Emulator

I did some more testing. With the Steam Link directly connected with Ethernet to my PC, the adapter is working perfectly well while being shared over usbip.

I very strongly suspect that the issue is with the 16ms timeout Dolphin uses for all the libusb_interrupt_transfer calls in ./Source/Core/InputCommon/GCAdapter.cpp.
If the adapter is connected over USB it's plenty of time for the call to succeed, but if the call has to be sent over the network then a timeout is likely to occur if the network adds too much latency (WiFi, Power-line communication, etc.).

I will try to recompile Dolphin with greater timeout values and see if it fixes the issue. Is there any reason why the timeout is set to 16ms and not something greater?

I recompiled Dolphin (latest commit in master) with a 100ms timeout value in all libusb_interrupt_transfer calls in ./Source/Core/InputCommon/GCAdapter.cpp
When the Steam Link is not directly connected to my PC, it still doesn't work.

My network config is : Steam Link (Ethernet) => power-line communication device ====> power-line communication device => switch => PC (Ethernet)

For both Dolphin version (5.0 and master with greater timeout value), when Dolphin is launched and as long as no game is launched, I see a lot of URB_INTERRUPT messages being sent back and forth (I'm using Wireshark with the kernel module usbmon). There is 5ms to 10ms of latency between the URB_SUBMIT and URB_COMPLETE messages.

Just after a game is launched, the flow of URB_INTERRUPT stops, and I get an error in the logs.

It might be related to a network packet drop or a similar issue. I will investigate more later!

Logs:

[Binaries]$ ./dolphin-emu
51:46:398 InputCommon/GCAdapter.cpp:110 N[SI]: GC Adapter scanning thread started
51:46:398 InputCommon/GCAdapter.cpp:238 N[SI]: Found GC Adapter with Vendor: 57E Product: 337 Devnum: 1
51:46:415 InputCommon/GCAdapter.cpp:506 I[SI]: Rumble state reset
51:46:415 InputCommon/GCAdapter.cpp:125 N[SI]: Using libUSB hotplug detection
51:46:532 InputCommon/ControllerInterface/evdev/evdev.cpp:54 N[SI]: evdev hotplug thread started
52:10:460 InputCommon/GCAdapter.cpp:407 N[SI]: New device connected to Port 1 of Type: 10
52:10:797 Core/HW/SI/SI_DeviceGCController.cpp:79 I[SI]: PAD - Get Origin
52:10:798 Core/HW/SI/SI_DeviceGCController.cpp:313 I[SI]: PAD 0 set to mode 3
52:10:798 Core/HW/SI/SI_DeviceGCController.cpp:79 I[SI]: PAD - Get Origin
52:10:798 Core/HW/SI/SI_DeviceGCController.cpp:313 I[SI]: PAD 3 set to mode 3
52:11:501 InputCommon/GCAdapter.cpp:396 E[SI]: error reading payload (size: 0, type: 21)
52:11:510 InputCommon/GCAdapter.cpp:370 N[SI]: GC Adapter detached

Edit of my previous message: I messed up, I did not apply the 100ms timeout value before compiling Dolphin. It's 4AM here! =P

After recompiling with the 100ms modification applied, and testing again from my Steam Link streaming the game, it works!
Of course, the latency is noticeable in game. But at least with the modification Dolphin now allows me to play with real GC controllers with the adapter.

So if there is no specific reason of setting the timeout value to 16ms (at least in my case it did not slow down the emulation), I would suggest increasing it to at least 100ms (or more? or just disabling the timeout?) so that people that share USB peripherals over the network can do so without issue (except input lag).
That would make Dolphin more compatible with VirtualHere which is included by default on Steam Links, in addition to the more "experimental" usbip approach. Especially with official GH controller adapters which don't have a "PC mode" (Xinput).

I'll probably do a pull request if there is no one against this idea here.

Make a Pull Request please so there can be discussion about it. I'm not 100% sure why it's set to 16, but if it is, someone will speak up in a PR. A lot of the core devs don't watch the issue tracker as much.

Done: https://github.com/dolphin-emu/dolphin/pull/5799

I've been having a similar issue to this execpt I am using the VirtualHere to pass the adapter through the steam link. It works fine for the most part but occasionally the c-stick will act weird and sometimes the left stick will get permantly stuck in one direction. I'm getting a similar error in the logs.

11:00:555 inputcommon\gcadapter.cpp:412 E[SI]: error reading payload (size: 0, type: 21)
11:00:558 inputcommon\gcadapter.cpp:386 N[SI]: GC Adapter detached
11:00:953 inputcommon\gcadapter.cpp:263 N[SI]: Found GC Adapter with Vendor: 57E Product: 337 Devnum: 1
11:00:963 inputcommon\gcadapter.cpp:423 N[SI]: New device connected to Port 1 of Type: 14
11:00:965 inputcommon\gcadapter.cpp:522 I[SI]: Rumble state reset

This happened multiple times thoughout me playing the game in dolphin and it directly corresponds to the glitches with the controllers. Im using version 5.0-9779 so I'm not sure whether your 100ms time out change has made it into the build or not. How did you edit the source code to change it?

So after messing around with it and some research I found this post https://bugs.dolphin-emu.org/issues/9486 and ended up trying their fix. I downloaded https://www.spice-space.org/download/windows/usbdk/UsbDk_1.0.15_x64.msi and installed it and my adapters been working fine.