MacOS custom Audio Driver doesn't override the default Driver

Question

We are developing a custom audio driver for a USB microphone in order to do simple processing (EQs) on the input audio stream (comparable to an APO for Windows). After some help, we managed to assign our Driver (based on the SimpleAudioDriver) to the right audio device. But we now have two devices showing up : one is assigned with our driver and the other one is assigned to the default driver. How can we override the original one with ours, to just have one device showing up ?

We already tried to add more IOKitPersonalities to have a better probe score but it's the same. We also read the logs and the original driver and our probe score are both at the maximum (100000).

Here is the current state of our info.plist file :

<plist version="1.0">
<dict>
    <key>IOKitPersonalities</key>
    <dict>
        <key>SimpleAudioDriver</key>
        <dict>
            <key>idProduct</key>
            <integer>49456</integer>
            <key>idVendor</key>
            <integer>1130</integer>
            <key>CFBundleIdentifier</key>
            <string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
            <key>IOClass</key>
            <string>IOUserService</string>
            <key>IOMatchCategory</key>
            <string>SimpleAudioDriver</string>
            <key>IOProviderClass</key>
            <string>IOUSBDevice</string>
            <key>IOResourceMatch</key>
            <string>IOKit</string>
            <key>IOUserAudioDriverUserClientProperties</key>
            <dict>
                <key>IOClass</key>
                <string>IOUserUserClient</string>
                <key>IOUserClass</key>
                <string>IOUserAudioDriverUserClient</string>
            </dict>
            <key>IOUserClass</key>
            <string>SimpleAudioDriver</string>
            <key>IOUserServerName</key>
            <string>com.example.apple-samplecode.SimpleAudio.Driver</string>
            <key>SimpleAudioDriverUserClientProperties</key>
            <dict>
                <key>IOClass</key>
                <string>IOUserUserClient</string>
                <key>IOUserClass</key>
                <string>SimpleAudioDriverUserClient</string>
            </dict>
        </dict>
    </dict>
</dict>
</plist>

If you have any hints, please get back to us.

Thanks.

Can you edit your question to add the dext's `Info.plist` and `ioreg` subtree for your device? You can get the latter by entering something like this on the Terminal: `ioreg -w 0 -irn "My USB Audio"` (Substitute with the name for your device, or use `ioreg -w 0 -irc IOUSBHostDevice` to dump *all* USB devices.) This should show us the device's USB interfaces and connected drivers, and hopefully give a clue as to what's going wrong. — pmdj, Feb 16 '23 at 09:57

pmdj · Accepted Answer · 2023-02-16T14:07:41.187

The sample driver doesn't match any real hardware, but instead attaches itself to the dummy "resources" nub:

            <key>IOProviderClass</key>
            <string>IOUserResources</string>

This is a generic object that does nothing other than exist so that virtual drivers can attach. By default, objects can only be successfully be matched by one client (i.e. driver), so virtual drivers would fight over access to this dummy object. So when matching this object only, you need to use match categories:

            <key>IOMatchCategory</key>
            <string>SimpleAudioDriver</string>

However, you must absolutely not use this key when matching real hardware, unless you have a very good reason. If you keep this in, your driver's match category will be different from Apple's default driver, so both will match the device, leading to chaos.

To avoid multiple drivers trying to claim the same device, simply remove the entire IOMatchCategory key/value pair to use the default category, which is also what the standard macOS USB audio driver uses, and therefore only one driver will succeed at matching. Which one wins can be controlled via the different probe score levels for different USB matching patterns or, in unusual cases, using an explicit probe score.

Hi @pmdj, thanks a lot for your help. I edited my post with our info.plist file. We replaced the dummy IOUserResources with "IOUSBDevice". I think we are actually in this "chaos" state you're mentioning. But then what should we use as the IOMatchCategory ? Thanks. — kinaar, Feb 16 '23 at 11:17
@pmdj I'm trying to match a Thunderbolt PCI device, but removing the IOMatchCategory makes it not get considered at all for matching. Is there something else I need to configure to get priority over the built-in driver? There's more context in this question: https://stackoverflow.com/questions/76183589/macos-driverkit-making-pci-dext-to-replace-built-in-driver — Mads Y, May 05 '23 at 16:10

MacOS custom Audio Driver doesn't override the default Driver

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