The document compares the kernel USB API and the userspace libusb API. It outlines the basic templates for using each, noting that the kernel API requires a USB ID table and passes the device handle while libusb must obtain the handle. It discusses the differences in asynchronous and synchronous calls between the kernel urb and libusb transfer models. Finally, it summarizes the pros and cons of each approach, noting userspace is better for prototyping but kernel provides better performance while being more difficult to debug.

1. Kernel USB API vs Userspace (libusb)
Vasily anarsoul Khoruzhick
email: anarsoul@gmail.com, skype: anarsoul
github.com/anarsoul
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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2. Template of (minimal) libusb app
libusb_init()
dev_handle = libusb_open_device_with_vid_pid(vid, pid)
libusb_claim_interface(dev_handle, intf_num)
libusb_set_interface_altsetting()
libusb_alloc_transfer() ... libusb_fill_*_transfer() ...
libusb_submit_transfer() ... completion callback ...
libusb_close(dev_handle)
libusb_exit()
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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3. Template of (simple) kernel driver for USB device
USB ID compatibility table (possible to match device class also)
driver probe function
usb_driver_claim_interface()
usb_set_interface()
usb_alloc_urb() ... usb_fill_*_urb() ... usb_submit_urb() ...
completion callback ... usb_free_urb()
driver removal function
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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4. Generic Kernel vs Userspace differences
No one cleanups after you in Kernel
Dereferenced NULL pointer? -> reboot
Corrupted some memory of other driver? -> reboot
Atomic/non-atomic context
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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5. Entry point
Userspace: should obtain device handle by itself
(libusb_open_device_with_vid_pid())
Kernel: contains USB ID compatibility table, device handle is passed
by driver core
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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6. Async API: libusb_transfer vs usb_urb
Represents single USB transfer
Contains source & destination (EP num, direction, pointer to buffer
and its size)
Completion callback
In kernel completion callback is called from atomic context
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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7. Async API libusb_submit_transfer() vs usb_urb_submit()
Almost no difference - both submit asynchronous transfer
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
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8. Sync API: libusb_{control,bulk,interrupt}_transfer vs
usb_{control,bulk,interrupt}_msg
Kernel: can’t use sync API in atomic context
Kernel: userspace: it’s convenient, but usually not a good idea to use
synchronous API
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
8 / 11

9. Userspace: Pros and Cons
Pros:
Good for prototyping, decent development speed
Easy to debug
Convenient synchronous API
Cons:
Performance and response latency is worse
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
9 / 11

10. Kernel: Pros and Cons
Pros:
Decent performance and response latency
Cons:
Development speed is slower
Not so easy to debug
One should always keep in mind that it’s a kernel:
Can’t sleep in completion callback
Can’t use synchronous API in atomic context
Be carefull with memory and other resources
Async stuff is complicated, sync stuff in kernel is evil.
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
10 / 11

11. The End
Thanks! Questions?
Vasily Khoruzhick, Synesis LLC
Kernel USB API vs Userspace (libusb)
October, 2013
11 / 11