On Monday 02 July 2007, Marc Pignat wrote: > Hello all! > > We're currently working with David Brownell and Remy Bohmer on implementation > of clocksource/events on the at91rm9200 processor (arm arch). > > While debugging our implementation, we came about a rounding problem, in the > setting of min_delta_ns field, and I think it can be in almost all > clock_event_device. > > Almost all clock_event_device initialize the field min_delta_ns this way: > clkevt32k.mult = div_sc(CLOCK_TICK_RATE, NSEC_PER_SEC, clkevt32k.shift); > clkevt32k.max_delta_ns = clockevent_delta2ns(0x000fffff, &clkevt32k); > clkevt32k.min_delta_ns = clockevent_delta2ns(1, &clkevt32k); > > The clockevent_delta2ns function rounds the result down, so the rounded > min_delta_ns > can be smaller than the real min_delta_ns.
For concrete numbers: CLOCK_TICK_RATE = 32768, making 30517.58 nsec per tick But min_delta_ns becomes 30517. (And the shift is 32 bits; in-kernel docs could stand to include guidance about how to choose a good shift value.) > When clockevents_program_event is called, this rounding problem shows up: > ... > if (delta < dev->min_delta_ns) > delta = dev->min_delta_ns; > > clc = delta * dev->mult; > clc >>= dev->shift; > > return dev->set_next_event((unsigned long) clc, dev) > > When delta is to small, min_delta_ns will be used, and with a wrong rounding, > will call set_next_event with a value smaller than the expected, and > set_next_event will fail. Will be passed a value of zero, that is -- rather troublesome. > This will result in an infinite loop when clockevents_program_event is called > from tick_program_event with a too short delay. Unless zero ticks gets special cased to return -ETIME and then the backup strategies kick in, eventually asking for more ticks... > In our driver, we fixed this problem using that: > - clkevt32k.min_delta_ns = clockevent_delta2ns(1, &clkevt32k); > + clkevt32k.min_delta_ns = clockevent_delta2ns(1, &clkevt32k)+1; > > I think other clock_event_device can have the same problem. > > Regards > > Marc > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
