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x86/PCI: VMD: Document code for maintainability

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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>
This commit is contained in:
Keith Busch 2016-03-02 15:31:03 -07:00 committed by Bjorn Helgaas
parent 92e963f50f
commit 83cc54a608
1 changed files with 17 additions and 0 deletions
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17
arch/x86/pci/vmd.c
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@ -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;