Re: [Xenomai-help] OT. A question about constraints in realtime

[Leopold Palomo Avellaneda <leo@domain.hid>]

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A Dimecres 30 Juliol 2008, Gilles Chanteperdrix va escriure:
> Leopold Palomo Avellaneda wrote:
> > A Dilluns 28 Juliol 2008, Gilles Chanteperdrix va escriure:
> >> Leopold Palomo Avellaneda wrote:
> >>> The sensable people claims that their driver works with some versions
> >>> of Linux (till fedora 4 and some suse) but I'm not be able to run it in
> >>> a debian etch. I don't know if the distros provide a kernel with some
> >>> kind of modifications that solve it, but I don't think so.
> >>>
> >>>
> >>> Well, thanks in advance and I'm sorry for the noise. Really, the
> >>> realtime stuff is something not obvious...
> >
> > First of all thanks for the answer.
> >
> >> The first thing to do is to get statistics about the scheduling latency
> >> of your driver. This means measuring the difference between the date
> >> when the driver (kernel-code) wakes up the task and the date when the
> >> task effectively runs. The bad news is that it will be hard with a
> >> binary-only driver. If the driver has some compilable glue, then you can
> >> hook into this glue, otherwise, you will have to resort to ugly hacks in
> >> the kernel code.
> >
> > Well, I cannot (or it's too difficult to do it with a _only_ binary
> > driver. This driver is a lib (.so) and the user use some functions of
> > that lib.
>
> It is hard, but not impossible. 

Sure, but the afford and the skill are not insignificant.

> You can hook into the sys_open function, 
> detect that you are opening the driver (using either the device path, or
>  current->comm), and in this case, replace the file operation pointer in
> the file descriptor with your own file operations pointers which does
> some tracing then calls the original file operation pointer. So, it is
> not impossible.

Well, the driver is for a device that uses the parallel port. It's a library 
that has implemented some kind of loop function and make the transformation 
from the values to send to the device and read the device. 

I guess that they have a very bad implementation in linux. But, really to me 
the question that intrigue me is why it works well in Windows and not in 
linux.

Leo



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