PowerPC FP handling weakness.

[Sergei Organov]

Eric Valette <valette at crf.canon.fr> writes:
> Joel Sherrill wrote:
> 
> > > I'm affraid deffered FPU context switching has never been tested on
> > > 60x/7xx. Maybe I can be wrong...
> > 
> > I think that in a system where all tasks are FP deferred switching is
> > equivalent to on every switch.  Think about it.  All deferred switching
> > says is that we won't save the context until another FP task is switched
> > in.  This is every time.  Not as clear as it should be but technically
> > OK.
> 
> No I disagree. FP context switching is performed when the newly tasks
> really use the FPU. Saying a thread is a FP task does not mean it will
> use the FP each time it is scheduled...
> 
> All I wanted to say is that deferred FP context saving remove the need
> of hardcoding whether a thread is FP or not and perform a better job by
> saving FP context only when needed.
> 
> Did I miss something?

Yes. RTEMS doesn't have "on-demand" FP context switch. Only "deferred" context 
switch. I.e. switch context only when new task is marked to be "floating
point" and doesn't already own the FP context.

Sergei.