[libhid-discuss] libhid use case question

[Charles Lepple]

On Apr 3, 2007, at 3:42 AM, Stefan Kost wrote:

> I currently work on a wrapper lib around hid-devices (/dev/input/ 
> eventX), lirc-devices, midi devices, ... - stuff that has buttonas  
> and controls basically. I want to use it to bind ardware  
> controllers to my application ui. I discover devices via HA right now.

not sure what "HA" is, with respect to hotplug or input events...

> For the hid-devices I started to work on the implementation when I  
> discovered libhid. Unfortunately I could not really see what it  
> offers. I need to query the device capabillities (how many controls  
> and which type they are) as well as open and read from them. Is  
> that waht the lib offers?

You can get to this information through the MGE hidparser component  
of libhid. However, you mentioned /dev/input/event*, and libhid  
operates one level lower than that. When you open a device in libhid,  
it eventually needs to be disconnected from the Linux kernel usbhid  
driver so that we can read the HID report descriptor (which is the  
collection of data that describes what buttons and features a HID- 
class device has).

libhid was originally written for HID UPSes that do not have /dev/ 
input/event* nodes, and at the time it was started, the /dev/hiddev*  
API was not complete enough to expose all of the different HID  
collections. It also requires you to poll the device from userspace,  
which is probably not as efficient as letting the kernel manage  
polling as it does when the usbhid driver is attached to your devices.

-- 
Charles Lepple