radeon: RFC speed cap detection on ppc64

The radeon driver does speed cap detection on the root PCI device for the maximum speed with which the adapter can communicate. On ppc64 systems, however, the root device belongs to the Hypervisor, so the current code would case a null pointer dereference.

I propose to look for the outmost bus with a parent node and get speed caps from it, though I suppose the cleaner way would be to inspect all devices along the way and choose the smallest speed cap.

Does anyone have suggestions for this?

Thanks

--

@@ -466,6 +466,19 @@
 }
 EXPORT_SYMBOL(drm_pci_exit);
 
+static struct pci_dev *drm_get_pcie_root_dev(struct pci_dev *dev)
+{
+	// Go up through all possible busses to get the info for the outmost bus
+	while (!pci_is_root_bus(dev->bus))
+		dev = dev->bus->parent->self;
+	
+	// In POWER architectures there's no PCI root device, so it should just read
+	// the caps from the device itself
+	if (dev->bus->self != NULL)
+		return dev->bus->self;
+	else
+		return dev;
+}
+
 int drm_pcie_get_speed_cap_mask(struct drm_device *dev, u32 *mask)
 {

struct pci_dev *root;
@@ -479,7 +492,7 @@
if (!pci_is_pcie(dev->pdev))
return -EINVAL;

- root = dev->pdev->bus->self;
+ root = drm_get_pcie_root_dev(dev->pdev);

pos = pci_pcie_cap(root);
if (!pos)

Lucas Kannebley Tavares
Software Engineer
IBM Linux Technology Center

That (walking all parent nodes) is probably the safest thing to do. I'm not sure whether it's optimal. It would likely depend on whether you can meaningfully have a bridge that's faster on the downstream side than on the upstream.

- ajax

This is architecture goo at heart - would this be better as a helper in the PCI and arch PCI code ?

Alan

Good point.

POWER is not the only architecture where host bridges do not appear as devices in PCI config space -- ia64 has that, too. So the comment is too specific.

The link is a point-to-point thing, so this should be a local negotiation between the radeon device and the upstream bridge. I don't see the point of walking any farther up the chain.

drm_pcie_get_speed_cap_mask() should also be changed to use pcie_capability_read_dword() to avoid any issues with v1/v2 PCI Express Capability structures.

Bjorn