[SGVLUG] More laptop fun--IRQ assignments

[David Lawyer]

On Thu, Dec 08, 2005 at 06:07:58AM -0800, Dustin wrote:
> On Tue, 6 Dec 2005, David Lawyer wrote:
> 
> > You must be using PCI which allows sharing IRQs.
> 
> PCI-Express, actually.  ISA is actually two generations out of date 
> now.... (and thank God for that--it sucked royally)

The sharing of IRQ is inefficient since the interrupt service routines
of many drivers must run at each IRQ.  You may have the LPC "bus" on
your laptop.  Look for a LPC interface controller (or the like) using
lspci.  Now LPC can use up a lot of interrupts and it shares a single
line for all interrupts if it has a SERIRQ line.  But it's better than
PCI because the interrupts are not shared, but are multiplexed
(serialized) on the same conductor.  This minimizes the number of
lines used for interrupts (1) while allowing for many distinct
interrupts to be transmitted.

"bus" means its not supposed to be a bus, but many legacy devices are
on it like serial & parallel ports and the floppy drive.  The bus
width is 4 bits used for address, data, and control.  Use of a SERIRQ
line is optional.  Now other Address/Data buses are 32/64 bits wide
(8/16 times wider).  The LPC is configured by the BIOS (it says so in
the specs which I downloaded).  There are no standards for setting
IRQs and each brand can do it differently, but it doesn't matter :-)
since only the BIOS need know how to configure devices on the LPC bus.  

Could the problem be that LPC has reserved a lot of interrupts?  I
would think that acpi should be able to get this info from the BIOS>

 
			David Lawyer