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SubjectRe: [RFC 0/1] ARM: print MHz in /proc/cpuinfo
On Sun, Jul 03, 2016 at 08:49:45PM +0200, Andrew Lunn wrote:
> On Sun, Jul 03, 2016 at 05:54:31PM +0100, Russell King - ARM Linux wrote:
> > Right, so having read all your email, there's no reason why we couldn't
> > just print:
> >
> > cpu MHz : 99999999.999
>
> Since it is a float, how about using the value NaN? I think that
> nicely summaries it is a useless value.

;)

> However, i agree, we first need a technical justification for needing
> a value at all.

The only case where we've had a userspace "failure" is with the Jack
audio server, which wanted to parse the cpu MHz value to use it in
this calculation:

static jack_time_t jack_get_microseconds_from_cycles (void) {
return get_cycles() / __jack_cpu_mhz;
}

Now, let's say that we did provide the CPU MHz, so __jack_cpu_mhz
reflects this value. Great, Jack can initialise this value, but
there's two fundamental problems:

1. Jack is not aware of cpufreq, so the CPU MHz value it read at
one point in time may not be the current CPU MHz - the CPU
frequency can change at any moment depending on the governor's
decisions.

2. get_cycles()... we have no userspace accessible CPU cycle counters
on 32-bit ARM, which means knowing the CPU MHz for use like this
gets you nowhere as get_cycles() can't return the number of CPU
cycles. It certainly can't return a number based on the CPU MHz.

Hence why this change to jack was the only sane thing to do:

https://github.com/jackaudio/jack2/commit/d425d8035b761b4a362c538c41eca874ff4

This wasn't even a kernel regression - the kernel never provided the
value on ARM, and in many cases the kernel doesn't know the right
value (as I've said previously in this thread.)

--
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.

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