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https://github.com/pikvm/pikvm
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55 lines
3.0 KiB
Markdown
55 lines
3.0 KiB
Markdown
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??? warning "USB limitations"
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**TL;DR: By default, you can add only one additional device to choose from, such as USB Ethernet, or USB Serial, or an extra Mass Storage Drive.**
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There is a hardware limit on the number of devices that can be emulated at the same time.
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Each USB device uses so-called endpoints to communicate with the host.
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Depending on the type, the device consumes a different number of endpoints,
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while their total number is limited by the capabilities of the chip, **for Raspberry Pi it is 8**.
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It is quite difficult to calculate the number of endpoints used, but in the case of PiKVM,
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you can focus on the following numbers:
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| Device | Endpoints |
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|--------|-----------|
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| Keyboard, mouse | 1 for each |
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| Mass Storage Drive | 2 for each |
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| USB Ethernet, USB Serial | 3 for each |
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V2 and V3 emulates one mouse by default, V4 emulates two mouses. Thus, V2 and V3 use 4 endpoints, and V4 uses 5 by default.
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Creating an axtra Mass Storage Drive consumes additional endpoints, as well as USB Serial and USB Ethernet,
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so only a limited number of devices can be selected for the final configuration, for example, one USB Ethernet.
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If you need something more non-standard, you can [disable the regular Mass Storage Drive](msd.md#disable-msd)
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and the additional mouse (on V4) to free up some extra endpoints.
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The `kvmd-otg` service is responsible for setting up USB emulation. If the endpoint limit is exceeded,
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the service will not be able to start and no emulated USB device will work.
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In the log it looks something like this:
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```
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# journalctl -u kvmd-otg
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...
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kvmd-otg[382]: kvmd.apps.otg INFO --- ===== Preparing complete =====
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kvmd-otg[382]: kvmd.apps.otg INFO --- Enabling the gadget ...
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kvmd-otg[382]: kvmd.apps.otg INFO --- WRITE --- /sys/kernel/config/usb_gadget/kvmd/UDC
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kvmd-otg[382]: OSError: [Errno 524] Unknown error 524
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kvmd-otg[382]: During handling of the above exception, another exception occurred:
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kvmd-otg[382]: Traceback (most recent call last):
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kvmd-otg[382]: File "/usr/bin/kvmd-otg", line 9, in <module>
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kvmd-otg[382]: main()
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kvmd-otg[382]: File "/usr/lib/python3.10/site-packages/kvmd/apps/otg/__init__.py", line 348, in main
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kvmd-otg[382]: options.cmd(config)
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kvmd-otg[382]: File "/usr/lib/python3.10/site-packages/kvmd/apps/otg/__init__.py", line 278, in _cmd_start
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kvmd-otg[382]: _write(join(gadget_path, "UDC"), udc)
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kvmd-otg[382]: File "/usr/lib/python3.10/site-packages/kvmd/apps/otg/__init__.py", line 83, in _write
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kvmd-otg[382]: with open(path, "w") as file:
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kvmd-otg[382]: OSError: [Errno 524] Unknown error 524
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systemd[1]: kvmd-otg.service: Main process exited, code=exited, status=1/FAILURE
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systemd[1]: kvmd-otg.service: Failed with result 'exit-code'.
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systemd[1]: Failed to start PiKVM - OTG setup.
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```
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In this case, you need to disable some of the previously enabled devices and restart PiKVM.
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