[PATCH] x86_64: avoid wasting IRQs

[PATCH] x86_64: avoid wasting IRQs

I suggest to change the way IRQs are handed out to PCI devices.

Currently, each I/O APIC pin gets associated with an IRQ, no matter if the
pin is used or not.  It is expected that each pin can potentually be
engaged by a device inserted into the corresponding PCI slot.  However,
this imposes severe limitation on systems that have designs that employ
many I/O APICs, only utilizing couple lines of each, such as P64H2 chipset.

It is used in ES7000, and currently, there is no way to boot the system
with more that 9 I/O APICs.

The simple change below allows to boot a system with say 64 (or more) I/O
APICs, each providing 1 slot, which otherwise impossible because of the IRQ
gaps created for unused lines on each I/O APIC.  It does not resolve the
problem with number of devices that exceeds number of possible IRQs, but
eases up a tension for IRQs on any large system with potentually large
number of devices.

I only implemented this for the ACPI boot, since if the system is this big
and using newer chipsets it is probably (better be!) an ACPI based system
:).  The change is completely "mechanical" and does not alter any internal
structures or interrupt model/implementation.  The patch works for both
i386 and x86_64 archs.  It works with MSIs just fine, and should not
intervene with implementations like shared vectors, when they get worked
out and incorporated.

To illustrate, below is the interrupt distribution for 2-cell ES7000 with
20 I/O APICs, and an Ethernet card in the last slot, which should be eth1
and which was not configured because its IRQ exceeded allowable number (it
actially turned out huge - 480!):

zorro-tb2:~ # cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3       CPU4       CPU5       CPU6       CPU7
  0:      65716      30012      30007      30002      30009      30010      30010      30010    IO-APIC-edge  timer
  4:        373          0        725        280          0          0          0          0    IO-APIC-edge  serial
  8:          0          0          0          0          0          0          0          0    IO-APIC-edge  rtc
  9:          0          0          0          0          0          0          0          0   IO-APIC-level  acpi
 14:         39          3          0          0          0          0          0          0    IO-APIC-edge  ide0
 16:        108         13          0          0          0          0          0          0   IO-APIC-level  uhci_hcd:usb1
 18:          0          0          0          0          0          0          0          0   IO-APIC-level  uhci_hcd:usb3
 19:         15          0          0          0          0          0          0          0   IO-APIC-level  uhci_hcd:usb2
 23:          3          0          0          0          0          0          0          0   IO-APIC-level  ehci_hcd:usb4
 96:       4240        397         18          0          0          0          0          0   IO-APIC-level  aic7xxx
 97:         15          0          0          0          0          0          0          0   IO-APIC-level  aic7xxx
192:        847          0          0          0          0          0          0          0   IO-APIC-level  eth0
NMI:          0          0          0          0          0          0          0          0
LOC:     273423     274528     272829     274228     274092     273761     273827     273694
ERR:          7
MIS:          0

Even though the system doesn't have that many devices, some don't get
enabled only because of IRQ numbering model.

This is the IRQ picture after the patch was applied:

zorro-tb2:~ # cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3       CPU4       CPU5       CPU6       CPU7
  0:      44169      10004      10004      10001      10004      10003      10004       6135    IO-APIC-edge  timer
  4:        345          0          0          0          0        244          0          0    IO-APIC-edge  serial
  8:          0          0          0          0          0          0          0          0    IO-APIC-edge  rtc
  9:          0          0          0          0          0          0          0          0   IO-APIC-level  acpi
 14:         39          0          3          0          0          0          0          0    IO-APIC-edge  ide0
 17:       4425          0          9          0          0          0          0          0   IO-APIC-level  aic7xxx
 18:         15          0          0          0          0          0          0          0   IO-APIC-level  aic7xxx, uhci_hcd:usb3
 21:        231          0          0          0          0          0          0          0   IO-APIC-level  uhci_hcd:usb1
 22:         26          0          0          0          0          0          0          0   IO-APIC-level  uhci_hcd:usb2
 23:          3          0          0          0          0          0          0          0   IO-APIC-level  ehci_hcd:usb4
 24:        348          0          0          0          0          0          0          0   IO-APIC-level  eth0
 25:          6        192          0          0          0          0          0          0   IO-APIC-level  eth1
NMI:          0          0          0          0          0          0          0          0
LOC:     107981     107636     108899     108698     108489     108326     108331     108254
ERR:          7
MIS:          0

Not only we see the card in the last I/O APIC, but we are not even close to
using up available IRQs, since we didn't waste any.

Signed-off-by: Natalie Protasevich <Natalie.Protasevich@unisys.com>
Acked-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>