x86/PCI: VMD: Document code for maintainability

Comment the less obvious portion of the code for setting up memory windows,
and the platform dependency for initializing the h/w with appropriate
resources.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

diff --git a/arch/x86/pci/vmd.c b/arch/x86/pci/vmd.c
index d57e48016f1578d8e1b04e22d0a30967faaf4e6e..29aebbcb510bd39850b4e1c40c9661c751af916c 100644 (file)
--- a/arch/x86/pci/vmd.c
+++ b/arch/x86/pci/vmd.c
@@ -532,6 +532,23 @@ static int vmd_enable_domain(struct vmd_dev *vmd)
                .flags = IORESOURCE_BUS | IORESOURCE_PCI_FIXED,
        };
 
+       /*
+        * If the window is below 4GB, clear IORESOURCE_MEM_64 so we can
+        * put 32-bit resources in the window.
+        *
+        * There's no hardware reason why a 64-bit window *couldn't*
+        * contain a 32-bit resource, but pbus_size_mem() computes the
+        * bridge window size assuming a 64-bit window will contain no
+        * 32-bit resources.  __pci_assign_resource() enforces that
+        * artificial restriction to make sure everything will fit.
+        *
+        * The only way we could use a 64-bit non-prefechable MEMBAR is
+        * if its address is <4GB so that we can convert it to a 32-bit
+        * resource.  To be visible to the host OS, all VMD endpoints must
+        * be initially configured by platform BIOS, which includes setting
+        * up these resources.  We can assume the device is configured
+        * according to the platform needs.
+        */
        res = &vmd->dev->resource[VMD_MEMBAR1];
        upper_bits = upper_32_bits(res->end);
        flags = res->flags & ~IORESOURCE_SIZEALIGN;