On Wed, Apr 30, 2014 at 05:56:53PM +0100, Sebastian Andrzej Siewior wrote: > * Will Deacon | 2014-04-30 14:26:28 [+0100]: > >I don't think that's the problem I was referring to. What I mean is that a > >clocksource might overflow at any number of bits, so the delay calculation > >needs to take this into account when it does: > > > > while ((get_cycles() - start) < cycles) > > > >because a premature overflow from get_cycles() will cause us to return > >early. The solution is to mask the result of the subtraction before the > >comparison to match the width of the clock. > > So I got this:
[...] > Is this what you had in mind? If so, there is one user of > register_current_timer_delay() which I didn't convert. That is > arch_timer_delay_timer_register(). It returns arch_counter_get_cntvct() > which seems to return an u64 (which is truncated to 32bit). However > arch_counter_register() registers the clocksource with 56bits. So this > does not look too good, right? That should be fine, I think there's only an issue if you can overflow twice during a single delay operation, so the thing would need to be ticking at quite a frequency for that to happen! > The other thing I noticed is > |arch/arm/include/asm/timex.h:typedef unsigned long cycles_t; > > This works for clocksource because timekeeping is using > |include/linux/clocksource.h:typedef u64 cycle_t; > > instead. > Do I assume correct, that the arch_timer is really providing a number > wider than 32bit? Shouldn't I then promote cycles_t to 64bit if that > timer is active? Unless you have better suggestions of course :) The architected timer is guaranteed to be at least 56 bits wide, but I think we can safely truncate delay sources to 32-bit. So actually, we only have a problem if people want to register delay clocks smaller than 32-bit. Maybe it's simpler to enforce at least 32-bit precision and don't bother with the registration if the clock is smaller than that? You could use sizeof(cycles_t) to parameterise that. Will -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
