Stuart Hughes wrote:
>
> Arnaud Westenberg wrote:
> >
> > Stuart Hughes wrote:
> >
> > > One point I want to make is that there is a good reason why often people
> > > would choose to have an ISR wake up a real-time task to carry out the
> > > bulk of a set of processing. On most other real-time operating systems,
> > > the task that gets woken from the ISR runs at a lower privilege level
> > > than an ISR (in terms of machine state), also interrupts are normally
> > > enabled. This means that the task can be pre-empted by other real-time
> > > tasks and also interrupted. In the Linux (hard) real-time
> > > implementations (RTL/RTAI) this is not currently true, as I believe that
> > > the task/thread runs at the same machine privilege level as the ISR.
> >
> > This is not correct, I asked this question recently. Here's the question
> > and victor's reply:
> >
> > Q: Does a "hard" handler stop a RT thread? (I would say yes) How are the
> > Q: "priorities" between hard handlers and RT threads determined by
> > RTLinux?
> >
> > A: Hard handlers preempt anything that is not running with hard
> > interrupts
> > A: disabled. There is no other priority mechanism.
> >
> > So RT threads will be interrupted by the handler.
> >
>
> Hi Arnaud,
>
> The real question is if I have an ISR whos only task is to wake up a
> thead, what happens:
>
> a) enter ISR, wakeup thread, thread code runs, return to ISR, exit ISR
>
> or:
>
> b) enter ISR, wakeup thread, exit ISR, thead code runs.
>
> I'm saying that b) is more conventional, and I would like to have this
> facility available.
>
> Regards, Stuart.
b) is the same as a) if one cares to wakeup the thread after having done
every thing that must be done in the ISR, acknowledge the PIC and set
enabled the CPU to take interrupts.
Making it as in a) is not too complex, but useless as the constraints
enatailed by b) are very mild, after all preemption is nothing but
scheduling from within interrupts. You can check Comer's book on XINU
for an explanation of why it is safe to reschedule from within
interrupts.
Ciao, Paolo.
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