Fixing The Deprecated Wireless Event In Your Driver

Hey folks! Let's dive into a common issue faced by developers – the use of deprecated features in Linux drivers. Specifically, we'll address the problem of the wireless_send_event function being deprecated and how to potentially migrate to the newer nl80211 API. This is super important because relying on deprecated features can lead to compatibility issues, especially when dealing with newer kernel versions like Fedora 42. So, let's get started and make sure our drivers are up-to-date and future-proof!

Understanding the Problem: Why wireless_send_event is a No-Go

Alright, first things first, let's understand why wireless_send_event is causing headaches. This function is part of the Linux Wireless Extensions (WEXT), which, as the kernel documentation (https://wireless.docs.kernel.org/en/latest/wext-statement.html) explicitly states, is deprecated. This means that WEXT is no longer actively maintained, and its functionality is gradually being replaced by the more modern nl80211 API. Using deprecated features in your driver can lead to build failures, as the original poster, hmtheboy154, experienced in their Fedora 42 environment. In their case, the error message ERROR: modpost: "wireless_send_event" [mt7902.ko] undefined! clearly indicates that the build process is unable to find the wireless_send_event function, which is no longer supported in the new kernel version.

So, what does this actually mean for you? Well, it means that your driver, which depends on WEXT, will likely fail to compile or function correctly on systems that have phased out or disabled WEXT support. This is a critical issue that needs immediate attention if you want your driver to work across different Linux distributions and kernel versions. By understanding the core problem, we are one step closer to making the necessary changes to ensure your driver is compatible and can function flawlessly.

Diving into the nl80211 API: The Future of Wireless

Now that we know the problem, let's talk about the solution: the nl80211 API. nl80211 is the successor to WEXT and offers a more robust, flexible, and modern way to interact with wireless devices. It provides a more comprehensive set of features and capabilities, making it the preferred choice for wireless driver development in modern Linux systems. The nl80211 API relies on Netlink sockets, which provide a more efficient and reliable communication channel between the kernel and user space. This not only improves performance but also enhances the overall security and stability of the system.

Migrating to nl80211 involves several steps: You will need to replace all instances of wireless_send_event and other WEXT-related functions with their nl80211 equivalents. This usually means rewriting parts of your driver to use nl80211's framework for event handling, configuration, and control of wireless devices. The nl80211 API offers a wide array of commands and attributes that allow you to manage and configure wireless interfaces, monitor network statistics, and handle various wireless events. While this migration process might seem daunting at first, it's a worthwhile effort that pays off in the long run by ensuring your driver's compatibility and long-term maintainability. Learning to use nl80211 will also open up new possibilities and provide you with more advanced features to implement in your driver, improving its functionality and performance.

Step-by-Step: Migrating from wireless_send_event to nl80211

Okay, guys, let's roll up our sleeves and get into the nitty-gritty of the migration process. Here's a simplified breakdown of how to swap out wireless_send_event for its nl80211 counterpart:

1. Identify Event Types: First, you need to identify the specific wireless events your driver is sending using wireless_send_event. These could be things like link up/down events, scanning events, or authentication events. Take note of all the event types, as they will be critical for the transition.

2. Lookup nl80211 Equivalents: Next, you have to look for the equivalent nl80211 counterparts for each of the identified events. nl80211 uses a different mechanism for sending these events, typically involving the use of Netlink messages. You'll need to figure out which Netlink messages or commands correspond to the specific events in your driver.

3. Replace wireless_send_event Calls: Then, the process is to replace the calls to wireless_send_event with code that constructs and sends the appropriate nl80211 Netlink messages. This typically involves several steps:

   • Get a Netlink Socket: You need to obtain a Netlink socket. This can be achieved by using functions like cfg80211_netlink_init().
   • Prepare the Message: Construct the Netlink message, filling in the necessary attributes for the event you are sending. This involves setting the Netlink message header and including the specific data related to the event, such as the interface index, event type, and any relevant data.
   • Send the Message: Use functions like nl80211_send_event() to actually send the constructed Netlink message. Make sure to specify the correct destination and other necessary parameters.

4. Handle Event Registration: Make sure you register any necessary event handlers in your driver. nl80211 often requires you to register callbacks for receiving events. This is similar to how you registered event handlers with the old WEXT API.

5. Testing and Debugging: After completing the code changes, thoroughly test your driver to ensure that the wireless events are sent and received correctly. You can use tools like iwconfig or iwlist to monitor the wireless interface and verify that the events are being generated. Debug any issues you find, as the transition to nl80211 can be tricky, and errors can be subtle.

Remember, this is a general guideline. The exact implementation details will depend on the specifics of your driver. Also, always consult the official Linux kernel documentation and examples when working with the nl80211 API.

Practical Example: Sending a Link Up/Down Event

Let's get practical, shall we? Here's a simplified example to illustrate how you might send a link up/down event using nl80211. This is a common event, so it's a great example to use for understanding the transition.

