Re: [Xenomai-core] [RFC] Fast Context Switch Extension for ARM Linux

[Jan Kiszka <jan.kiszka@domain.hid>]

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Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>  > Richard Cochran wrote:
>  > > I know that this is a bit off topic, yet perhaps some on this list
>  > > have interest or knowledge in this area. I am also posting this idea
>  > > to the linux-arm-kernel list.
>  > > 
>  > > I would like to implement the ARM FCSE under Linux, using the Intel
>  > > IXP425 board. Before I start, I would like to ask:
>  > > 
>  > > 1. Has any work already been done in this area?
>  > > 
>  > > 2. Is there something about the current Linux MM that would make this
>  > >    impractical?
>  > 
>  > Nothing is impossible (specifically as FASS proves its feasibility),
>  > it's just a question how much work it is to implement it AND maintain it
>  > or merge it into standard Linux. And I cannot imagine ARM or its
>  > licensees dropping this feature without an acceptable alternative once
>  > Linux started to use it broadly...
> 
> The feature was already dropped, it is deprecated in ARMv6.
> 

I'm not claiming to be an ARM insider, but browsing through some manuals
makes me think that "The c13, Context ID Register, [...] replaces the
FCSE PID register. Use of the FCSE PID Register is deprecated."
Moreover: "The ARM1136JF-S processor implements the Fast Context Switch
Extension (FCSE) and high vectors extension that are required to run
Microsoft WindowsCE." Given this popular user, it is highly unlikely
that a feature comparable to FCSE is going to be removed from the ARM
architecture - at least as long as its cache is located at such an
unpleasant place.

Jan


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