ensure we don't need a 64-bit division when converting the ts64 to a 32-bit microsecond value, and combined with the conversion is still no slower than do_gettimeofday(), and it still avoids the double bookkeeping because it uses a monotonic timebase that is robust against settimeofday.
Two other approaches that occurred to me later:
- introduce common ktime_get_ms(), ktime_get_us(), ktime_get_real_ms() and ktime_get_real_is() interfaces, to match the other interfaces we already provide. These could be done as efficiently or better than what aoe does manually today.
- change the timebase that is used for the computations in aoe to use scaled nanoseconds instead of microseconds. Using
u32 time = ktime_get_ns() >> 10;
would give you a similar range and precision as microseconds, but completely avoid integer division. You could also use a different shift value to either extend the range beyond 71 minutes, or the extend the precision to something below a microsecond. This would be the most efficient implementation, but also require significant changes to the driver.
Arnd