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 d57e480..29aebbc 100644
--- a/arch/x86/pci/vmd.c
+++ b/arch/x86/pci/vmd.c
@@ -532,6 +532,23 @@
.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;