[PATCH] PCI/ASPM: Enable L0s/L1 for removable devices when BIOS didn't configure ASPM

[Mario Limonciello]

When comparing lspci output between Windows and Linux for hotplugged
Thunderbolt 5 eGPU devices, Windows enables ASPM L1 but Linux doesn't:

  Windows: LnkCtl: ASPM L1 Enabled
  Linux:   LnkCtl: ASPM Disabled

This difference in ASPM configuration can cause behavioral differences
between the two operating systems for the same hardware.

The root cause is that Linux's default ASPM policy (POLICY_DEFAULT) relies
on firmware/BIOS configuration. For hotplugged devices like Thunderbolt/USB4
eGPUs, the BIOS may not have configured ASPM since the device wasn't present
at boot time. As a result, link->aspm_enabled is 0, link->aspm_default is
set to 0, and Linux never enables ASPM for these devices.

Devicetree platforms already have special handling to enable L0s/L1 by
default regardless of firmware configuration. Extend this same logic to
removable devices when firmware hasn't configured any ASPM states.

This makes Linux behavior more consistent with Windows for hotplugged
Thunderbolt/USB4 devices.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=221319
Assisted-by: Claude Opus 4.6 <noreply@xxxxxxxxxxxxx>
Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
---
 drivers/pci/pcie/aspm.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index 925373b98dff0..77497d90be0b7 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -804,8 +804,15 @@ static void pcie_aspm_override_default_link_state(struct pcie_link_state *link)
 	struct pci_dev *pdev = link->downstream;
 	u32 override;
 
-	/* For devicetree platforms, enable L0s and L1 by default */
-	if (of_have_populated_dt()) {
+	/*
+	 * For devicetree platforms, enable L0s and L1 by default.
+	 *
+	 * For removable devices (e.g., Thunderbolt/USB4), enable L0s and L1
+	 * by default if BIOS didn't configure any ASPM states. This handles
+	 * hotplugged devices where firmware may not have configured ASPM.
+	 */
+	if (of_have_populated_dt() ||
+	    (dev_is_removable(&pdev->dev) && !link->aspm_enabled)) {
 		if (link->aspm_support & PCIE_LINK_STATE_L0S)
 			link->aspm_default |= PCIE_LINK_STATE_L0S;
 		if (link->aspm_support & PCIE_LINK_STATE_L1)
-- 
2.43.0