PCI: Add comments about ROM BAR updating
[ Upstream commit 0b457dde ] pci_update_resource() updates a hardware BAR so its address matches the kernel's struct resource UNLESS it's a disabled ROM BAR. We only update those when we enable the ROM. It's not obvious from the code why ROM BARs should be handled specially. Apparently there are Matrox devices with defective ROM BARs that read as zero when disabled. That means that if pci_enable_rom() reads the disabled BAR, sets PCI_ROM_ADDRESS_ENABLE (without re-inserting the address), and writes it back, it would enable the ROM at address zero. Add comments and references to explain why we can't make the code look more rational. The code changes are from 755528c8 ("Ignore disabled ROM resources at setup") and 8085ce08 ("[PATCH] Fix PCI ROM mapping"). Link: https://lkml.org/lkml/2005/8/30/138Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com> Signed-off-by: Sasha Levin <alexander.levin@verizon.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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