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PCI/VGA: Fail pci_set_vga_state() if VGA decoding not supported

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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>
[bhelgaas: subject, add comment about VGA Enable writability]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260307173538.763188-5-Simon.Richter@hogyros.de
This commit is contained in:
Simon Richter 2026-03-08 02:35:37 +09:00 committed by Bjorn Helgaas
parent 94555ea9a0
commit 5b6471fc72
1 changed files with 12 additions and 0 deletions
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12
drivers/pci/pci.c
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@ -6197,6 +6197,18 @@ 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);
/*
* VGA Enable may not be writable if bridge doesn't
* support it.
*/
if (decode) {
pci_read_config_word(bridge, PCI_BRIDGE_CONTROL,
&cmd);
if (!(cmd & PCI_BRIDGE_CTL_VGA))
return -EIO;
}
}
bus = bus->parent;
}