Date: Mon, 30 Jul 2012 14:33:28 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Alexander Motin <mav@FreeBSD.org> Cc: freebsd-acpi@FreeBSD.org Subject: Re: Using bintime() in acpi_cpu_idle()? Message-ID: <20120730141426.D1219@besplex.bde.org> In-Reply-To: <50154C58.4060408@FreeBSD.org> References: <5014DD00.3000307@FreeBSD.org> <20120729175031.U2084@besplex.bde.org> <50150CF5.4070605@FreeBSD.org> <20120729221526.H2941@besplex.bde.org> <50154C58.4060408@FreeBSD.org>
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On Sun, 29 Jul 2012, Alexander Motin wrote: > On 29.07.2012 15:26, Bruce Evans wrote: >> On Sun, 29 Jul 2012, Alexander Motin wrote: >> >>> On 29.07.2012 11:37, Bruce Evans wrote: >>> ... >>>> binuptime() is more accurate than uncalibrated scaling. Is accuracy >>>> required? >>> >>> Accuracy is not required at all. +-20% is not a problem. >>> >>>> If not, the CPU ticker might work, and is faster than HPET, >>>> and and is not under user control for perverse settings. It normally >>>> reduces to readtsc() with no serializing instruction even in proposed >>>> changes. This is good enough for process times (not very good) and >>>> depends on the CPU not changing. Its calibration is very accurate >>>> (similar to timecounters) modulo bugs, but not always up to date. >>> >>> Problem with ticker that it may stop during idle periods, and idle is >>> exactly what happens here. Unlike timecounter usage here we don't need >>> CPU synchronicity, but we need it working during deep sleeps. >> >> The ticker is the same as the timecounter in many cases of interest. If >> the TSC stops then it cannot be used for timecounting unless timecounting >> is reinitialized. Timecounting should be reinitialized after deep sleeps, >> but you say you need it to work during deep sleeps. > > Timecounter already has detection logic to disable TSC in cases where it is > unreliable. I don't want to replicate it here. I need not precise and not > synchronized by reliable and fast time source. Yes, this logic gives exactly what you don't want (an inefficient timecounter), by preventing use of the TSC for the timecounter, although the TSC is perfectly usable for the ticker and here. >> I wouldn't trust timecounters for some time after waking up after a >> deep sleep. If their clock stopped then the times read might only be >> very out of date. If their clock didn't stop, then they might have >> wrapped or otherwise overflowed and the times read would be garbage. >> Is there any locking or ordering to prevent them being used before they >> are reinitialized? > > I am not sure what reinitialization are you talking about. IIRC, there is no > any waking up code for TSC. None other time counters have problems with > C-states. It is the timecounter code that needs reinitializing. If the TSC stops, or wraps mod 2**32, then its counts become garbage for the purpose of timecounting. Maybe it is not used for timecounting in either of these cases. But these cases shouldn't prevent its use for timecounting. The 2**32 number is because timecounters only use 32 bits of hardware counters (for efficiency). So even if the hardware has some magic to not stop the TSC while sleeping (maybe it fakes not stopping it be reloading on wakeup), it is still unusable by timecounters after sleeping for a second or 2 so that it wraps. The software needs similar faking to reload the timecounter on wakeup. This makes use of timecounters in sleep/wakeup code fragile. At boot time there is a dummy timecounter that returns bogo-times. Apparently sleeping doesn't occur before the timecounter is switched to a real one. The dummy timecounter isn't switched back to after boot time. But it probably should be, since the hardware timecounter may have stopped or wrapped. Sleeping could just set a flag to indicate this state, but then you would have to provide a fake time anyway on finding the flag set. Boot time just points to the dummy timecounter so as not to check this flag in all early timecounter "hardware" calls. Bruce
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