On 06.11.2012 23:46, Russell King - ARM Linux wrote: > * round_jiffies - function to round jiffies to a full second > > This is probably it. With mine, it's a 32.768kHz clock, so limiting > it to 16-bit gives a wrap period of 2 seconds exactly. We take 10% > off, so the timer would be asked to fire every 1.8s, which would be > rounded up to 2 seconds. That's a little too close for comfort... Confirmed. - if I artificially change my timer code to act as a 16-bit one, I get wrap-arounds. Not always, but there are definitely some during the bootup (where maybe the tick is sometimes delayed a tad more) - if I then remove the round_jiffies and only leave jiffies + wrap_ticks, the wrap-arounds go away > I think in this case, we need a version of round_jiffies() which _always_ > rounds down. Unfortunately, it doesn't exist. Thomas? What are the > options here? What is actually the reason of round_jiffies there? The http://kernel.org/doc/htmldocs/device-drivers/API-round-jiffies.html mentions saving power, is it the only one? I'd probably just leave the round_jiffies out at least for wrap_ticks < around 16*HZ. Above that the error by possibly rounding up can be ignored. There is also a round_jiffies_up - unless I am too tired, as long as we tick no faster than once per second, subtracting (HZ-1) and rounding up should be the same as rounding down. But: is the round_jiffies* safe at all for sub-second precision at jiffies around 0xffffffff? From quick looking it does a modulo, 0xffffffff % say 250 is 45, the next jiffy is at 0... Regards -- Stano -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
