Re: AW: [MiNT] analysing syscall.spp

[Johan Klockars <rand@cd.chalmers.se>]

> > FN>It's not a good idea to make the complete GEMDOS callable from an
> > FN>interrupt.
> > I don't even think that it's possible for all calls.
> 
> Well, MagiC for some degree proves that it is possible.

Note that it said 'all calls' above.
What would MagiC do if a potentially blocking call, such as a semaphore
wait (or for that matter some file system things) was called from inside
an interrupt?
Under MiNT, none of the routines that rely on the original TOS could be
called from interrupts either, of course.

Making it safe to call MiNT from interrupts will require that interrupts
are disabled here and there in the system, but that (or semaphore
locking, but then we _really_ need faster semaphores) is needed anyway if
the kernel is to be reentrant at the process level.

> > Especially for the file system calls it should be very difficult or
> > impossible (and not worth the work)
> 
> Then you won't be able to make use of background DMA.

Why would you need to use the file system for background DMA?
I don't know how the mechanism in HDDriver work, but it can hardly
be file system level, since that's handled by MiNT/MagiC drivers.

It could work something like this, for example:
1 The file system tells the device driver (HDDriver) that it wants to
  transfer a block of data and be notified when it's done.
2 The FS code blocks on a semaphore or similar, thereby leaving all
  processor time for other tasks.
3 When a transfer is completed, the device driver wakes up the FS again,
  which may initiate another transfer (goto 1) or return to the caller.

Step 1 might require a call to a variation of RWABS (XBIOS).
In step 2 it would obviously be best if as much of the operating system
as possible is still callable. Some semaphore protection on the actual
disk read/write routines might be enough.
Since HDDriver is not MiNT-aware, I guess the way to deal with step 3
is to make the file system code wait for the 'DMA-complete' interrupt
itself. IIRC, MiNT already can treat interrupts like signals, so it should
be possible to handle that.

None of this require any OS calls whatsoever from an interrupt handler.

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