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I would like to ask for guidance on how to ideally communicate with a custom USB HID device on MacOS.

Use case Modify a microphone volume via an external USB HID device.

Question Can I use DriverKit (HIDDriverKit) for that or I need to use IOKit? I have read something here about audio limitation, but not sure what exactly is not supported.

DriverKit doesn’t support USB devices that manipulate audio or that communicate wirelessly over Bluetooth or Wi-Fi. For those types of devices, create a kernel extension using IOKit. — Source

Would DriverKit still work in my case as I am not sending audio streams but controlling volume only?

Many thanks! Cheers, Tom

Tom Kupka
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If I understand you correctly, you wouldn't even need to use DriverKit. (from experience: avoid it if you can!)

You can communicate with HID-compliant devices directly from user space processes. User space processes can generally also control the volume on audio devices.

So by far the easiest option would be to have a launch agent which uses IOKit matching as its launch condition so it starts up when your device is connected. Your agent can communicate with the device using the IOHIDManager API to receive events when your buttons are pressed, and then use the regular Core Audio APIs to control volume.

It doesn't have to be a launch agent, incidentally: a regular Cocoa app with a UI can do all of this as well. (And indeed, you may want to show some form of UI as feedback to the user pressing the buttons.)

pmdj
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  • Hey Phil! Thanks so much. Actually, I have currently a Cocoa app, and it works great as you mentioned, the issue comes when I bring another window into focus, e.g. switching focus to Safari and it stops listening to the buttons, then I switch back to my app and it works again. It seems like the process is paused or something. That is why I thought, that the communication with HID device must run somewhere deeper in the background. I would like that it works like a mouse or keyboard no matter what app is in the forefront. Do you know some tricks on how to make the connection alive? :-) – Tom Kupka May 18 '21 at 20:14
  • Ah, interesting, I've not come across this issue, it probably depends on the subtype of HID device. Have you tried using the [`kIOHIDOptionsTypeSeizeDevice` option](https://developer.apple.com/documentation/iokit/1556660-anonymous/kiohidoptionstypeseizedevice)? If this doesn't help, you could try to find out what happens if you handle the events in a process running as root. (`sudo`) If that helps, moving the driver aspect to a launch *daemon* should work. (These run as root.) – pmdj May 18 '21 at 20:41
  • I will definitely try these options! The issue is probably because I use the native SDK Mac libraries provided by SiliconLabs (the HID USB bridge producer). The device is HID-compliant so the methods you suggest may work. Thank you for the direction! – Tom Kupka May 19 '21 at 09:20