Re: ARM Linux Suitability for Real-time Application

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Wed, Jun 22, 2005 at 12:55:45PM +0400, Eugeny S. Mints wrote:
> The bottom line is that interrupt and preemption latencies for a kernel 
> with RT patch are inbetween 15-150us. But of course even with real-time 
> patch you have to perform _real_ fine tuning of your system to achieve 
> such hard constraints you identified.

If you're just after some background process to run off interrupts with
minimal interrupt latency, the good news is that you don't have to modify
the kernel on ARM, and you certainly don't need any RT patches.

If you use the FIQ, then your FIQ latency will be the time it takes the
CPU to enter your FIQ function.  Since the kernel _never_ disables FIQs
in any way, FIQs have ultimate priority over everything else in the
system.

This does mean that you have to be careful if you want data to cross
the boundary between FIQ and the rest of the kernel since you can't use
any of the normal locking functions from FIQ mode.  Also, note that FIQ
code can not reside in modules.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core