Hello Cedric,
On 08.02.24 10:53, Cedric Berger wrote:
Hello,
I've a question: does RTEMS really wants to support FPU operations in ISRs?
Because if the answer is "no", then I believe that we could simplify the
RTEMS code (and for me the mental model of the whole thing) by running
the FPU with both FPCCR.ASPEN and FPCCR.LSPEN = 0.
This mean that during IRQ/exception, the simple exception frame (32
bytes) would always be used instead of a combination of the simple and
extended frame (32 or 116 bytes).
This would then improve the real-time guarantees of the system, by
having a shorter and more deterministic IRQ response time.
if you don't use the FPU in ISRs, then the overhead is just a space
overhead with the lazy FPU save/restore. If you switch from an ISR to
another thread you have to save/restore the volatile FPU context anyway.
This would also simplify the context switching code, by centralizing of
the saving of the FPU context in RTEMS only, and enabling optimisation
like only saving/restoring the FPU when switching between tasks defined
with RTEMS_FLOATING_POINT.
What do you think? I'm missing something? would it be a good idea?
From experience, working with the RTEMS_FLOATING_POINT in applications
is quite annoying. Is there really a measurable and significant
performance improvement if you enable the deferred FPU switching? Can
you guarantee that the compiler will not generate FPU or vector
instructions for integer operations? In this version or a GCC release in
the future?
I would be willing to work on that is there is some kind of agreement here.
If you change the ARMv7-M CPU port to use the deferred FPU switching,
then you surely break existing applications which then have to use
RTEMS_FLOATING_POINT. I would do this only if there would be a clear and
measurable performance improvement. For the measurements we would need a
benchmark.
--
embedded brains GmbH & Co. KG
Herr Sebastian HUBER
Dornierstr. 4
82178 Puchheim
Germany
email: sebastian.hu...@embedded-brains.de
phone: +49-89-18 94 741 - 16
fax: +49-89-18 94 741 - 08
Registergericht: Amtsgericht München
Registernummer: HRB 157899
Vertretungsberechtigte Geschäftsführer: Peter Rasmussen, Thomas Dörfler
Unsere Datenschutzerklärung finden Sie hier:
https://embedded-brains.de/datenschutzerklaerung/
_______________________________________________
devel mailing list
devel@rtems.org
http://lists.rtems.org/mailman/listinfo/devel
