ocxl: fix pci device refcount leak when calling get_function_0()

get_function_0() calls pci_get_domain_bus_and_slot(), as comment
says, it returns a pci device with refcount increment, so after
using it, pci_dev_put() needs be called.

Get the device reference when get_function_0() is not called, so
pci_dev_put() can be called in the error path and callers
unconditionally. And add comment above get_dvsec_vendor0() to tell
callers to call pci_dev_put().

Fixes: 87db7579 ("ocxl: control via sysfs whether the FPGA is reloaded on a link reset")
Suggested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221121154339.4088935-1-yangyingliang@huawei.com

drivers/misc/ocxl/config.c

@@ -193,6 +193,18 @@ static int read_dvsec_vendor(struct pci_dev *dev)
 	return 0;
 }
 
+/**
+ * get_dvsec_vendor0() - Find a related PCI device (function 0)
+ * @dev: PCI device to match
+ * @dev0: The PCI device (function 0) found
+ * @out_pos: The position of PCI device (function 0)
+ *
+ * Returns 0 on success, negative on failure.
+ *
+ * NOTE: If it's successful, the reference of dev0 is increased,
+ * so after using it, the callers must call pci_dev_put() to give
+ * up the reference.
+ */
 static int get_dvsec_vendor0(struct pci_dev *dev, struct pci_dev **dev0,
 			     int *out_pos)
 {
@@ -202,10 +214,14 @@ static int get_dvsec_vendor0(struct pci_dev *dev, struct pci_dev **dev0,
 		dev = get_function_0(dev);
 		if (!dev)
 			return -1;
+	} else {
+		dev = pci_dev_get(dev);
 	}
 	pos = find_dvsec(dev, OCXL_DVSEC_VENDOR_ID);
-	if (!pos)
+	if (!pos) {
+		pci_dev_put(dev);
 		return -1;
+	}
 	*dev0 = dev;
 	*out_pos = pos;
 	return 0;
@@ -222,6 +238,7 @@ int ocxl_config_get_reset_reload(struct pci_dev *dev, int *val)
 
 	pci_read_config_dword(dev0, pos + OCXL_DVSEC_VENDOR_RESET_RELOAD,
 			      &reset_reload);
+	pci_dev_put(dev0);
 	*val = !!(reset_reload & BIT(0));
 	return 0;
 }
@@ -243,6 +260,7 @@ int ocxl_config_set_reset_reload(struct pci_dev *dev, int val)
 		reset_reload &= ~BIT(0);
 	pci_write_config_dword(dev0, pos + OCXL_DVSEC_VENDOR_RESET_RELOAD,
 			       reset_reload);
+	pci_dev_put(dev0);
 	return 0;
 }