PCI: Add DMA alias iterator

In a mixed PCI/PCI-X/PCIe topology, bridges can take ownership of
transactions, replacing the original requester ID with their own.
Sometimes we just want to know the resulting device or resulting alias;
other times we want each step in the chain.  This iterator allows either
usage.  When an endpoint is connected via an unbroken chain of PCIe
switches and root ports, it has no alias and its requester ID is visible to
the root bus.  When PCI/X get in the way, we pick up aliases for bridges.

The reason why we potentially care about each step in the path is because
of PCI-X.  PCI-X has the concept of a requester ID, but bridges may or may
not take ownership of various types of transactions.  We therefore leave it
to the consumer of this function to prune out what they don't care about
rather than attempt to flatten the alias ourselves.

Tested-by: George Spelvin <linux@horizon.com>
Tested-by: Pat Erley <pat-lkml@erley.org>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/search.c

@@ -17,6 +17,76 @@
 DECLARE_RWSEM(pci_bus_sem);
 EXPORT_SYMBOL_GPL(pci_bus_sem);
 
+/*
+ * pci_for_each_dma_alias - Iterate over DMA aliases for a device
+ * @pdev: starting downstream device
+ * @fn: function to call for each alias
+ * @data: opaque data to pass to @fn
+ *
+ * Starting @pdev, walk up the bus calling @fn for each possible alias
+ * of @pdev at the root bus.
+ */
+int pci_for_each_dma_alias(struct pci_dev *pdev,
+			   int (*fn)(struct pci_dev *pdev,
+				     u16 alias, void *data), void *data)
+{
+	struct pci_bus *bus;
+	int ret;
+
+	ret = fn(pdev, PCI_DEVID(pdev->bus->number, pdev->devfn), data);
+	if (ret)
+		return ret;
+
+	for (bus = pdev->bus; !pci_is_root_bus(bus); bus = bus->parent) {
+		struct pci_dev *tmp;
+
+		/* Skip virtual buses */
+		if (!bus->self)
+			continue;
+
+		tmp = bus->self;
+
+		/*
+		 * PCIe-to-PCI/X bridges alias transactions from downstream
+		 * devices using the subordinate bus number (PCI Express to
+		 * PCI/PCI-X Bridge Spec, rev 1.0, sec 2.3).  For all cases
+		 * where the upstream bus is PCI/X we alias to the bridge
+		 * (there are various conditions in the previous reference
+		 * where the bridge may take ownership of transactions, even
+		 * when the secondary interface is PCI-X).
+		 */
+		if (pci_is_pcie(tmp)) {
+			switch (pci_pcie_type(tmp)) {
+			case PCI_EXP_TYPE_ROOT_PORT:
+			case PCI_EXP_TYPE_UPSTREAM:
+			case PCI_EXP_TYPE_DOWNSTREAM:
+				continue;
+			case PCI_EXP_TYPE_PCI_BRIDGE:
+				ret = fn(tmp,
+					 PCI_DEVID(tmp->subordinate->number,
+						   PCI_DEVFN(0, 0)), data);
+				if (ret)
+					return ret;
+				continue;
+			case PCI_EXP_TYPE_PCIE_BRIDGE:
+				ret = fn(tmp,
+					 PCI_DEVID(tmp->bus->number,
+						   tmp->devfn), data);
+				if (ret)
+					return ret;
+				continue;
+			}
+		} else {
+			ret = fn(tmp, PCI_DEVID(tmp->bus->number, tmp->devfn),
+				 data);
+			if (ret)
+				return ret;
+		}
+	}
+
+	return ret;
+}
+
 /*
  * find the upstream PCIe-to-PCI bridge of a PCI device
  * if the device is PCIE, return NULL

include/linux/pci.h

@@ -1795,6 +1795,10 @@ static inline struct eeh_dev *pci_dev_to_eeh_dev(struct pci_dev *pdev)
 }
 #endif
 
+int pci_for_each_dma_alias(struct pci_dev *pdev,
+			   int (*fn)(struct pci_dev *pdev,
+				     u16 alias, void *data), void *data);
+
 /**
  * pci_find_upstream_pcie_bridge - find upstream PCIe-to-PCI bridge of a device
  * @pdev: the PCI device