[PATCH 7/9] x86/pci: Comment pci_mmconfig_insert() obscure MCFG dependency

[Bjorn Helgaas <helgaas@kernel.org>]

From: Bjorn Helgaas <bhelgaas@google.com>

In pci_mmconfig_insert(), there's no reference to "addr" between locking
pci_mmcfg_lock and testing "addr", so it *looks* like we should move the
test before the lock.

But 07f9b61c3915 ("x86/PCI: MMCONFIG: Check earlier for MMCONFIG region at
address zero") did that, which broke things by returning -EINVAL when
"addr" is zero instead of -EEXIST.

So 07f9b61c3915 was reverted by 67d470e0e171 ("Revert "x86/PCI: MMCONFIG:
Check earlier for MMCONFIG region at address zero"").

Add a comment about this issue to prevent it from happening again.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
---
 arch/x86/pci/mmconfig-shared.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/pci/mmconfig-shared.c b/arch/x86/pci/mmconfig-shared.c
index b36c10e86505..459e95782bb1 100644
--- a/arch/x86/pci/mmconfig-shared.c
+++ b/arch/x86/pci/mmconfig-shared.c
@@ -786,6 +786,10 @@ int pci_mmconfig_insert(struct device *dev, u16 seg, u8 start, u8 end,
 		return -EEXIST;
 	}
 
+	/*
+	 * Don't move earlier; we must return -EEXIST, not -EINVAL, if
+	 * pci_mmconfig_lookup() finds something
+	 */
 	if (!addr) {
 		mutex_unlock(&pci_mmcfg_lock);
 		return -EINVAL;
-- 
2.34.1