[LAU] RTC/HPET timer permissions

[Gene Heskett]

On Sunday 25 November 2012 08:44:57 Clemens Ladisch did opine:

> Brent Busby wrote:
> > Is it still necessary or recommended these days for proper operation
> > of audio apps to have a user-accessible /dev/rtc?
> 
> Not really.  Using the RTC or HPET devices directly is necessary only if
> the kernel does not use them for timers.  Nowadays, kernels do, and
> applications use the proper timer APIs.
> 
> You'd need this only if you're using some audio application that hasn't
> been updated for many years.
> 
> > Is there a security risk?
> 
> The risk was that handling interrupts at a frequency of 8 kHz would not
> leave enough CPU time for applications.  I doubt that you have any CPU
> where this is actually the case.
> 
> 
> Regards,
> Clemens

Chuckle.  You are worried about an 8 kilohertz IRQ?

I have an app on this machine that uses about a 25khz IRQ, Actually slow 
because this quad core phenom at 2.1Ghz, has lousy latency test figures, 
and were this machine better suited, the app could ask for 50 kilohertz or 
better.

It is however, tied to this kernel version:

2.6.32-122-rtai

Because RTAI, when enabled, actually becomes the top dog to do that, with 
the kernel playing 2nd fiddle, all other apps get what time is left over.  

But everything else on this machine still continues to function albeit at a 
slightly slower speed. So it is actually running right now as I type this.

The reason for its existence on this machine is that I do have 2 machines 
out in the shop that are suitable for running at a 40+ kilohertz loop rate, 
my lathe and my desktop milling machine are both running this same install.
They are running it on Intel "atom" D525MW boards with 2Gb of dram. On that 
board, absolute worst case latency is 7 microseconds that might occur while 
opening something like firefox.  Normal runs 2 to 3. Microseconds.  Great 
mobo for us, very low power and dirt cheap.  But no expansion facilities to 
plug in any fancy sound cards.  But it does have decent onboard video and 
sound, but I haven't tested the sound, it would be hard to hear well when a 
6 pack of nema 23 sized double and triple stack stepper motors are growling 
as they go about their work, carving steel to thousandth of an inch 
accuracies.

FWIW, if I have any xruns or stutters in the sound here, its generally 
because my net connection is only a 3 megabit from cable & that can get a 
bit stuttery of an evening when the cable folks big pipe gets loaded up.  
It not a big enough problem to make me yell at the cable folks.

Point is, low latency CAN be done.  On 260 USD machines that are ready to 
run OOTB. 250Gb SATA hard drive, and an optical dvd writer/reader.

But now I am curious.  The cd to install this is available from 

<http://www.linuxcnc.org/index.php/english/download>

I'd be interested in finding out if this approach would do you folks any 
good with your latency related problems.

Cheers, Gene
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