Date: Fri, 5 Jun 2015 07:58:37 +0200
From: Ingo Molnar <mingo@kernel.org>
To: George Spelvin <linux@horizon.com>
Cc: adrian.hunter@intel.com, ak@linux.intel.com, hpa@zytor.com,
        linux-kernel@vger.kernel.org, luto@amacapital.net, tglx@linutronix.de,
        torvalds@linux-foundation.org
Subject: Re: [PATCH RFC] x86, tsc: Allow for high latency in
 quick_pit_calibrate()
Message-ID: <20150605055837.GA15407@gmail.com>
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 <20150604163816.6684.qmail@ns.horizon.com>
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* George Spelvin <linux@horizon.com> wrote:

> Ingo Molnar wrote:
> > - Alternatively, I also tried a different method: to set up the RTC
> >   periodic IRQ during early boot, but not have an IRQ handler, polling
> >   RTC_PF in the rtc_cmos_read(RTC_INTR_FLAGS) IRQ status byte.
> >
> >   Unfortunately when I do this then PIO based RTC accesses can take
> >   tens of thousands of cycles, and the resulting jitter is pretty bad
> >   and hard to filter:
> 
> Did you use rtc_cmos_read()?  [...]

Yeah, so initially I did, but then after I noticed the overhead I introduced:

+unsigned char rtc_cmos_read_again(void)
+{
+       return inb(RTC_PORT(1));
+}
+

which compiles to a single INB instruction.

This didn't change the delay/cost behavior.

The numbers I cited, with tens of thousands of cycles per iteration, were from 
such an optimized poll loop already.

Thanks,

	Ingo
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