Re: [PATCH RFC] x86, tsc: Allow for high latency in quick_pit_calibrate()

["H. Peter Anvin" <hpa@zytor.com>]

On 06/03/2015 11:29 AM, George Spelvin wrote:
> 
> Indeed, it's the only one which is guaranteed a separate crystal.
> Many low-cost chipsets generate ALL other frequencies from one crystal
> with PLLs.
> 

Not guaranteed either, and I know for a fact there are platforms out
there which synthesize the RTC clock.

> But as I mentioned earlier, you *can* get higher frequencies with
> interrupts *or* polling.  When you program the periodic event frequency
> (from 2 to 8192 Hz), it does three things at that rate:
> 
> 1) Periodic interrupts (if enabled),
> 2) Square wave output (if enabled, and relevant to discrete chips only), and
> 3) Sets the PE bit (register C, bit 6), which is auto-cleared on read.

Ah, I wasn't aware of the PF (not PE) bit.  That suddenly makes it a lot
more interesting.  So polling for the PF bit suddenly makes sense, and
is probably the single best option for calibration.

> So if you're willing to poll the device (which the TSC calibration does
> already), you can get high resolution tick edges without interrupts.
> 
> Because it's only one read (port 0x71), it's slightly faster than the PIT.
> 
> (I also wish we could use all those TSC reads for initial entropy seeding
> somehow.)

Well, on x86 hopefully the entropy problem should soon be history...

	-hpa