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

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

#21

Post by yellowapple »

Perfect timing. I'm expecting to get my hands on a MOTU MIDI Express XT from my local Guitar Center within the next couple days (I paid for it when it arrived there a couple weeks ago, but they have a mandatory waiting period on used equipment to make sure it ain't stolen), which unfortunately uses some weird proprietary protocol instead of class-compliant MIDI-over-USB — so any use over USB from my PCs (nearly all of which are running Linux, OpenBSD, Haiku, or something other than Windows or macOS) is a no-go. This is okay for my immediate use cases (I just need it to route between some synth modules and controllers, without necessarily needing the PC to do any processing in-between), but it'd be cool to get the PC side of things working, too.

There's an existing out-of-tree Linux driver¹ that looks promising, but AFAICT it only does the bare minimum of exposing the MIDI ports for use with e.g. JACK, and it's also unclear how stable it is and whether it really does support the XT (the README says the kernel panic got fixed, but there are open issues about it; the README says the XT's supported, but there are open issues about that, too). I'd like to be able to create new routing presets and stuff like what the proprietary companion app can do, and I'd also like to be able to use the thing without needing to shove extra drivers into my kernel, and I'd also like to be able to use the thing on my OpenBSD and Haiku boxen, so I've been perusing LibUSB docs since a userspace USB driver that then presents the relevant MIDI ports and some tooling to reroute the MIDI ports as desired seems like something useful. This article happens to be exactly what I've been looking for w.r.t. a starting point for such a userspace driver.

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¹: https://github.com/vampirefrog/motu

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

#24

Post by grawlinson »

yellowapple wrote: post #21

Perfect timing. I'm expecting to get my hands on a MOTU MIDI Express XT from my local Guitar Center within the next couple days (I paid for it when it arrived there a couple weeks ago, but they have a mandatory waiting period on used equipment to make sure it ain't stolen), which unfortunately uses some weird proprietary protocol instead of class-compliant MIDI-over-USB — so any use over USB from my PCs (nearly all o…

I packaged that driver on the AUR, since I’ve got the same device. I can’t get the binary blob to work (admittedly, I haven’t tried very hard) and it’s not high on my list of priorities so I’m okay using it as a dumb MIDI tool.

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

#26

Post by varispeed »

Ages ago when I was trying to create a simple USB device, I found that there is very much zero information how to do it - e.g. how to correctly write descriptors and so on. The typical advice was: find similar device to what you want to make, copy its descriptors and adapt to your own device using trial and error.

Sounds like USB is a wonderful standard. Am I wrong?

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

#29

Post by kam »

pjc50 wrote: post #13
Neywiny wrote: post #6

Earlier quoted context omitted.

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'…

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.

No need to pretend to be HID. Windows has WinUSB for userspace USB drivers that don't need special signing.

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

#30

Post by kam »

brcmthrowaway wrote: post #27

Dumb question.. do USB devices support DMA? Is it done through the host? Or does the USB device always push/pull data to host memory?

USB devices cannot directly address host memory like PCIe or FireWire, but the XHCI controller does DMA to/from host memory, and most USB device controllers have some kind of DMA between USB and the device's RAM.
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