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I am trying to understand how devices are communicating over USB in linux, specifically for mobile android devices, so I can connect a DIY project to my phone via USB. After doing some search, I only found some general explanations of USB drivers specific for certain devices.

I would like to know a few things:

  1. how can I find for a specific device connected to a mobile device which driver is used? Where can I find it's code?
  2. how does the USB stack looks like? Couldn't find a good documentation.
  3. How can I develop drivers for USB devices? (Again, for android)
  4. how can I embed these drivers into the kernel or load them dynamically on my phone?

Thanks

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Partial answer: USB drivers generally are kernel modules that are loaded for specific USB devices by looking at vendor/device id's and other patterns embedded into the module. You can find this information for existing drivers with modinfo, e.g.

$ sudo modinfo usbhid
filename:       /lib/modules/4.12.13/kernel/drivers/hid/usbhid/usbhid.ko
license:        GPL
description:    USB HID core driver
author:         Jiri Kosina
author:         Vojtech Pavlik
author:         Andreas Gal
alias:          usb:v*p*d*dc*dsc*dp*ic03isc*ip*in*
depends:        hid,usbcore
intree:         Y
vermagic:       4.12.13 SMP preempt mod_unload modversions 686 
parm:           mousepoll:Polling interval of mice (uint)
parm:           jspoll:Polling interval of joysticks (uint)
parm:           ignoreled:Autosuspend with active leds (uint)
parm:           quirks:Add/modify USB HID quirks by specifying  
quirks=vendorID:productID:quirks where vendorID, productID, and quirks are all in 0x-prefixed hex (array of charp)

There are also various tools that can show for an existing device which driver is in use. I believe these all end up using the information from the /sys hierarchy eventually.

So if you want to write a new driver, you need to learn how to write a kernel module (google, there are tutorials). Pick an existing driver and modify it.

You can also write "drivers" (or rather appliation programs) in user space with a library like libusb.

The kernel documentation also includes information about the USB API, and by extension, information how the USB stack works.

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