PCI: Indicate context lost if L1SS exit is broken during resume from system suspend

Per PCIe v7.0, sec 5.5.3.3.1, when exiting L1.2 due to an endpoint
asserting CLKREQ# signal, the REFCLK must be turned on within the latency
advertised in the LTR message.  This requirement applies to L1.1 as well.

On some platforms like Qcom, these requirements are satisfied during OS
runtime, but not while resuming from the system suspend. This happens
because the PCIe RC driver may remove all resource votes and turn off the
PHY analog circuitry during suspend to maximize power savings while keeping
the link in L1SS.

Consequently, when the endpoint asserts CLKREQ# to wake up, the RC driver
must restore the PHY and enable the REFCLK.  When this recovery process
exceeds the L1SS exit latency time (roughly L10_REFCLK_ON + T_COMMONMODE),
the endpoint may treat it as a fatal condition and trigger Link Down (LDn).
This results in a reset that destroys the internal device state.

So to indicate this platform limitation to the client drivers, introduce a
new flag 'pci_host_bridge::broken_l1ss_resume' and check it in
pci_suspend_retains_context(). If the flag is set by the RC driver, the API
will return 'false' indicating the client drivers that the device context
may not be retained and the drivers must be prepared for context loss.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260519-l1ss-fix-v2-2-b2c3a4bdeb15@oss.qualcomm.com

1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 86961551ec51..2f7e7e186b47 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2910,6 +2910,8 @@ void pci_config_pm_runtime_put(struct pci_dev *pdev)
  */
 bool pci_suspend_retains_context(struct pci_dev *pdev)
 {
+	struct pci_host_bridge *bridge = pci_find_host_bridge(pdev->bus);
+
 	/*
 	 * If the platform firmware (like ACPI) is involved at the end of
 	 * system suspend, device context may not be retained.
@@ -2917,6 +2919,16 @@ bool pci_suspend_retains_context(struct pci_dev *pdev)
 	if (pm_suspend_via_firmware())
 		return false;
 
+	/*
+	 * Some host bridges power off the PHY to enter deep low-power
+	 * modes during system suspend. Exiting L1SS from this condition
+	 * may violate timing requirements and result in Link Down (LDn),
+	 * which causes a reset of the device.  On such platforms, the
+	 * endpoint must be prepared for context loss.
+	 */
+	if (bridge && bridge->broken_l1ss_resume)
+		return false;
+
 	/* Assume that the context is retained by default */
 	return true;
 }