Commit fbe2b31b authored Jun 18, 2009 by Bjorn Helgaas Committed by Len Brown Jun 20, 2009

ACPI: pci_root: check _CRS, then _BBN for downstream bus number

To find a host bridge's downstream bus number, we currently look at _BBN
first. If _BBN returns a bus number we've already seen, we conclude that
_BBN was wrong and look for a bus number in _CRS.

However, the spec[1] (figure 5-5 and the example in sec 9.12.1) and an ACPI
FAQ[2] suggest that the OS should use _CRS to discover the bus number
range, and that _BBN is really intended to bootstrap _CRS methods that
reference PCI opregions.

This patch makes us always look at _CRS first. If _CRS doesn't supply a
bus number, we look at _BBN. If _BBN doesn't exist, we default to zero.
This makes the behavior consistent regardless of device discovery order.
Previously, if A and B had duplicate _BBNs and we found A first, we'd only
look at B's _CRS, whereas if we found B first, we'd only look at A's _CRS.

I'm told that Windows discovers host bridge bus numbers using _CRS, so
it should be fairly safe to rely on this BIOS functionality.

This patch also removes two misleading messages: we printed the "Wrong _BBN
value, reboot and use option 'pci=noacpi'" message before looking at _CRS,
so we would likely find the bus number in _CRS, the system would work fine,
and the user would be confused. The "PCI _CRS %d overrides _BBN 0" message
incorrectly assumes _BBN was zero, and it's useless anyway because we
print the segment/bus number a few lines later.

References:
[1] http://www.acpi.info/DOWNLOADS/ACPIspec30b.pdf
[2] http://www.acpi.info/acpi_faq.htm _BBN/_CRS discussion
http://download.microsoft.com/download/9/8/f/98f3fe47-dfc3-4e74-92a3-088782200fe7/TWAR05005_WinHEC05.ppt (slide 17)
http://bugzilla.kernel.org/show_bug.cgi?id=1662 ASUS PR-DLS
http://bugzilla.kernel.org/show_bug.cgi?id=1127 ASUS PR-DLSW
http://bugzilla.kernel.org/show_bug.cgi?id=1741

ASUS PR-DLS533

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reviewed-by: Alex Chiang <achiang@hp.com>
CC: Shaohua Li <shaohua.li@intel.com>
CC: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Len Brown <len.brown@intel.com>

parent 07a2039b

drivers/acpi/pci_root.c

+16 −38

Original line number	Diff line number	Diff line
		@@ -365,12 +365,12 @@ static int __devinit acpi_pci_root_add(struct acpi_device *device)
		{
		int result = 0;
		struct acpi_pci_root *root = NULL;
		struct acpi_pci_root *tmp;
		acpi_status status = AE_OK;
		unsigned long long value = 0;
		acpi_handle handle = NULL;
		struct acpi_device *child;
		u32 flags, base_flags;
		int bus;


		if (!device)
		@@ -420,46 +420,24 @@ static int __devinit acpi_pci_root_add(struct acpi_device *device)
		/*
		* Bus
		* ---
		* Obtained via _BBN, if exists, otherwise assumed to be zero (0).
		* Check _CRS first, then _BBN. If no _BBN, default to zero.
		*/
		status = acpi_evaluate_integer(device->handle, METHOD_NAME__BBN, NULL,
		&value);
		switch (status) {
		case AE_OK:
		status = try_get_root_bridge_busnr(device->handle, &bus);
		if (ACPI_SUCCESS(status))
		root->id.bus = bus;
		else {
		status = acpi_evaluate_integer(device->handle, METHOD_NAME__BBN, NULL, &value);
		if (ACPI_SUCCESS(status))
		root->id.bus = (u16) value;
		break;
		case AE_NOT_FOUND:
		ACPI_DEBUG_PRINT((ACPI_DB_INFO, "Assuming bus 0 (no _BBN)\n"));
		else if (status == AE_NOT_FOUND)
		root->id.bus = 0;
		break;
		default:
		else {
		ACPI_EXCEPTION((AE_INFO, status, "Evaluating _BBN"));
		result = -ENODEV;
		goto end;
		}

		/* Some systems have wrong _BBN */
		list_for_each_entry(tmp, &acpi_pci_roots, node) {
		if ((tmp->id.segment == root->id.segment)
		&& (tmp->id.bus == root->id.bus)) {
		int bus = 0;
		acpi_status status;

		printk(KERN_ERR PREFIX
		"Wrong _BBN value, reboot"
		" and use option 'pci=noacpi'\n");

		status = try_get_root_bridge_busnr(device->handle, &bus);
		if (ACPI_FAILURE(status))
		break;
		if (bus != root->id.bus) {
		printk(KERN_INFO PREFIX
		"PCI _CRS %d overrides _BBN 0\n", bus);
		root->id.bus = bus;
		}
		break;
		}
		}

		/*
		* Device & Function
		* -----------------