Bug 17493

The HP Vectra XU 5/90 (Pentium SMP, Neptune chipset) will hang with the 
default RH 6.2 installation and most PCI cards in either of it's two PCI 
slots when the card is accessed at the same time as the onboard SCSI 
controller (interrupt conflict problem).

I have had a chance to do a lot of investigation into this problem and 
have discovered what causes the problem and I have a proposal for how to 
fix it. Unfortunately, I don't have enough experience modifying the kernel 
code to fully implement my proposal.

The problem -- simply put, the machine has an empty MP IRQ table. So the 
kernel runs "construct_default_ISA_mptable()". The computer is a PCI/ISA 
system, so mpc_default_type==5 and the default table created assigns ISA-
style interrupts to everything. This means that the PCI interrupts get 
assigned type EDGE.

That is what causes the machine to hang. In my case I have a Netgear 
FA310TX ethernet card in one PCI slot. Using the ethernet card during disk 
access will cause the onboard SCSI controller to hang. Due to the design 
of the MB, both PCI slots and the onboard SCSI controller share an 
interrupt so there is no way to reassign interrupts or move the card to 
avoid the conflict.

The solution -- the machine, even though it is a PCI/ISA combo, supports 
the EISA ELCR (EISA Level/Edge Control Register). As a quick hack, making 
the following change to io_apic.c:

938c938
<                             mpc_default_type == 6) ? MP_BUS_EISA : 
MP_BUS_ISA;
---
>                             mpc_default_type == 5 || mpc_default_type == 
6) ?
MP_BUS_EISA : MP_BUS_ISA;

will read the ELCR and assign the correct interrupt trigger types (EDGE 
for everything but the PCI interrupts). In fact, looking back at old 
kernel source, it appears the kernel used to do this (it didn't have the 
comparison and used EISA for everything) until around 2.2.5 or 2.2.6 (I 
don't remember exactly).

The proposal -- I propose changing construct_default_ISA_mptable() to do 
the following. If mpc_default_type==5 (an ISA/PCI board), then it *should* 
have a MP IRQ table. The fact that it doesn't means it's slightly broken 
(or really old). If mpc_default_type==5, then there is at least the 
possibility of PCI devices in the system, so could the kernel check and 
see if a valid ELCR exists and use it if it does?

I don't know what would be a good sanity check for the ELCR. Maybe check a 
known IRQ (like 2) and see that it makes sense (EDGE trigger) and then go 
from there? Or make sure that the values aren't all 0x00 or 0xff or 
something obviously incorrect?

If the ELCR passes the sanity test then use it to assign the interrupt 
triggers for the MB. The impact should be minimal since any 
mpc_default_type other than 5 would not be affected, and if it really is 
ISA/PCI, how else can it communicate the interrupt triggers?

By using the ELCR on my XU 5/90, it can boot the 2.2.17SMP kernel and has 
been running for two weeks now without any problems and under 
(occassionally) heavy disk and network load.

Thank you for your patience.