On 22/05/11 01:33, J. William Campbell wrote: > On 5/21/2011 5:38 AM, Graeme Russ wrote: >> Hi All,
>> 4. Implement microsecond API - get_usec_timer() >> ----------------------------------------------- >> - Useful for profiling >> - A 32-bit microsecond counter wraps in 71 minutes - Probably OK for most >> U-Boot usage scenarios >> - By default could return get_timer() * 1000 if hardware does not support >> microsecond accuracy - Beware of results> 32 bits! > Hi All, > I think the multiply overflowing an unsigned long is ok here as long > as the timeout value you desire is less than 71 seconds. This assumes that > the CPU returns the correct lower 32 bits when overflow occurs, but I think > this is the "normal" behavior(?) I think you mean 71 minutes - 2^32 = 4294967296 (usec) = 4294967.296 (msec) = 4294.967296 s = 71.582788267 minutes So provided any millisecond timer using a 32-bit microsecond timer as a time base is called every 71 minutes, a 32-bit microsecond timer should suffice to keep the msb's accurately maintained by software >> - If hardware supports microsecond resolution counters, get_timer() could >> simply use get_usec_timer() / 1000 > I think this actually is NOT equivalent to the "current" API in > that the full 32 bits of the timer is not available and as a result the > "wrapping" properties of a 32 bit subtract for delta times will not work > properly. If a "larger" counter is available in hardware, then it is > certainly possible to do a 64 by 32 bit divide in get_timer, but probably > you don't want to do that either. As previously discussed, it is possible > to extract a 32 bit monotonic counter of given resolution (microsecond or > millisecond resolution) from a higher resolution counter using a shift to > "approximately" the desired resolution followed by a couple of multiply/add > functions of 32 bit resolution. To do this with a microsecond resolution, > a 42 bit or larger timer is required. The "extra" bits can be provided in Of course, how silly of me - To downscale the microsecond timer down to milliseconds you need to have at least 1000 times more resolution (9.965784285 bits) - It was late ;) > software as long as the get_timer/get_usec_timer routines are called more > often than every 71/2 sec, so that a correct delta in microseconds can be > obtained. Note that when the timer is not actively in use (not called > often enough), the millisecond timer msb would stop updating, but that > wouldn't matter. Minutes - see above > If the hardware supports sub-microsecond accuracy in a "longer" > register, say 64 bits, you can just convert the 64 bit hardware timer to 32 > bit microseconds or milliseconds by a shift and 32 bit multiplies Yes > > Good luck with this effort. I think getting the timer API and also > the method of implementation of the interface to the hardware to be the > same across all u-boot architectures is a very good idea, and it is > possible. However, it is a quite a bit of work and I am glad you are brave > enough to try! It's all there already - it just needs a little bit of housekeeping :) Regards, Graeme _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
