On Mon, 20 Apr 2009, Sarah Sharp wrote: > Say you have a USB device that wants to default to a new USB class that > may or may not have a driver for it yet. The USB device wants the newer > behavior whenever possible, but it can also use an older class. > > Would the device work on an older system if it used alt setting 0 for > the new class, and alt setting 1 for the old class? Will the USB core > switch the USB device over to the second alt setting and load the > old class driver? The device should use altsetting 0 for the old class and altsetting 1 for the new class. That's because the kernel always installs altsetting 0 when a device is first configured, so a legacy kernel would still be able to work with the device. The new class driver should be written so that it can bind to an interface belonging to the old class and switch it to altsetting 1. > Or is the device forced to define the old and new class driver endpoints > in the same interface? Then you'd face the problem of which class to advertise in the interface descriptor. Another alternative is to use multiple configurations and have a udev rule to install the appropriate config. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
