[PATCH v6 2/4] PCI: Modify extend_bridge_window() to set resource size directly

[Nicholas Johnson]

Background
==========================================================================

In the current state, the PCI allocation could fail with Thunderbolt
under certain unusual circumstances, because add_list resources are
"optional". Guaranteed allocation requires guaranteed resource sizes.

It is difficult to give examples of these failures - because without the
previous patch in the series, the symptoms of the problem are hidden by
larger problems. This patch has been split from the previous patch and
makes little sense on its own - as it is almost impossible to see the
effect of this patch without first fixing the problems addressed by the
previous patch. So the evidence I put forward for making this change is
that because add_list resources are "optional", there could be any
number of unforeseen bugs that are yet to be encountered if the kernel
decides not to assign all of the optional size. In kernel development,
we should not play around with chance.

Moving away from add_size also allows for use of pci=hpmemsize to assign
resources. Previously, when using add_size and not allowing the add_size
to shrink, it made it impossible to distribute resources. If a hotplug
bridge has size X, and below it is some devices with non-zero size Y and
a nested hotplug bridge of same size X, fitting X+Y into size X is
mathematically impossible.

This patch solves this by dropping add_size and giving each bridge the
maximum size possible without failing resource assignment. Using
pci=hpmemsize still works as pci_assign_unassigned_root_bus_resources()
does not call pci_bus_distribute_available_resources(). At boot,
pci_assign_unassigned_root_bus_resources() is used, instead of
pci_bridge_distribute_available_resources().

By allowing to use pci=hpmemsize, it removes the reliance on the
firmware to declare the window resources under the root port, and could
pay off in the future with USB4 (which is backward-compatible to
Thunderbolt devices, and not specific to Intel systems). Users of
Thunderbolt hardware on unsupported systems will be able to specify the
resources in the kernel parameters. Users of official systems will be
able to override the default firmware window sizes to allocate much
larger resource sizes, potentially enabling Thunderbolt support for
devices with massive BARs (with a few other problems solved by later
patches in this series).

Patch notes
==========================================================================

Modify extend_bridge_window() to remove the resource from add_list and
change the resource size directly.

Modify extend_bridge_window() to reset resources that are being assigned
zero size. This is required to prevent the bridge not being enabled due
to resources with zero size. This is a direct requirement to prevent the
change away from using add_list from introducing a regression - because
before, it was not possible to end up with zero size.

Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@xxxxxxxxxxxxxx>
---
 drivers/pci/setup-bus.c | 42 ++++++++++++++++++++++++++---------------
 1 file changed, 27 insertions(+), 15 deletions(-)

diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
index 1b5b851ca..5675254fa 100644
--- a/drivers/pci/setup-bus.c
+++ b/drivers/pci/setup-bus.c
@@ -1810,28 +1810,40 @@ void __init pci_assign_unassigned_resources(void)
 }
 
 static void extend_bridge_window(struct pci_dev *bridge, struct resource *res,
-				 struct list_head *add_list,
-				 resource_size_t available)
+			struct list_head *add_list, resource_size_t new_size)
 {
-	struct pci_dev_resource *dev_res;
+	resource_size_t add_size;
 
 	if (res->parent)
 		return;
 
-	if (resource_size(res) >= available)
-		return;
-
-	dev_res = res_to_dev_res(add_list, res);
-	if (!dev_res)
-		return;
+	if (new_size >= resource_size(res)) {
+		add_size = new_size - resource_size(res);
+		pci_dbg(bridge, "bridge window %pR extended by %pa\n", res,
+			&add_size);
+	} else {
+		add_size = resource_size(res) - new_size;
+		pci_dbg(bridge, "bridge window %pR shrunken by %pa\n", res,
+			&add_size);
+	}
 
-	/* Is there room to extend the window? */
-	if (available - resource_size(res) <= dev_res->add_size)
-		return;
+	/*
+	 * Resources requested using add_size in additional resource lists are
+	 * considered optional when allocated. Guaranteed size of allocation
+	 * is required to guarantee successful resource distribution. Hence,
+	 * the size of the actual resource must be adjusted, and the resource
+	 * removed from add_list to prevent any additional size interfering.
+	 */
+	res->end = res->start + new_size - 1;
+	remove_from_list(add_list, res);
 
-	dev_res->add_size = available - resource_size(res);
-	pci_dbg(bridge, "bridge window %pR extended by %pa\n", res,
-		&dev_res->add_size);
+	/*
+	 * If we have run out of bridge resources, we may end up with a
+	 * zero-sized resource which may cause its bridge to not be enabled.
+	 * Disabling the resource prevents any such issues.
+	 */
+	if (!new_size)
+		reset_resource(res);
 }
 
 static void pci_bus_distribute_available_resources(struct pci_bus *bus,
-- 
2.20.1