[PATCH v4] PCI: Accept AtomicOps already enabled by the hypervisor

[Nikola Prica <nikprica@amd.com>]

From: Nikola Prica <nikola.prica@amd.com>

pci_enable_atomic_ops_to_root() currently fails when no Root Port is
visible. That is common in passthrough guests (ESXi, Hyper-V): the
endpoint is assigned to the VM, but the guest topology has no Root
Port above it.

In those setups the hypervisor may already have enabled AtomicOp
Requester Enable on the device. If PCI_EXP_DEVCTL2_ATOMIC_REQ is set,
treat AtomicOps as already enabled and return success instead of
failing the Root Port walk.

After 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port
supports them"), pci_enable_atomic_ops_to_root() always fails if the
Root Port is not visible.  On systems where the Root Port is not
visible but *does* support AtomicOps, this is a regression: prior to
1ae8c4ce1570, it enabled AtomicOps in the endpoint and returned
success.

Fixes: 1ae8c4ce1570 ("PCI: Enable AtomicOps only if Root Port supports them")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
Signed-off-by: Nikola Prica <nikola.prica@amd.com>

---
v4: Updates commit log to make regression point clear
v3: Add Fixes tag
v2: Be more strict and only apply logic in headless device case.
---
 drivers/pci/pci.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index b2879a6be5f8..62729ade496f 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3769,8 +3769,18 @@ int pci_enable_atomic_ops_to_root(struct pci_dev *dev, u32 cap_mask)
 	}
 
 	root = pcie_find_root_port(dev);
-	if (!root)
+	if (!root) {
+		/*
+		 * A hypervisor may expose a headless topology with no
+		 * visible root port.  If it has already set AtomicOp
+		 * Requester Enable, there is nothing more to do.
+		 */
+		pcie_capability_read_dword(dev, PCI_EXP_DEVCTL2, &ctl2);
+		if (ctl2 & PCI_EXP_DEVCTL2_ATOMIC_REQ)
+			return 0;
+
 		return -EINVAL;
+	}
 
 	pcie_capability_read_dword(root, PCI_EXP_DEVCAP2, &cap);
 	if ((cap & cap_mask) != cap_mask)
-- 
2.34.1