#include <linux/nl80211.h>
#include <net/cfg80211.h>

// Assuming you have a struct representing your driver's state
struct my_driver_data {
    struct wiphy *wiphy;
    struct net_device *netdev;
    // Other driver-specific data
};

static void send_link_event(struct my_driver_data *drv_data, bool up)
{
    struct sk_buff *skb;
    void *msg_head;
    int err;

    skb = cfg80211_alloc_event_skb(drv_data->wiphy, 0);
    if (!skb) {
        printk(KERN_ERR "Failed to allocate skb for link event\n");
        return;
    }

    msg_head = nl80211hdr_put(skb, drv_data->netdev->ifindex, 0, NL80211_CMD_NEW_IFACE, NL80211_MCAST_USER);
    if (!msg_head) {
        kfree_skb(skb);
        printk(KERN_ERR "Failed to put nl80211 header\n");
        return;
    }

    if (nla_put_u32(skb, NL80211_IFTYPE, drv_data->netdev->type) ||
        nla_put_u8(skb, NL80211_IF_OPERSTATE, up ? NL80211_IF_OPERSTATE_UP : NL80211_IF_OPERSTATE_DOWN)) {
        kfree_skb(skb);
        printk(KERN_ERR "Failed to put nla attributes\n");
        return;
    }

    err = cfg80211_send_event(drv_data->wiphy, skb, GFP_KERNEL);
    if (err) {
        printk(KERN_ERR "Failed to send link event: %d\n", err);
    }
}

// Example usage:  In your driver's link up/down handling
void my_driver_link_state_change(struct my_driver_data *drv_data, bool up) {
    send_link_event(drv_data, up);
    // ... other actions for link up/down ...
}

Explanation:

• Headers: Include necessary headers like <linux/nl80211.h> and <net/cfg80211.h>. These are essential for using the nl80211 API and related functions.
• cfg80211_alloc_event_skb(): This function allocates a socket buffer (skb) for the Netlink message. It is the beginning for creating the frame to send a Netlink message.
• nl80211hdr_put(): Puts the nl80211 header in the skb. This is essential for proper formatting of the message. The interface index, which identifies the network interface, is added using the ifindex of the network device to properly identify the network interface associated with the event.
• nla_put_u32() and nla_put_u8(): These functions add Netlink attributes to the message. In this example, the interface type (NL80211_IFTYPE) and operational state (NL80211_IF_OPERSTATE) are added to the message.
• cfg80211_send_event(): This function sends the event to the appropriate userspace applications.

This simple example demonstrates how to send a link up/down event, showcasing the structure of an nl80211 event message. While your specific implementation may vary depending on your driver's functionality, this basic example provides a solid starting point for understanding how to use the nl80211 API.

Troubleshooting Common Issues

During the migration to the nl80211 API, you might encounter a few common issues. Let's look at some of them and how you can resolve them:

• Build Errors: One of the main issues, as we saw in the original post, is build errors. The most common cause of build errors is the use of deprecated or incorrect function calls. Ensure that you are using the correct functions from the nl80211 API and that you have included all the necessary header files. Carefully review the kernel documentation to make sure you have the correct function signatures and parameters.
• Event Not Being Received: Another common issue is that the events are not being received by the user space applications. Double-check that you've correctly registered the necessary event handlers and that the attributes in the Netlink messages are correct. Use tools like tcpdump to capture the Netlink messages and verify that they are being sent and received correctly. This will help you identify any problems with the message format or content.
• Interface Not Showing Up: If the wireless interface is not showing up or not functioning properly, check the interface configuration and make sure that it has been correctly initialized. Verify that the correct driver is being loaded and that the wireless device is properly configured. Also, make sure that the regulatory domain is set correctly, and the interface has the proper permissions.
• Memory Issues: Always pay attention to memory management when working with the kernel. Make sure you correctly allocate and free any memory that you use. Use kernel debugging tools, like printk and kmemleak, to identify and fix any memory leaks or corruption issues that may arise.

Best Practices and Resources

To make your migration process as smooth as possible, keep these best practices in mind:

• Read the Documentation: The Linux kernel documentation is your best friend. Make sure to consult the official documentation for the nl80211 API and the kernel itself. This will provide you with the most up-to-date information, examples, and function descriptions.
• Study Existing Drivers: Look at existing drivers that use the nl80211 API. This can provide valuable insights into how to implement the API in your own driver. The Linux kernel source code contains many examples that you can reference.
• Test Thoroughly: Test your driver extensively after the migration. Test all possible scenarios and configurations. Use various tools to monitor the functionality of the driver and verify that it's working as expected. This will help you catch any subtle errors before they cause real-world issues.
• Stay Updated: Keep up-to-date with kernel changes. The nl80211 API may evolve over time, and kernel updates can introduce changes. By staying updated, you can ensure that your driver remains compatible and that you're taking advantage of any new features or improvements.

Conclusion: Future-Proof Your Driver!

Alright, folks, that's a wrap! By understanding the need to replace wireless_send_event and transitioning to the nl80211 API, you're taking a crucial step in maintaining your wireless driver's functionality and compatibility with modern Linux kernels. Remember to consult the official documentation, study existing drivers, and test your changes thoroughly. While the migration might take some time and effort, the end result will be a more robust and future-proof driver. Stay awesome, and keep those drivers updated!