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, 5 insertions, 2 deletions

diff --git a/Documentation/driver-api/fpga/fpga-bridge.rst b/Documentation/driver-api/fpga/fpga-bridge.rst
index 604208534095..833f68fb0700 100644
--- a/Documentation/driver-api/fpga/fpga-bridge.rst
+++ b/Documentation/driver-api/fpga/fpga-bridge.rst
@@ -6,9 +6,12 @@ API to implement a new FPGA bridge
 
 * struct fpga_bridge - The FPGA Bridge structure
 * struct fpga_bridge_ops - Low level Bridge driver ops
-* fpga_bridge_register() - Create and register a bridge
+* __fpga_bridge_register() - Create and register a bridge
 * fpga_bridge_unregister() - Unregister a bridge
 
+The helper macro ``fpga_bridge_register()`` automatically sets
+the module that registers the FPGA bridge as the owner.
+
 .. kernel-doc:: include/linux/fpga/fpga-bridge.h
    :functions: fpga_bridge
 
@@ -16,7 +19,7 @@ API to implement a new FPGA bridge
    :functions: fpga_bridge_ops
 
 .. kernel-doc:: drivers/fpga/fpga-bridge.c
-   :functions: fpga_bridge_register
+   :functions: __fpga_bridge_register
 
 .. kernel-doc:: drivers/fpga/fpga-bridge.c
    :functions: fpga_bridge_unregister