FAQ

According to USB 2.0 specification, USB hubs can advertise no power switching,

ganged (all ports at once) power switching or per-port (individual) power switching.

Note that `uhubctl` will only detect USB hubs which support per-port power switching.

You can find what kind of power switching your hardware supports by using `sudo lsusb -v`:

No power switching:

wHubCharacteristic 0x000a

No power switching (usb 1.0)

Per-port overcurrent protection

Ganged power switching:

wHubCharacteristic 0x0008

Ganged power switching

Per-port overcurrent protection

Per-port power switching:

wHubCharacteristic 0x0009

Per-port power switching

Per-port overcurrent protection

1. Run `sudo uhubctl`. If your hub is not listed, it is not supported.

Alternatively, you can run `sudo lsusb -v` and check for

`Per-port power switching` - if you cannot see such line in lsusb output,

hub is not supported.

2. Check for VBUS (voltage) off support: plug a phone, USB light

or USB fan into USB port of your hub.

Try using `uhubctl` to turn power off on that port, and check

that phone stops charging, USB light stops shining or USB fan stops spinning.

If VBUS doesn't turn off, your hub manufacturer did not include circuitry

to actually cut power off. Such hub would still work

to cut off USB data connection, but it cannot turn off power,

and we do not consider this supported device.

3. If tests above were successful, please report your hub

by opening new issue at https://github.com/mvp/uhubctl/issues,

so we can add it to list of supported devices.

After powering down USB port, udev does not get any event, so it keeps the device files around.

However, trying to access the device files will lead to an IO error.

This is Linux kernel issue. It may be eventually fixed in kernel, see more discussion [here](https://bit.ly/2JzczjZ).

Basically what happens here is that kernel USB driver knows about power off,

but doesn't send notification about it to udev.

You can find workaround for this in article https://goo.gl/qfrmGK.

Some device drivers in kernel are surprised by USB device being turned off and automatically try to power it back on.

You can use option `-r N` where N is some number from 10 to 1000 to fix this -

`uhubctl` will try to turn power off many times in quick succession, and it should suppress that.

This may be eventually fixed in kernel, see more discussion [here](https://bit.ly/2JzczjZ).

Many hub manufacturers build their USB hubs using basic 4 port USB chips.

E.g. to make 7 port hub, they daisy-chain two 4 port hubs - 1 port is lost to daisy-chaining,

so it makes it 4+4-1=7 port hub. Simularly, 10 port hub could be built as 3 4-port hubs

daisy-chained together, which gives 4+4+4-2=10 usable ports.

Note that you should never try to change power state for ports used to daisy-chain internal hubs together.

Doing so will confuse internal hub circuitry and will cause unpredictable behavior.

This is limitation of Raspberry Pi hardware design.

For reference, Raspberry Pi models have following internal USB topology:

* B+/2B/3B: one USB hub `1-1`, with port `1` for Ethernet+wifi, and ports `2-5` ganged, controlled by port `2`.

(Trying to control ports 3,4,5 won't do anything).

* 3B+: 2 hubs - main hub `1-1`, all 4 ports ganged, all controlled by port `2`.

Second hub `1-1.1` (daisy-chained to main): 3 independently controlled ports, `1` is used for Ethernet+wifi,

and ports `2,3` are available with proper per-port power switching.

In other words, 2 out of total 4 ports wired outside do support independent power switching on 3B+.

As a workaround, you can buy any external USB hub from supported list,

attach it to any USB port of Raspberry Pi, and control power on its ports independently.