IRQ Conflict
 

I run dual boot Windows95/Linux Redhat 9 on a computer with a PnP Bios.

Under Linux there is an IRQ conflict between the serial mouse on COM1 and the PnP modem on COM3 where both want to use IRQ 4. There is no conflict under Windows, where the modem uses IRQ 10.

I would like to change the IRQ for either the mouse or the modem. I have tried using setserial to change IRQ for ttyS0 (mouse on COM1), but get an error message that the mouse is in use and so the IRQ cannot be changed.

Linux does not allow me to make changes to the file /proc/isapnp where the IRQ 4 is listed for the modem.

Please has anyone suggestions for resolving the conflict.
Thank you.

I had this sort of thing in the past before retiring my modem and ISA soundcard. I think I found some setting in the BIOS which reserved certain IRQs for PnP or hardwired devices, and fixed the problem.

There is also a kernel parameter 'isapnp_reserve_irq=' which I don't recall ever using, but looks promising.

Finally there is a set of tools called 'isapnp' or 'isapnptools' which use a configuration file to negotiate with any PnP devices on start-up. I seem to recall I found understanding this pretty heavy going.

Be careful with this advice, possibly wait for someone to comment on it before acting:

As far as I can tell

/dev/modem

is a symbolic link to a com/serial port

you can ls -l /dev/modem to see which one it's linked to at the moment probably ttyS0 same as your mouse or possibly ttyS2

As it's a symbolic link my guessing is you can delete it and recreate it again pointing to the a different serial port

ln -s /dev/ttyS1 /dev/modem

or ttyS3


There's probably a way to do it graphically but as I say this is a hunch and might not even work, hang in there and wait for an other opinion

Please tell me in which file the isapnp_reserve_irq statement should go.

Thank you.

what is on com 2? if nothing, put your mouse there.
or
change the bios setting "Plug n' Play OS installed" to no
or
change the bios setting for serial port 1 to a different irq
or
change the bios settings to reserve irq's 3 and 4.

Thanks whansard for your suggestions. Unfortunately COM 2 is not available for the mouse. I would prefer not to change the bios settings as linux is part of a dual boot computer.

Can the COM1 irq setting be changed without changing the bios and before resources are assigned to the Mouse?

the plug n play os setting in the bios does the following.
if set to yes, then plug n' play devices are not initialized.
if set to no, they are initialized and given the resources
they need.

most of these settings will not mess up windows, but
windows will likely go through a hardware detection
when you start it next time.

I had the same problem. My modem jumpers were set to IRQ 3 and port 4, and this was the only way it worked in Windows. But it didn't under Linux.

I used BIOS setup to use IRQ 4 for both internal serial ports, and it worked, but things got a bit messed up because now COM 2 became COM 3

Serial driver version 5.05c (2001-07-08) with MANY_PORTS MULTIPORT SHARE_IRQ SERIAL_PCI enabled
ttyS0 at 0x03f8 (irq = 4) is a 16550A
ttyS2 at 0x03e8 (irq = 4) is a 16550A
ttyS3 at 0x02e8 (irq = 3) is a 16550A

I have a drawing tablet attached to port 2 so I had to change everything in my XF86Config from /dev/ttyS1 to /dev/ttyS2. But under windows 98 and 2000 both the modem and the tablet continued to work perfectly.