Messages in this thread | | | Date | Fri, 21 Jan 2011 11:57:25 -0500 | From | Alan Ott <> | Subject | Re: [PATCH 0/1] HID: Report Descriptor in Sysfs |
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On 01/21/2011 11:14 AM, Jiri Kosina wrote: > On Fri, 21 Jan 2011, Alan Ott wrote: > > >> Well, what I really want is the Usage Page and Usage of the device. For some >> background, I maintain a library called hidapi[1] for accessing HID devices in >> a cross platform way. There are currently four backends, Linux-hidraw, >> Linux-libusb, Mac OS, and Windows. >> >> I've recently received requests for supporting composite HID devices. Since a >> composite device will show up as multiple devices with the same VID/PID, one >> needs a way to differentiate between its different interfaces. On Windows and >> Mac, the platform HID libraries support getting the Usage Page and Usage of >> each interface. On Linux/libusb I can request the HID report descriptor and >> parse it myself, but I have to claim the interface to do it, and to do that, I >> have to detach the kernel driver. Needless to say, detaching the kernel driver >> is not good, especially when the library is supposed to be just scanning for >> devices. >> >> For these reasons, it would be really convenient to get the report descriptor >> from sysfs. >> > In this case, you can still get the report descriptor from hidraw (which > is parallel to any other HID-bus based kernel driver). Woudl that suit > your needs? > >
Not exactly, because not all HID devices have a hidraw interface (because some are blacklisted). These devices are still accessible through libusb. Plus, getting it from hidraw involves having permissions to open the device[1]. Maybe that's less of a limitation, but since I can get everything else from sysfs (vid/pid, strings, device version), it seemed to make sense to me to have the hid report descriptor in the same place.
Again, if I'm completely wrong about how I should be going about this, just let me know.
Alan.
[1] Last year, I suggested adding some interfaces to hidraw to get things like the serial number, and it was suggested to me on linux-usb[2] that I use sysfs (through libudev) instead. I extrapolated that the proper paradigm was to use sysfs to get information about the device, and to use the device node itself to actually communicate with the device. I could be wrong again.
[2] I didn't realize at the time that I really should have asked on linux-input instead of linux-usb.
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