linux-kernel - Re: [RFC PATCH] Make quick_pit

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Date:	9 Jun 2015 05:54:11 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	adrian.hunter@...el.com, linux@...izon.com, tglx@...utronix.de
Cc:	ak@...ux.intel.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	luto@...capital.net, mingo@...nel.org
Subject: Re: [RFC PATCH] Make quick_pit_calibrate more robust

Adrian Hunter wrote:
> I am not really sure what problem you are trying to solve.

I'm solving the problem I ran into testing variants on your patch!
Some I fairly normal hardware that I have which fails reliably with the
old quick code:
-> tsc: Fast TSC calibration failed: code=0 i=30 j=0 d=12453,103342771544
-> tsc: Unable to calibrate against PIT
-> tsc: using HPET reference calibration
-> Using user defined TSC freq: 2500.210 MHz
-> tsc: Detected 2500.210 MHz processor

(The "code=0" is some debugging stuff I added.)

And works with this:
-> tsc: Fast TSC calibration using PIT: 254(12315)..159(12480)
-> Using user defined TSC freq: 2500.210 MHz
-> tsc: Detected 2500.210 MHz processor


> I tried this patch on my problem hardware but it failed both with
> ONE_BYTE_PIT set to 0 and set to 1.  I am not sure it addresses the
> 'really-long-latency to read the counter' problem that I have.

I'm sure it *doesn't* address your problem; it's solving something else.
It works if the read latency is only *sometimes* slow.  If it's *always*
slow (your problem), your solution or some variant would be required.

> A bigger issue for my case is that "slow" calibration is not that slow,
> taking only 10ms anyway which is much better than the 50ms max for so-called
> "quick" calibration.

I need to study how that part works and what it does.
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