On Fri, Jan 26, 2018 at 3:20 PM, Baolin Wang <baolin.w...@linaro.org> wrote: > On 26 January 2018 at 22:00, Daniel Thompson <daniel.thomp...@linaro.org> > wrote: >> On Fri, Jan 26, 2018 at 10:21:58AM +0100, Arnd Bergmann wrote: >>> On Fri, Jan 26, 2018 at 4:03 AM, Baolin Wang <baolin.w...@linaro.org> wrote: >>> >>> Using ktime_get_seconds() avoids locking problems, but I wonder what >>> would happen if we trigger the 'WARN_ON(timekeeping_suspended)' >>> from kdb. Is that a problem? If it is, we have to use >>> ktime_get_mono_fast_ns() >>> and div_u64() instead. >> >> Normally a WARN_ON() doesn't triggered a kgdb_breakpoint() so (apart >> from bugs) we can start executing the warning. Unfortunately >> kdb_trap_printk isn't set when we call ktime_get_seconds() so printing >> the warning isn't safe. >> >> If we had no choice of time function we could work around by >> enabling printk() trapping for the call but since ktime_get_mono_fast_ns() >> already exists its probably best just to use that. >> > > If timekeeping_suspended is set, which means the system had been in > suspend state. So now we still need debugger the system? But cores > were already powered down.
I'm not using kdb myself, but I would assume that trapping into the debugger during a suspend/resume bug is a very important scenario. > The ktime_get_mono_fast_ns() will access the the clocksource driver, > if the timekeeping is suspended following system suspend and the > clocksource is not SUSPEND_NONSTOP, we may meet some unexpected issue > to access the timer's register without clock. So I am not sure if > ktime_get_mono_fast_ns() can work well for this case. I misread the code the same way before, but as Thomas Gleixner pointed out, ktime_get_mono_fast_ns() will not call the clocksource driver when timekeeping is suspended. See halt_fast_timekeeper(). Arnd