Some Qualcomm PCIe devices (WCN6855/WCN7850 WiFi cards, SDX62/SDX65 modems)
lack working reset methods for VFIO passthrough scenarios. These devices
have no FLR capability, advertise NoSoftRst+ (blocking PM reset), and have
broken bus reset.

The problem manifests in VFIO passthrough scenarios:

- WCN6855 WiFi card (17cb:1103): Normal VM operation works fine, including
  clean shutdown/reboot. However, when the VM terminates uncleanly
  (crash, force-off), VFIO attempts to reset the device before it can
  be assigned to another VM. Without a working reset method, the device
  remains in an undefined state, preventing reuse.

- WCN7850 WiFi card (17cb:1107): Same behavior as WCN6855.

- SDX62/SDX65 5G modems (17cb:0308): Never successfully initialize even
  on first VM assignment without proper reset capability.

Add device-specific reset entries for these Qualcomm devices using D3hot
power cycling. Testing shows that despite advertising NoSoftRst+, D3hot
transition provides sufficient reset for VFIO reuse, particularly after
unexpected VM termination. While not a complete reset (BARs preserved),
it provides the only viable reset mechanism for these devices.

Testing was performed on desktop platforms with M.2 WiFi and modem cards
using M.2-to-PCIe adapters, including extensive force-reset cycling to
verify stability.

Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
---
v9:
  - Complete redesign based on maintainer feedback (Alex Williamson, Bjorn
    Helgaas, Rafael Wysocki): dropped general d3cold infrastructure entirely
    and now just a single patch: the proven D3hot reset for specific
    Qualcomm devices (device-specific reset)
  - Previous v8 patch 1/3 (general d3cold) dropped: concerns about ACPI
    portability, bridge issues, runtime PM, and lack of _PR3 hardware for
    testing.
  - Previous v8 patch 3/3 (quirk_no_bus_reset) already merged for v7.2
v8: https://lore.kernel.org/all/20260609163649.319755-1-jtornosm@redhat.com/

 drivers/pci/quirks.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
index 431c021d7414..bac1edb6c2dc 100644
--- a/drivers/pci/quirks.c
+++ b/drivers/pci/quirks.c
@@ -4240,6 +4240,41 @@ static int reset_hinic_vf_dev(struct pci_dev *pdev, bool probe)
 	return 0;
 }
 
+/*
+ * Device-specific reset method for certain Qualcomm devices via D3hot power
+ * cycle.
+ *
+ * These specific Qualcomm devices lack FLR capability, advertise NoSoftRst+
+ * (blocking PM reset), and have broken bus reset. Despite advertising
+ * NoSoftRst+, testing shows that D3hot transition provides sufficient reset
+ * for VFIO reuse, particularly after unexpected VM termination where the
+ * device would otherwise remain in an undefined state. While not a complete
+ * reset (BARs are preserved), it provides the only viable reset mechanism for
+ * these devices in the commented situations.
+ */
+static int reset_qualcomm_d3hot(struct pci_dev *dev, bool probe)
+{
+	int ret;
+
+	if (probe)
+		return 0;
+
+	if (dev->current_state != PCI_D0)
+		return -EINVAL;
+
+	ret = pci_set_power_state(dev, PCI_D3hot);
+	if (ret)
+		return ret;
+	msleep(200);
+
+	ret = pci_set_power_state(dev, PCI_D0);
+	if (ret)
+		return ret;
+	msleep(200);
+
+	return 0;
+}
+
 static const struct pci_dev_reset_methods pci_dev_reset_methods[] = {
 	{ PCI_VENDOR_ID_INTEL, PCI_DEVICE_ID_INTEL_82599_SFP_VF,
 		 reset_intel_82599_sfp_virtfn },
@@ -4255,6 +4290,9 @@ static const struct pci_dev_reset_methods pci_dev_reset_methods[] = {
 		reset_chelsio_generic_dev },
 	{ PCI_VENDOR_ID_HUAWEI, PCI_DEVICE_ID_HINIC_VF,
 		reset_hinic_vf_dev },
+	{ PCI_VENDOR_ID_QCOM, 0x1103, reset_qualcomm_d3hot },  /* WCN6855 */
+	{ PCI_VENDOR_ID_QCOM, 0x1107, reset_qualcomm_d3hot },  /* WCN7850 */
+	{ PCI_VENDOR_ID_QCOM, 0x0308, reset_qualcomm_d3hot },  /* SDX62/SDX65 */
 	{ 0 }
 };
 
-- 
2.54.0