[PATCH 2/2] PCI/ASPM: Clear ASPM Control on links without common ASPM support

[Stefan Roese <stefan.roese@mailbox.org>]

pcie_aspm_cap_init() returns early when the two ends of a link share
no ASPM state. It then never touches Link Control, so ASPM Control
keeps whatever the device came out of reset with. A device that
enables ASPM by default, against a port that cannot do it, keeps ASPM
enabled, which the spec does not allow.

The Renesas uPD720201 xHCI (1912:0014) is such a device: it resets
with LnkCtl 0x0003 (L0s and L1 enabled) and sits behind the CPM Root
Port of AMD Versal, which supports no ASPM. It only works because the
chip clears ASPM Control itself during its firmware download, and it
stops doing so once the host has written Link Control.

Clear ASPM Control on every function of such a link, downstream
component first, and only where it is set.

Before (lspci -vvv -s 01:00.0):
  LnkCtl: ASPM L0s L1 Enabled; RCB 64 bytes, LnkDisable- CommClk-

After:
  pci 0000:01:00.0: ASPM: link supports no ASPM, clearing ASPM Control
  LnkCtl: ASPM Disabled; RCB 64 bytes, LnkDisable- CommClk-

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Stefan Roese <stefan.roese@mailbox.org>
---
 drivers/pci/pcie/aspm.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
index f8e64971c6c3..13d7a6d3f6ad 100644
--- a/drivers/pci/pcie/aspm.c
+++ b/drivers/pci/pcie/aspm.c
@@ -931,6 +931,18 @@ static void pcie_aspm_override_default_link_state(struct pcie_link_state *link)
 	}
 }
 
+static void pcie_aspm_clear_aspmc(struct pci_dev *pdev)
+{
+	u16 lnkctl;
+
+	pcie_capability_read_word(pdev, PCI_EXP_LNKCTL, &lnkctl);
+	if (!(lnkctl & PCI_EXP_LNKCTL_ASPMC))
+		return;
+
+	pci_info(pdev, "ASPM: link supports no ASPM, clearing ASPM Control\n");
+	pcie_capability_clear_word(pdev, PCI_EXP_LNKCTL, PCI_EXP_LNKCTL_ASPMC);
+}
+
 static void pcie_aspm_cap_init(struct pcie_link_state *link, int blacklist)
 {
 	struct pci_dev *child = link->downstream, *parent = link->pdev;
@@ -947,11 +959,17 @@ static void pcie_aspm_cap_init(struct pcie_link_state *link, int blacklist)
 
 	/*
 	 * If ASPM not supported, don't mess with the clocks and link,
-	 * bail out now.
+	 * bail out now. A device may still come out of reset with ASPM
+	 * Control set, which the rest of this code never touches for
+	 * such a link. Clear it, downstream component first.
 	 */
 	if (!(parent->aspm_l0s_support && child->aspm_l0s_support) &&
-	    !(parent->aspm_l1_support && child->aspm_l1_support))
+	    !(parent->aspm_l1_support && child->aspm_l1_support)) {
+		list_for_each_entry(fn, &linkbus->devices, bus_list)
+			pcie_aspm_clear_aspmc(fn);
+		pcie_aspm_clear_aspmc(parent);
 		return;
+	}
 
 	/* Configure common clock before checking latencies */
 	pcie_aspm_configure_common_clock(link);
-- 
2.56.0