linux-kernel - Re: [PATCH 0/1] HID: Report Descriptor in Sysfs

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Date:	Fri, 21 Jan 2011 11:57:25 -0500
From:	Alan Ott <alan@...nal11.us>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] HID: Report Descriptor in Sysfs

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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