Messages in this thread | | | Date | Sun, 3 Jul 2016 20:10:51 +0100 | From | Russell King - ARM Linux <> | Subject | Re: [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.
| |