Rodrigo Rosenfeld Rosas wrote: > Em Sexta 10 Março 2006 15:32, Jan Kiszka escreveu: > >> Rodrigo Rosenfeld Rosas wrote: >>> Em Quinta 09 Março 2006 17:33, Jan Kiszka escreveu: >>>> Rodrigo Rosenfeld Rosas wrote: >>>>> Hi Jan, >>>>> >>>>> I'm still concerned about the future of RTDM and timer functions. I >>>>> think there should be some function for starting the timer manually, >>>>> since the automatic feature don't work great for RTDM drivers. >>>>> >>>>> It is not nice to have to run the latency (or any other) program for >>>>> starting the timer before I can load my driver. And it is not suffice to >>>>> run it once I booted. After I open/close my rtdm device and reload my >>>>> driver the problem will occur again and I'll have to re-run the latency >>>>> program. >>>> Sorry I don't see the problem here. >>>> >>>> # modprobe xeno_nucleus; cat /proc/xenomai/timer >>>> status=off:setup=1392:tickval=0:jiffies=0 >>>> >>>> # modprobe xeno_rtdm; cat /proc/xenomai/timer >>>> status=oneshot:setup=1392:tickval=1:jiffies=8113917792696 >>>> >>>> So the timer is running right since when rtdm is loaded?! >>> Yes, here too. >>> >>>> And that simple heartbeat rtdm example on my rt-addon homepage now >>>> cleanly runs even without any further helper to start some timer. >>> Yes, here too. You are right, once the timer is in oneshot mode. My driver >>> loads correctly without the helper. Then I start a user application that >>> changes the timer to periodic mode and uses my driver. When I reload my >>> driver, now in periodic mode, the problem raises. >> What happens if you make the periodic timer the default one in the >> kernel configuration? > > The same behaviour. > >>> It seems, there is no problem when the timer is set to oneshot. But when >>> turning it to periodic, at least one of rtdm_task_busy_sleep() or >>> rtdm_clock_read() doesn't seem to work. See below: >>> >>> cat /proc/xenomai/timer >>> status=periodic:setup=188:tickval=100000:jiffies=19972453 >>> >>> start_time = rtdm_clock_read(); >>> rtdm_task_busy_sleep(84000); >>> temp_time = rtdm_clock_read(); >>> rtdm_printk(KERN_INFO "Should be near 84000: %u\n", (unsigned int) >>> (temp_time-start_time)); >>> >>> Sometimes the result is "Should be near 84000: 100000", that is kind of >>> correct, since the tickval is 100000, although I think that those >>> functions in the RTDM driver context should be independent of the tick >>> value set by the user program... Maybe using oneshot in the driver calls >>> and periodic in the application... I really don't know what would be the >>> best approach here... >> rtdm_clock_read always uses the nucleus clock. > > Do you mean that rtdm_clock_read will always read a multiple of tickval > value? > If so, I think it would be good to make it clear on its documentation. "Get > system time" isn't enough for getting this information, IMHO.
Please have a look at the two sentences of documentation I added to SVN (didn't make it into the release). I gave your scenario a try, and I was able to verify that rtdm_task_busy_sleep works correctly under both timer modes. Indeed, the behaviour of rtdm_clock_read may have been confusing due to lacking information, but it was also correct. > >> Using something different >> (e.g. always TSC) would break applications specifying absolute times >> derived from the return values of other skins' functions. > > I did not understand. I'm talking about using TSC only for these two > functions. I can not see why shouldn't it be possible... I mean, I think the > driver should not depend on the userspace program timer for these two > functions. > >>> But the worst case is that sometimes I get "Should be near 84000: 0" which >>> clearly is a incorrect result. >> That might be a rounding issue somewhere, as the sleep than clearly did >> not wait at least one tick. Will have to check this when time permits. >> >>> After I run the latency program, the timer turns to be oneshot again and >>> everything goes right. >>> >>> What can I do to solve this problem? >> Use oneshot mode in the meantime - or even longer ;). > > That is what I'll gonna do, but I know it is not a definite solution. Since > I'm providing a framework, the user should decide which approach is better > for him/her, oneshot or periodic mode. > >> Why do you prefer >> periodic mode for your application? Another workaround: reduce the tick >> interval. > > I have some loops in my userspace programs that a common 100us tick would > satisfy them all. I think the overhead would be lower than using the > aperiodic oneshot mode... I'm not pretty sure about that. But that is not the > question. My application is just an use case of my framework (actually I > didn't even started building it). The final user should decide what is the > best approach for him/her, not me. So, I would prefer that the driver be > independent from the timer source chosen by the user program. > I see your point. But when the user decides to pick a low-precision timer source to reduce overhead, (s)he has to live with the side-effect. There is no such thing like "user vs. kernel" timer source - it is always the same. Thus, also the precision of time stamping in drivers suffers. Jan
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