On Tuesday 12 May 2015 11:44:21 Arnd Bergmann wrote:
There are of course multiple ways to do this. One way would be toTwo other approaches that occurred to me later:
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.
- 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.