Re: How to talk to DAQ Hardware?

[Carsten Emde <Carsten.Emde@osadl.org>]

Guennadi Liakhovetski wrote:
> On Sat, 25 Aug 2007, Calin Culianu wrote:
>> Now, there is no way to raise the priority of particular driver's ISRs is
>> there?  So that a specific ISR can preempt anything including a hard realtime
>> process?
> It's the second time you say "hard" real-time, so, just wanted to warn - 
> it is not hard. It is still only soft real time, but a pretty good one, I 
> think.
I don't know what exactly "soft real-time" is, and I don't think there 
is an exact definition of it. The term "hard real-time", however, is 
well defined. It is the same as "real-time". It means that a system 
never ever exceeds a previously defined maximum latency. If, for 
example, you define 100 microseconds as such, you run your system over a 
long time period (several days), and you observe several billions of 
single external events that trigger a user space process at real-time 
priority, then there must be no single event that fails to do so within 
100 microseconds. We did this constantly and successfully with an -rt 
kernel. So why are we not allowed to call this "hard real-time"?

If you are aware of a situation where this is not the case, then this 
may be a specific problem related to a specific situation. Please help 
us to improve the -rt patches further and submit a bug report. I am 
confident that it will be fixed as soon as possible.

	--cbe