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USB for Software Developers: An introduction to writing userspace USB drivers

96 points by WerWolv 3 hours ago | 15 comments

Neywiny 3 hours ago

But this kinda expects that your USB driver is the application code too, no? This is less of a driver and more of a library + program. If I have, say, a USB to Ethernet device, how do I hook this into the ethernet adapter subsystem?

pjc50 2 hours ago

Things which are relatively standard tend to get good generic support: Ethernet devices will generally be USB/CDC/ECM or RNDIS, for example. That may Just Work (tm) if it has the right descriptors.

The userland approach is much more useful for weird or custom devices. In particular, on Windows you can do one of these user space "drivers" without having to bother with driver signing, and if you use libusb it will be portable too.

(I maintain a small USB DFU based tool for work)

Neywiny 2 hours ago

DFU - great example. If you have a USB device that has a DFU class that needs a custom driver, can dfu-util and the like hook into these userspace drivers? Or do you also need to maintain the application part?

pjc50 2 hours ago

Dfu-util is one of those "user space drivers", so if you have a nonstandard protocol you'd have to add it directly to dfu-util. There's no intermediate API.

It's not easy to set up a fake or "remapped" USB device on most OS as far as I'm aware, if you were trying to write an adapter program that modified USB packets.

WerWolv 2 hours ago

dfu-util actually also just uses libusb under the hood! Any class or device that doesn't have a driver baked into the OS can be implemented like this. And if you'd need the DFU functionality in a different application, you may be able to just simply link parts of the dfu-util tool into your application

WerWolv 2 hours ago

On Linux you could create a tun/tap device from your application and translate data sent over that to requests sent to the ethernet adapter.

Of course, when you're doing these things in userspace you either need some way of communicating with the Kernel or for the other subsystems to be in userspace as well.

Neywiny 2 hours ago

Not to be too facetious but a great place for communicating with the kernel where there are a ton of other driver subsystems is... the kernel.

Possibly a good addition to the article would be parallel development of an lkm. I guess it wouldn't have that windows interop but I would also be interested to see how this driver would be implemented on Windows. If it's idk 10x as many lines in the kernel vs userspace, that's a great benefit to the userspace approach.

pjc50 2 hours ago

Driver signing is a killer issue on Windows; if you put your machine into dev/unsigned mode you get an ugly banner that can't be turned off.

Much easier to design the device to avoid that. E.g. by abusing USB-HID. The desktop USB missile launcher toy is USB HID, for example.

WerWolv 2 hours ago

Arguably all these other subsystems shouldn't be in the Kernel either but that's a different topic :)

There are quite a few benefits to doing these things in userspace over the Kernel, not really necessarily just because of the code size:

- The code is much easier to write and debug, you just write code like you always would.

- Bugs don't have the possibility to taking down your entire system or introduce vulnerabilities

- Especially on Windows, everyone can do this without requiring an impossible to get driver signing certificate

fleahunter 2 hours ago

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dist-epoch 2 hours ago

In HFT user-space networking drivers have a long history - there is too much latency induced by switching from kernel to user space to handle networking.

> OpenOnload: A user-space network stack that intercepts socket calls to bypass the kernel network stack, accelerating standard socket operations for faster networking.

> Netmap: A framework providing a simple API for high-speed packet I/O in user space, bypassing much of the kernel overhead for efficient packet forwarding and filtering.

https://dysnix.com/blog/high-frequency-trading-infrastructur...

tosti 2 hours ago

The C++ looks messed up. I have yet to come across a keyboard that can type an arrow.

Something1234 55 minutes ago

Some developers like ligature based fonts. They combine 2 characters into one glyph

tosti 47 minutes ago

Thank you and the others who were kind enough to explain this. I've avoided such fonts like the plague. Didn't know it did arrows like that.

quietbritishjim 54 minutes ago

It's just a programming font ligature. If you copy and paste it you'll see the actual characters e.g.

   auto main() -> int {

(It's also modern C++ trailing return type.)

bheadmaster 52 minutes ago

It's just "->" - the ligature font just renders it as a unitary arrow

webdevver 2 hours ago

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