[PATCH 2.6.28 1/3] PCI-quirks: Unhide MCH5/6 memory controller configuration device

[Michał Mirosław]

Some BIOSes hide 'overflow' device (dev #6) for i82875P/PE chipsets.
The same happens for i82865P/PE. Add a quirk to enable this device.
This allows i82875 EDAC driver to bind to chipset's dev #6 and not
dev #0 as the latter is used by AGP driver.

After testing this patch for couple of days on my laptop (i82856P)
it looks like something is resetting device 0 (MCH) config register
0xF4 to zero and effectively disabling the device again. The delay
looks random to me. I can easily update the register using
'hexedit /sys/bus/pci/devices/0000\:00\:00.0/config' and see
correct values in lspci output afterwards. This is probably
BIOS's fault. This changes nothing as far as i82875P EDAC driver
is concerned as it has the same assumption that BIOS is well behaved.

In case some really broken BIOS is found, this can be wrapped around
some new Kconfig #ifdef.

Signed-off-by: Michał Mirosław <mirq-linux@xxxxxxxxxxxx>

diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
index a47db02..0ca02c7 100644
--- a/drivers/pci/quirks.c
+++ b/drivers/pci/quirks.c
@@ -1787,6 +1787,28 @@ DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_BROADCOM,
 			PCI_DEVICE_ID_NX2_5709S,
 			quirk_brcm_570x_limit_vpd);
 
+/* Originally in EDAC sources for i82875P:
+ * Intel tells BIOS developers to hide device 6 which
+ * configures the overflow device access containing
+ * the DRBs - this is where we expose device 6.
+ * http://www.x86-secret.com/articles/tweak/pat/patsecrets-2.htm
+ */
+static void __devinit quirk_unhide_mch_dev6(struct pci_dev *dev)
+{
+	u8 reg;
+
+	if (pci_read_config_byte(dev, 0xF4, &reg) == 0 && !(reg & 0x02)) {
+		dev_info(&dev->dev, "Enabling MCH 'Overflow' Device\n");
+		pci_write_config_byte(dev, 0xF4, reg | 0x02);
+	}
+}
+
+DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82865_HB,
+                       quirk_unhide_mch_dev6);
+DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82875_HB,
+                       quirk_unhide_mch_dev6);
+
+
 #ifdef CONFIG_PCI_MSI
 /* Some chipsets do not support MSI. We cannot easily rely on setting
  * PCI_BUS_FLAGS_NO_MSI in its bus flags because there are actually
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