linux-kernel - Re: CPUfreq - udelay() interaction issues

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Date:	Fri, 23 Apr 2010 12:22:59 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	cpufreq <cpufreq@...r.kernel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Thomas Renninger <trenn@...e.de>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: CPUfreq - udelay() interaction issues

On Fri, 23 Apr 2010 14:40:42 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> [CCing Arjan, who seems to have played a lot with ondemand lately]
> 
> * Saravana Kannan (skannan@...eaurora.org) wrote:
> > Resending email to "cc" the maintainers.
> >
> > Maintainers,
> >
> > Any comments?
> >
> > -Saravana
> >
> > Saravana Kannan wrote:
> >> Hi,
> >>
> >> I think there are a couple of issues with cpufreq and udelay  
> >> interaction. But that's based on my understanding of cpufreq. I
> >> have worked with it for sometime now, so hopefully I not
> >> completely wrong. So, I will list my assumptions and what I think
> >> is/are the issue(s) and their solutions.
> >>
> >> Please correct me if I'm wrong and let me know what you think.
> >>
> >> Assumptions:
> >> ============
> >> * Let's assume ondemand governor is being used.
> >> * Ondemand uses one timer per core and they have CPU affinity set.
> >> * For SMP, CPUfreq core expects the CPUfreq driver to adjust the 
> >> per-CPU jiffies.
> >> * P1 indicates for lower CPU perfomance levels and P2 indicates a
> >> much higher CPU pref level (say 10 times faster).
> >>


so in reality, all hardware that does coordination between cores/etc
like this also has a tsc that is invariant of the actual P state.
If there are exceptions, those have a problem, but I can't think of any
right now.
Once the TSC is invariant of P state, udelay() is fine, since that goes
of the tsc, not of some delay loop kind of thing....
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