x86/PCI: Add "pci=big_root_window" option for AMD 64-bit windows

Only try to enable a 64-bit window on AMD CPUs when "pci=big_root_window"
is specified.

This taints the kernel because the new 64-bit window uses address space we
don't know anything about, and it may contain unreported devices or memory
that would conflict with the window.

The pci_amd_enable_64bit_bar() quirk that enables the window is specific to
AMD CPUs.  The generic solution would be to have the firmware enable the
window and describe it in the host bridge's _CRS method, or at least
describe it in the _PRS method so the OS would have the option of enabling
it.

Signed-off-by: Christian König <christian.koenig@amd.com>
[bhelgaas: changelog, extend doc, mention taint in dmesg]
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6571fbfdb2a1..619638362416 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3094,6 +3094,12 @@
 		pcie_scan_all	Scan all possible PCIe devices.  Otherwise we
 				only look for one device below a PCIe downstream
 				port.
+		big_root_window	Try to add a big 64bit memory window to the PCIe
+				root complex on AMD CPUs. Some GFX hardware
+				can resize a BAR to allow access to all VRAM.
+				Adding the window is slightly risky (it may
+				conflict with unreported devices), so this
+				taints the kernel.
 
 	pcie_aspm=	[PCIE] Forcibly enable or disable PCIe Active State Power
 			Management.