USB is different [LWN.net]

USB is different

Posted Jan 29, 2004 16:04 UTC (Thu) by HalfMoon (guest, #3211)
Parent article: User-space device drivers

This Gelato approach would seem to be focussed on a particular style of userspace driver, which works on non-virtualized devices. Specifically, it aims for PCI (or ISA) style devices, where the device driver needs to touch "real hardware".

USB is a good example of a different style driver, one where the device driver can't touch the hardware. In fact you can view USB drivers as the clients in a client/server framework, with the "server" being the device. The bus is really a special purpose network link, used to exchange packets between device and host. (And always initiated by the host, not the device/"server".) So userspace USB drivers work with virtualized devices ... they start with a formalized protocol to talk with the hardware, which doesn't involve register access, IRQs, or memory mapping except indirectly.

There are two approaches right now to userspace USB drivers.

The original "usbfs" (or "usdevfs"), which is in sore need of replacement. It's ioctl-heavy, and multiplexes I/O streams (up to 32 per device) into one file descriptor. And it defines its own async I/O primitives. So while a Java API exists, it's awkward.
"gadgetfs" runs inside USB devices. It's got hardly any ioctls, and uses one file descriptor per I/O stream. Current versions of gadgetfs (not yet in Linux 2.6) use standard AIO calls to support data streaming from userspace.

There are discussions underway to create "usbfs2", which should eventually replace "usbfs". It'll look much more like gadgetfs than "usbfs", with few ioctls and using the standard AIO framework.

So that's something else to keep in mind. This Gelato framework doesn't seem like it'd work well with USB, or with other device models that have already upleveled and virtualized their hardware.

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