[PATCH v3 4/5] pci: check if VGA decoding was really activated

[Simon Richter <Simon.Richter@hogyros.de>]

PCI bridges are allowed to refuse activating VGA decoding, by simply
ignoring attempts to set the bit that enables it, so after setting the bit,
read it back to verify.

One example of such a bridge is the root bridge in IBM PowerNV, but this is
also useful for GPU passthrough into virtual machines, where it is
difficult to set up routing for legacy IO through IOMMU.

Signed-off-by: Simon Richter <Simon.Richter@hogyros.de>
---
 drivers/pci/pci.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 8479c2e1f74f..e60b948f8576 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6197,6 +6197,12 @@ int pci_set_vga_state(struct pci_dev *dev, bool decode,
 				cmd &= ~PCI_BRIDGE_CTL_VGA;
 			pci_write_config_word(bridge, PCI_BRIDGE_CONTROL,
 					      cmd);
+			if (decode) {
+				pci_read_config_word(bridge, PCI_BRIDGE_CONTROL,
+						     &cmd);
+				if(!(cmd & PCI_BRIDGE_CTL_VGA))
+					return -EIO;
+			}
 		}
 		bus = bus->parent;
 	}
-- 
2.47.3