Re: [PATCH 0/2] external interrupts

[Brent Casavant <bcasavan@sgi.com>]

On Sun, 21 Aug 2005, Pavel Machek wrote:

> > Here is a set of patches that implements an external interrupt capability
> > in Linux, along with a device driver for a specific hardware device.  I
> > submitted the patches several weeks ago, and they drew no comments, which
> > I take to be a good sign.  Anyway, I'm 
> It was not good sign in this particular case. My reaction was "this is _so_
> overengineered tjat he's probably joking".

Laughter was not wholly unexpected, though I wasn't joking.  I'm trying
to be realistic about the lifetime of any given hardware, and IOC4 is
several years old at this point.  Couple that with a sincere desire to
preserve application source compatability when (not if) new hardware
appears, and an abstraction layer seemed to be a logical choice.  I'm
more than happy to discuss problems in the abstraction layer's interface
and make appropriate changes -- I'm nothing if not obliging.

I do recognize that the code is a bit excessive given that there's
only one known device (IOC4) that is currently supported.  However,
the abstraction isn't really all that complicated.  The low-level
driver simply provides to the abstraction layer a structure containing
a handful of functions pointers: get/set output modes and timings,
get/set interrupt sources, get timing roundoff information and device
ID.  The abstraction layer provides a function to call whenever an
interrupt occurs.  Then add in a bit of registration/deregistration
glue so that kernel modules can load and unload.  That's pretty much it
-- nothing fancier than needed to decouple hardware implementation details
from the programming interface.

Yes, I suppose I could implement completely seperate hardware drivers
with a consistent interface, and do away with the abstraction layer.
However, that leads to code duplication, which has its own downfalls.
Personally, if I had to pick one poison or the other I'd take
overengineering over maintaining divergent implementations, but the
patch apparently made my predeliction obvious :).

Thanks,
Brent Casavant

-- 
Brent Casavant                          If you had nothing to fear,
bcasavan@sgi.com                        how then could you be brave?
Silicon Graphics, Inc.                    -- Queen Dama, Source Wars
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