1USB port LED trigger
 2====================
 3
 4This LED trigger can be used for signalling to the user a presence of USB device
 5in a given port. It simply turns on LED when device appears and turns it off
 6when it disappears.
 7
 8It requires selecting USB ports that should be observed. All available ones are
 9listed as separated entries in a "ports" subdirectory. Selecting is handled by
10echoing "1" to a chosen port.
11
12Please note that this trigger allows selecting multiple USB ports for a single
13LED. This can be useful in two cases:
14
151) Device with single USB LED and few physical ports
16
17In such a case LED will be turned on as long as there is at least one connected
18USB device.
19
202) Device with a physical port handled by few controllers
21
22Some devices may have one controller per PHY standard. E.g. USB 3.0 physical
23port may be handled by ohci-platform, ehci-platform and xhci-hcd. If there is
24only one LED user will most likely want to assign ports from all 3 hubs.
25
26
27This trigger can be activated from user space on led class devices as shown
28below:
29
30  echo usbport > trigger
31
32This adds sysfs attributes to the LED that are documented in:
33Documentation/ABI/testing/sysfs-class-led-trigger-usbport
34
35Example use-case:
36
37  echo usbport > trigger
38  echo 1 > ports/usb1-port1
39  echo 1 > ports/usb2-port1
40  cat ports/usb1-port1
41  echo 0 > ports/usb1-port1