fpga: bridge: add owner module and take its refcount

The current implementation of the fpga bridge assumes that the low-level
module registers a driver for the parent device and uses its owner pointer
to take the module's refcount. This approach is problematic since it can
lead to a null pointer dereference while attempting to get the bridge if
the parent device does not have a driver.

To address this problem, add a module owner pointer to the fpga_bridge
struct and use it to take the module's refcount. Modify the function for
registering a bridge to take an additional owner module parameter and
rename it to avoid conflicts. Use the old function name for a helper macro
that automatically sets the module that registers the bridge as the owner.
This ensures compatibility with existing low-level control modules and
reduces the chances of registering a bridge without setting the owner.

Also, update the documentation to keep it consistent with the new interface
for registering an fpga bridge.

Other changes: opportunistically move put_device() from __fpga_bridge_get()
to fpga_bridge_get() and of_fpga_bridge_get() to improve code clarity since
the bridge device is taken in these functions.

Fixes: 21aeda950c5f ("fpga: add fpga bridge framework")
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Xu Yilun <yilun.xu@intel.com>
Reviewed-by: Russ Weight <russ.weight@linux.dev>
Signed-off-by: Marco Pagani <marpagan@redhat.com>
Acked-by: Xu Yilun <yilun.xu@intel.com>
Link: https://lore.kernel.org/r/20240322171839.233864-1-marpagan@redhat.com
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/include/linux/fpga/fpga-bridge.h b/include/linux/fpga/fpga-bridge.h
index 223da48a6d18..94c4edd047e5 100644
--- a/include/linux/fpga/fpga-bridge.h
+++ b/include/linux/fpga/fpga-bridge.h
@@ -45,6 +45,7 @@ struct fpga_bridge_info {
  * @dev: FPGA bridge device
  * @mutex: enforces exclusive reference to bridge
  * @br_ops: pointer to struct of FPGA bridge ops
+ * @br_ops_owner: module containing the br_ops
  * @info: fpga image specific information
  * @node: FPGA bridge list node
  * @priv: low level driver private date
@@ -54,6 +55,7 @@ struct fpga_bridge {
 	struct device dev;
 	struct mutex mutex; /* for exclusive reference to bridge */
 	const struct fpga_bridge_ops *br_ops;
+	struct module *br_ops_owner;
 	struct fpga_image_info *info;
 	struct list_head node;
 	void *priv;
@@ -79,10 +81,12 @@ int of_fpga_bridge_get_to_list(struct device_node *np,
 			       struct fpga_image_info *info,
 			       struct list_head *bridge_list);
 
+#define fpga_bridge_register(parent, name, br_ops, priv) \
+	__fpga_bridge_register(parent, name, br_ops, priv, THIS_MODULE)
 struct fpga_bridge *
-fpga_bridge_register(struct device *parent, const char *name,
-		     const struct fpga_bridge_ops *br_ops,
-		     void *priv);
+__fpga_bridge_register(struct device *parent, const char *name,
+		       const struct fpga_bridge_ops *br_ops, void *priv,
+		       struct module *owner);
 void fpga_bridge_unregister(struct fpga_bridge *br);
 
 #endif /* _LINUX_FPGA_BRIDGE_H */