[U-Boot] [RFC] Review of U-Boot timer API

[Graeme Russ <graeme.russ@gmail.com>]

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