src/kernel/linux/v4.19/Documentation/usb/chipidea.txt

1. How to test OTG FSM(HNP and SRP)
-----------------------------------
To show how to demo OTG HNP and SRP functions via sys input files
with 2 Freescale i.MX6Q sabre SD boards.

1.1 How to enable OTG FSM
---------------------------------------
1.1.1 Select CONFIG_USB_OTG_FSM in menuconfig, rebuild kernel
Image and modules. If you want to check some internal
variables for otg fsm, mount debugfs, there are 2 files
which can show otg fsm variables and some controller registers value:
cat /sys/kernel/debug/ci_hdrc.0/otg
cat /sys/kernel/debug/ci_hdrc.0/registers
1.1.2 Add below entries in your dts file for your controller node
	otg-rev = <0x0200>;
	adp-disable;

1.2 Test operations
-------------------
1) Power up 2 Freescale i.MX6Q sabre SD boards with gadget class driver loaded
   (e.g. g_mass_storage).

2) Connect 2 boards with usb cable with one end is micro A plug, the other end
   is micro B plug.

   The A-device(with micro A plug inserted) should enumerate B-device.

3) Role switch
   On B-device:
   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req

   B-device should take host role and enumerate A-device.

4) A-device switch back to host.
   On B-device:
   echo 0 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req

   or, by introducing HNP polling, B-Host can know when A-peripheral wish
   to be host role, so this role switch also can be trigged in A-peripheral
   side by answering the polling from B-Host, this can be done on A-device:
   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_req

   A-device should switch back to host and enumerate B-device.

5) Remove B-device(unplug micro B plug) and insert again in 10 seconds,
   A-device should enumerate B-device again.

6) Remove B-device(unplug micro B plug) and insert again after 10 seconds,
   A-device should NOT enumerate B-device.

   if A-device wants to use bus:
   On A-device:
   echo 0 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_drop
   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_req

   if B-device wants to use bus:
   On B-device:
   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req

7) A-device power down the bus.
   On A-device:
   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/a_bus_drop

   A-device should disconnect with B-device and power down the bus.

8) B-device does data pulse for SRP.
   On B-device:
   echo 1 > /sys/bus/platform/devices/ci_hdrc.0/inputs/b_bus_req

   A-device should resume usb bus and enumerate B-device.

1.3 Reference document
----------------------
"On-The-Go and Embedded Host Supplement to the USB Revision 2.0 Specification
July 27, 2012 Revision 2.0 version 1.1a"

2. How to enable USB as system wakeup source
-----------------------------------
Below is the example for how to enable USB as system wakeup source
at imx6 platform.

2.1 Enable core's wakeup
echo enabled > /sys/bus/platform/devices/ci_hdrc.0/power/wakeup
2.2 Enable glue layer's wakeup
echo enabled > /sys/bus/platform/devices/2184000.usb/power/wakeup
2.3 Enable PHY's wakeup (optional)
echo enabled > /sys/bus/platform/devices/20c9000.usbphy/power/wakeup
2.4 Enable roothub's wakeup
echo enabled > /sys/bus/usb/devices/usb1/power/wakeup
2.5 Enable related device's wakeup
echo enabled > /sys/bus/usb/devices/1-1/power/wakeup

If the system has only one usb port, and you want usb wakeup at this port, you
can use below script to enable usb wakeup.
for i in $(find /sys -name wakeup | grep usb);do echo enabled > $i;done;