generic method to assign IRQs

generic method to assign IRQs

[Florin Andrei]

Is there any generic method to manually assign IRQs to devices? Not
something that applies to one kernel module or another, but something
that works in general.

It happens quite often, especially on multimedia workstations, when
multiple devices get assigned the same IRQ, the performance goes down
the toilet, and users experience strange things like "my video capture
application stutters when my system sends/receives traffic on the
network card."
In such cases, the usual recommendation is to "shuffle the PCI cards
around." Sometimes that works, sometimes it doesn't. It definitely
doesn't apply to laptops.
Another trick is to enable APIC in the kernel. While this is not a
direct solution, it helps sometimes by providing a larger IRQ space. In
some rare cases it makes the systems less stable.

However, quite often i've heard people saying "i wish i could just
manually assign IRQs to devices, just like i do on That Other Operating
System."

This issue may not matter much on "normal" systems, but it matters a
whole bunch on multimedia machines. Not being able to untangle like five
or six devices assigned to the same IRQ may render an otherwise powerful
system totally unusable for any decent media purpose (i'm talking here
about simple tasks such as watching movies, not necessarily of
professional stuff, which is even more demanding).

Any suggestions?

Thanks,

-- 
Florin Andrei

http://florin.myip.org/

Re: generic method to assign IRQs

[Mike Dresser]

On 13 Jun 2003, Florin Andrei wrote:

> This issue may not matter much on "normal" systems, but it matters a
> whole bunch on multimedia machines. Not being able to untangle like five
> or six devices assigned to the same IRQ may render an otherwise powerful
> system totally unusable for any decent media purpose (i'm talking here
> about simple tasks such as watching movies, not necessarily of
> professional stuff, which is even more demanding).

Some of the problem is that motherboard manufacturers setup their hardware
so that slots HAVE to share IRQ's no matter what you do.  I've seen
motherboards that have shared IRQ's even if there are no cards plugged in.

Mike

Re: generic method to assign IRQs

[Trever L. Adams]

On Sat, 2003-06-14 at 00:37, Mike Dresser wrote:
> Some of the problem is that motherboard manufacturers setup their hardware
> so that slots HAVE to share IRQ's no matter what you do.  I've seen
> motherboards that have shared IRQ's even if there are no cards plugged in.
> 
> Mike

Valid data point: Asus A7N8X motherboard.  Then again, it does have
everything, including the sink.

Trever
--
"Blessed is the man, who having nothing to say, abstains from giving
wordy evidence of the fact." -- George Eliot 

Re: generic method to assign IRQs

[Florin Andrei]

On Fri, 2003-06-13 at 21:37, Mike Dresser wrote:
> On 13 Jun 2003, Florin Andrei wrote:
> 
> > This issue may not matter much on "normal" systems, but it matters a
> > whole bunch on multimedia machines. Not being able to untangle like five
> > or six devices assigned to the same IRQ may render an otherwise powerful
> > system totally unusable for any decent media purpose (i'm talking here
> > about simple tasks such as watching movies, not necessarily of
> > professional stuff, which is even more demanding).
> 
> Some of the problem is that motherboard manufacturers setup their hardware
> so that slots HAVE to share IRQ's no matter what you do.  I've seen
> motherboards that have shared IRQ's even if there are no cards plugged in.

I think i see what you mean.

Still, then what's the explanation for this thing: if i run a non-APIC
kernel, lots of devices are on the same IRQ. Just enable APIC in the
kernel, and change nothing else, and the busy IRQ becomes less busy.
In either case, there are tons of spare IRQs, which just sit there idle,
unused.

If the problem would be entirely in hardware, i would say APIC shouldn't
make a big difference.

Do i miss something?

-- 
Florin Andrei

http://florin.myip.org/