Re: [PARPORT] Re: problems with parallel port IRQ detection

David Campbell (
Sun, 1 Feb 1998 07:47:17 +0000

> To: "Jerod E. Tufte" <>
> Cc:,
> Subject: [PARPORT] Re: problems with parallel port IRQ detection
> Date: Sat, 31 Jan 1998 00:07:52 +0000
> From: Philip Blundell <>

> >does anyone have a solution for this? in 2.0.x, both my palmtop PC110
> >and my desktop machine have no problems using PLIP. if I boot either
> >to 2.1.x (I've tried for most x>60) PLIP reports detecting IRQ 8 for
> >the first port (should be 0x378/IRQ7) on both of them. trying to use
> >modules for all the parallel port stuff and plip and forcing the irq
> >as an option to insmod doesn't seem to work either. What useful
> >information can I report to help the parallel port gurus to track this
> >down?
> The problem is that the ECP/EPP code is too smart about detecting IRQs. David
> Campbell is probably the man to fix it. If it can't be made to work reliably
> then that autodetect should be taken out.

I have a few ideas to fix this problem, however I am extremely busy at
work for the next two weeks.

When I wrote the original code it was to handle the situation of IRQ
sharing (I have a machine with full port count 3 LPT + 4 COM). This
required a slightly different IRQ probing scheme to that used previously
in Linux (which assumes that each IRQ is mapped to at most one device

Rumor has it that with *some* ASUS main boards (I am most likely to be
wrong here) that if the onboard PS/2 mouse port is disabled then the IRQ
is stuck active.

What needs to be done is to adjust the IRQ code to either:
a) Use conventional Linux IRQ probing techniques.
b) Check for IRQs which are either stuck active or periodically active
   Then mask these IRQs from the results.

I favour method b (flame me if you like, it is bushfire season here in
Asutralia), as the average PC does not have enough IRQs for all the
*useful* hardware => 2 IDE, 2 serial, 1 parallel, 1 network card, 1 sound
card, 1 video card + several other system devices [keyboard, timer, x87
interupt] THEN start adding SCSI cards.....

Using the current Linux model it is impossible to have a SoundBlaster card
AND use the IRQ for printing. (Arrgg!!!)

I will spend some time today (Sunday, 1st Feb) and grab the latest 2.1.x
kernel and make the necessary mods to the IRQ detection. A patch will be
distributed via the parport mailing list.

David Campbell
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