On Tue, 2011-06-07 at 11:40 +0200, Armin Steinhoff wrote:

Hi,

when I read all these confusing statements here ( in german it looks 
like an "Eiertanz")  ... I can only say:

- do the basic stuff in a minimal kernel driver
- use UIO (or VFIO for PCI devices)

I see no requirement for any of those horrid things to be used. You can
write a full on proper kernel driver, it just cannot set kernel thread
priorities to a sane value (let them all default to 50 or so).

Then have a user space script or whatever set the kthread priorities.

and you get clean control about your real-time priorities.

I think changing the priorities of "interrupt threads" inside the kernel 
could lead to strange race conditions in the kernel.

No, changing the priority in the kernel is a perfectly sound operation,
it just doesn't make any sense to do so since its impossible to
determine a proper priority.

Therefore setting a priority is a pure user policy and should not be
done by the driver itself -- it simply cannot do it right so why bother
doing it.