Hi Tim,


On Thu, Dec 26, 2013 at 10:54 AM, tim Rowledge <tim at rowledge.org> wrote:

>
> On 26-12-2013, at 7:11 AM, marcelo Cortez <jmdc_marcelo at yahoo.com.ar>
> wrote:
>
> > hi
> >
> > I'm developing over  Raspberry pi  with squeak and i need access to GPIO.
> >
> > Questions:
> >
> > How load FFI on Raspberry pi?
> > Any link,paper , advice , pdf.
> > be appreciated.
>
> To get the FFI working for Pi (and indeed other ARM *nix machines)
> somebody would need to write any needed ARM assembler bits to interface to
> the apicall and/or libffi and/or whatever the current facility is called.
> There is some information on what is needed in the vm sources; see
> http://squeakvm.org/cgi-bin/viewvc.cgi/squeak/trunk/platforms/unix/plugins/SqueakFFIPrims/and start with 00README. Probably the best place to ask more specific
> questions about FFI would be the squeak vm-dev list (sign up at
> http://lists.squeakfoundation.org/mailman/listinfo/vm-dev)
>

Actually it is simpler than this, provided one starts from
ThreadedFFIPlugin.  This is a replacement for the old FFIPlugin that uses
very little assembler, and that in the form of asm statements.  The old
FFIPlugin has two main problems, first that it needs a lots of assembler
support, and second that it is not reentrant.  The latter follows from the
former, in that the marshalling is done to a static buffer, and so making a
call while nested will screw the pooch.

ThreadedFFIPlugin in contrast uses alloca to allocate space for outgoing
arguments and is hence reentrant and written in C with two asm statements,
getsp and setsp.  Better still ThreadedFFIPlugin was written with ARM (and
PowerPC) in mind.  It has three subclasses, ThreadedIA32FFIPlugin,
ThreadedARMFFIPlugin, & ThreadedPPCBEFFIPlugin.  The first is fully
functional (and has been in Cog since its release).  The second two are
only a few days away from completion.  Let me invite anyone interested to
have a go.

The secret to an alloca based implementation is that alloca, answering a
stack-allocated array, provides space for outgoing arguments, so that
subsequently calling a function pointer supplies he called function with
its arguments.  This is slightly more complicated on an ABI with register
arguments, such as ARM or PowerPC.  The extra work is to also marshal into
a struct that holds the arguments, and instead of just calling the function
pointer, one calls it with enough arguments to fill the register arguments,
taken from the struct.

All the structure is there.  All it needs is someone with am ARM and some
gumption to fill in the missing pieces.  I know all this works because I
wrote (and/or rewrite) the VisualWorks VM's FFI to use alloca, including
for ARM and PowerPC platforms.

It’s definitely a job I have on my list of TO DO, but not high and there’s
> plenty of work piled up above it so please feel free to tackle it!
>
> It *might* be easier in the short term to write a vm plugin specifically
> for the Pi GPIO interface. You can find out quite a bit about the gpio
> programming fro mthe Pi forums and particularly from the Scratch forum
> where simplesi has done a great deal of clever stuff by using python to
> provide a connection for the Scratch remote sensor connection subsystem,
> and then driving the gpio from pythin. It’s certainly time there was as
> more direct interface for him to use.
>

It might.  But someone who understands the ARM ABI could probably get
the ThreadedARMFFIPlugin working in a week or less.

I'll happily answer any and all questions in getting this to work.
-- 
best,
Eliot
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