[PATCH] APIC/LAPIC hanging problems on nForce2 system

current state:
Systems with Nforce2 could freeze on high disk i/o activity in APIC mode
when CPU Disconnect is enabled. If bios doesn't fix this, current kernel
fix changes the registers according to follwing table:

      * Chip  Old value   New value
      * C17   0x1F0FFF01  0x1F01FF01
      * C18D  0x9F0FFF01  0x9F01FF01

But this is only done, if cpu disconnect has been enabled in bios.

why change this:
If CPU disconnect is not enabled in bios, and bios is broken (some
manufacturers like Abit don't care about their customers and even the
latest bios doesn't fix this; I have an Abit mainboard), the kernel
doesn't apply the fix, so if cpu disconnect is enabled at a later stage
(in userspace), the system will be unstable and most likely freeze.

new behaviour:
The fix is now applied regardless of cpu disconnect being enabled at
boot time, or not. As you only have to change byte 3 to 0x01, reading
out chipset version isn't needed, so the patch simplifies the fix. Now
turning cpu disconnect on, at later stage won't break the system, and if
it was already enabled, it gets fixed, as the old version did.
Signed-off-by: Prakash Punnoor <prakashp@arcor.de>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/pci/fixup.c

@@ -227,10 +227,7 @@ DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_ANY_ID, pci_fixup_transparent_
  */
 static void __init pci_fixup_nforce2(struct pci_dev *dev)
 {
-	u32 val, fixed_val;
-	u8 rev;
-
-	pci_read_config_byte(dev, PCI_REVISION_ID, &rev);
+	u32 val;
 
 	/*
 	 * Chip  Old value   New value
@@ -240,17 +237,14 @@ static void __init pci_fixup_nforce2(struct pci_dev *dev)
 	 * Northbridge chip version may be determined by
 	 * reading the PCI revision ID (0xC1 or greater is C18D).
 	 */
-	fixed_val = rev < 0xC1 ? 0x1F01FF01 : 0x9F01FF01;
-
 	pci_read_config_dword(dev, 0x6c, &val);
 
 	/*
-	 * Apply fixup only if C1 Halt Disconnect is enabled
-	 * (bit28) because it is not supported on some boards.
+	 * Apply fixup if needed, but don't touch disconnect state
 	 */
-	if ((val & (1 << 28)) && val != fixed_val) {
+	if ((val & 0x00FF0000) != 0x00010000) {
 		printk(KERN_WARNING "PCI: nForce2 C1 Halt Disconnect fixup\n");
-		pci_write_config_dword(dev, 0x6c, fixed_val);
+		pci_write_config_dword(dev, 0x6c, (val & 0xFF00FFFF) | 0x00010000);
 	}
 }
 DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_NVIDIA, PCI_DEVICE_ID_NVIDIA_NFORCE2, pci_fixup_nforce2);