[PATCH v5] PCI: Allow PCI bridges to go to D3Hot on all Devicetree based platforms

[Brian Norris]

From: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>

Unlike ACPI based platforms, there are no known issues with D3Hot for
the PCI bridges in Device Tree based platforms. Past discussions (Link
[1]) determined the restrictions around D3 should be relaxed for all
Device Tree systems. So let's allow the PCI bridges to go to D3Hot
during runtime.

To match devm_pci_alloc_host_bridge() -> devm_of_pci_bridge_init(), we
look at the host bridge's parent when determining whether this is a
Device Tree based platform. Not all bridges have their own node, but the
parent (controller) should.

Link: https://lore.kernel.org/linux-pci/20240227225442.GA249898@bhelgaas/ [1]
Link: https://lore.kernel.org/linux-pci/20240828210705.GA37859@bhelgaas/ [2]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>
[Brian: look at host bridge's parent, not bridge node; rewrite
description]
Signed-off-by: Brian Norris <briannorris@xxxxxxxxxxxx>
---
Based on prior work by Manivannan Sadhasivam that was part of a bigger
series that stalled:

[PATCH v5 4/4] PCI: Allow PCI bridges to go to D3Hot on all Devicetree based platforms
https://lore.kernel.org/linux-pci/20240802-pci-bridge-d3-v5-4-2426dd9e8e27@xxxxxxxxxx/

I'm resubmitting this single patch, since it's useful and seemingly had
agreement. I massaged it a bit to relax some restrictions on how the
Device Tree should look.

Changes in v5:
- Pulled out of the larger series, as there were more controversial
  changes in there, while this one had agreement (Link [2]).
- Rewritten with a relaxed set of rules, because the above patch
  required us to modify many device trees to add bridge nodes.

 drivers/pci/pci.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index e278861684bc..5d898f5ea155 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3018,6 +3018,8 @@ static const struct dmi_system_id bridge_d3_blacklist[] = {
  */
 bool pci_bridge_d3_possible(struct pci_dev *bridge)
 {
+	struct pci_host_bridge *host_bridge;
+
 	if (!pci_is_pcie(bridge))
 		return false;
 
@@ -3038,6 +3040,15 @@ bool pci_bridge_d3_possible(struct pci_dev *bridge)
 		if (pci_bridge_d3_force)
 			return true;
 
+		/*
+		 * Allow D3 for all Device Tree based systems. We assume a host
+		 * bridge's parent will have a device node, even if this bridge
+		 * may not have its own.
+		 */
+		host_bridge = pci_find_host_bridge(bridge->bus);
+		if (dev_of_node(host_bridge->dev.parent))
+			return true;
+
 		/* Even the oldest 2010 Thunderbolt controller supports D3. */
 		if (bridge->is_thunderbolt)
 			return true;
-- 
2.47.0.338