PCI/CXL: Add 'cxl_bus' reset method for devices below CXL Ports

By default Secondary Bus Reset (SBR) is masked for CXL Ports (see CXL r3.1,
sec 8.1.5.2).

Add cxl_reset_bus_function() (method "cxl_bus") to set the "Unmask SBR" bit
in the upstream CXL Port before performing the bus reset and restore the
original value afterwards.

This method allows the user to perform a bus reset on a CXL device without
needing to set the "Unmask SBR" bit via a user tool.

Link: https://lore.kernel.org/r/20240502165851.1948523-5-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
[bhelgaas: simplify commit log, invert condition to avoid negation]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Author: davejiang

drivers/pci/pci.c

@@ -4980,6 +4980,44 @@ static int pci_reset_bus_function(struct pci_dev *dev, bool probe)
 	return pci_parent_bus_reset(dev, probe);
 }
 
+static int cxl_reset_bus_function(struct pci_dev *dev, bool probe)
+{
+	struct pci_dev *bridge;
+	u16 dvsec, reg, val;
+	int rc;
+
+	bridge = pci_upstream_bridge(dev);
+	if (!bridge)
+		return -ENOTTY;
+
+	dvsec = cxl_port_dvsec(bridge);
+	if (!dvsec)
+		return -ENOTTY;
+
+	if (probe)
+		return 0;
+
+	rc = pci_read_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL, &reg);
+	if (rc)
+		return -ENOTTY;
+
+	if (reg & PCI_DVSEC_CXL_PORT_CTL_UNMASK_SBR) {
+		val = reg;
+	} else {
+		val = reg | PCI_DVSEC_CXL_PORT_CTL_UNMASK_SBR;
+		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL,
+				      val);
+	}
+
+	rc = pci_reset_bus_function(dev, probe);
+
+	if (reg != val)
+		pci_write_config_word(bridge, dvsec + PCI_DVSEC_CXL_PORT_CTL,
+				      reg);
+
+	return rc;
+}
+
 void pci_dev_lock(struct pci_dev *dev)
 {
 	/* block PM suspend, driver probe, etc. */
@@ -5064,6 +5102,7 @@ static const struct pci_reset_fn_method pci_reset_fn_methods[] = {
 	{ pci_af_flr, .name = "af_flr" },
 	{ pci_pm_reset, .name = "pm" },
 	{ pci_reset_bus_function, .name = "bus" },
+	{ cxl_reset_bus_function, .name = "cxl_bus" },
 };
 
 static ssize_t reset_method_show(struct device *dev,

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 7
+#define PCI_NUM_RESET_METHODS 8
 
 #define PCI_RESET_PROBE		true
 #define PCI_RESET_DO_RESET	false