[PATCH v3 1/2] PCI: Add D3cold as general reset method

[Jose Ignacio Tornos Martinez <jtornosm@redhat.com>]

Add D3cold power cycle as a general PCI reset method, available for
single-function devices. This provides a more robust reset mechanism
than D3hot for devices where a full power cycle is beneficial.

The implementation uses pci_set_power_state(dev, PCI_D3cold), which
automatically handles platform differences:
- Platforms WITH _PR3 ACPI power resources: true D3cold (power cycle)
- Platforms WITHOUT _PR3: automatic fallback to D3hot transition

D3cold reset is placed at the end of the reset hierarchy as a last
resort before giving up, since it provides a strong reset when other
methods are unavailable or broken.

Reset hierarchy with this change:
1. device_specific
2. acpi
3. flr
4. af_flr
5. pm (D3hot via config space)
6. bus (SBR)
7. cxl_bus
8. d3cold (NEW - power cycle with D3hot fallback)

This benefits devices that:
- Have broken or missing FLR
- Advertise NoSoftRst+ incorrectly (blocking D3hot PM reset)
- Have broken bus reset implementations
- Need power cycle for reliable reset
- Are used in VFIO passthrough scenarios

Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
---
v3: Implement d3cold as a general PCI core reset method instead of
  device-specific quirk approach from v2 (Alex Williamson suggestion)
v2: https://lore.kernel.org/all/20260508145153.717641-2-jtornosm@redhat.com/

 drivers/pci/pci.c   | 41 +++++++++++++++++++++++++++++++++++++++++
 include/linux/pci.h |  2 +-
 2 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 8f7cfcc00090..6da8feff7ccc 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -4491,6 +4491,46 @@ static int pci_pm_reset(struct pci_dev *dev, bool probe)
 	return ret;
 }
 
+/**
+ * pci_d3cold_reset - Put device into D3cold (or D3hot) and back to D0 for reset
+ * @dev: PCI device to reset
+ * @probe: if true, check if D3cold reset is supported; if false, perform reset
+ *
+ * Attempt to reset the device by transitioning through D3cold and back to D0.
+ * On platforms with ACPI _PR3 power resources, this performs a true D3cold
+ * power cycle (actual power removal). On platforms without _PR3 support,
+ * pci_set_power_state() automatically falls back to D3hot, providing a
+ * D3hot->D0 reset transition.
+ *
+ * Only available for single-function devices to avoid affecting other
+ * functions in multi-function devices.
+ *
+ * Returns 0 if device can be/was reset this way, -ENOTTY if not supported,
+ * or other negative error code on failure.
+ */
+static int pci_d3cold_reset(struct pci_dev *dev, bool probe)
+{
+	int ret;
+
+	if (dev->multifunction)
+		return -ENOTTY;
+
+	if (probe) {
+		if (!pci_pr3_present(dev))
+			pci_dbg(dev, "d3cold reset: no _PR3 support, will use D3hot fallback\n");
+		return 0;
+	}
+
+	if (dev->current_state != PCI_D0)
+		return -EINVAL;
+
+	ret = pci_set_power_state(dev, PCI_D3cold);
+	if (ret)
+		return ret;
+
+	return pci_set_power_state(dev, PCI_D0);
+}
+
 /**
  * pcie_wait_for_link_status - Wait for link status change
  * @pdev: Device whose link to wait for.
@@ -5065,6 +5105,7 @@ const struct pci_reset_fn_method pci_reset_fn_methods[] = {
 	{ pci_pm_reset, .name = "pm" },
 	{ pci_reset_bus_function, .name = "bus" },
 	{ cxl_reset_bus_function, .name = "cxl_bus" },
+	{ pci_d3cold_reset, .name = "d3cold" },
 };
 
 /**
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 2c4454583c11..1ca7b880ead7 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -51,7 +51,7 @@
 			       PCI_STATUS_PARITY)
 
 /* Number of reset methods used in pci_reset_fn_methods array in pci.c */
-#define PCI_NUM_RESET_METHODS 8
+#define PCI_NUM_RESET_METHODS 9
 
 #define PCI_RESET_PROBE		true
 #define PCI_RESET_DO_RESET	false
-- 
2.53.0