Development of a User-Space Application for an HID Device, Using libhid
Snooping can be done with a number of utilities. This is where I learned about the discrepancies between what the Matrix documentation says and what actually happens:
[5037 ms] <<< URB 647 coming back <<< -- URB_FUNCTION_CONTROL_TRANSFER: PipeHandle = 8180c814 TransferFlags = 00000002 (DIRECTION_OUT) TransferBufferLength = 00000005 TransferBuffer = 92a137ed TransferBufferMDL = fe9876e8 UrbLink = 00000000 SetupPacket = 00000000: 21 09 00 02 00 00 05 00 [5038 ms] <<< URB 645 coming back <<< -- URB_FUNCTION_BULK_OR_INTERRUPT_TRANSFER: PipeHandle = fe9876a0 [endpoint 0x81] TransferFlags = 00000003 (DIRECTION_IN) TransferBufferLength = 00000005 TransferBuffer = fefeef08 TransferBufferMDL = 81a18f48 00000000: 00 20 00 00 1a UrbLink = 00000000 [5038 ms] >>> URB 648 going down >>> -- URB_FUNCTION_BULK_OR_INTERRUPT_TRANSFER: PipeHandle = fe9876a0 [endpoint 0x81] TransferFlags = 00000003 (DIRECTION_IN) TransferBufferLength = 00000005 TransferBuffer = fefeef08 TransferBufferMDL = 00000000 UrbLink = 00000000
From the snoop log, we see the control message sent to the device at the start, followed by a series of interrupt reads. According to the documentation, “The Host sends [a] poll to request information from Matrix at a periodic rate. Matrix answers to the poll and reports all the happening events.” So, my interpretation of this was to send periodic control write messages to the device and read the responses from the interrupt endpoint. Also according to the documentation, the format of the write message is five bytes in length, so with this information, I used the test_libhid.c file included with libhid to see what happens. I found that functions within libhid give error codes if they fail and that the /var/log/messages file, with the extra DEBUG information from the modified kernel file, reports useful errors.
Upon closer inspection of the snoop log, I saw that the control write was, in fact, eight bytes in length. See SetupPacket in snoop log output. The five bytes described in the documentation seemed to represent the first five bytes of the packet, and the last three bytes seemed to be padding. That is, changing these last three bytes doesn't seem to affect the operation. Subsequent error-free testing, with the packet set to eight bytes, confirmed that the documentation had been misleading.
In terms of where to start with this project, I found the mailing list for libhid to be helpful. The libusb mailing list also provided guidelines. The Linux usbutils are quite useful in determining what interfaces are available on the device and the meaning of the descriptors.
The libhid source code, still in constant development, also is a source of help. Because the code constantly is being developed, it is a good idea to keep an eye on the Subversion repository for changes, including documentation changes such as helpful comments in the code.
Special thanks to Charles Lepple and Arnaud Quette, the original authors of libhid, and also to Martin F. Krafft, who later joined and led the rewrite. They all provided me with a lot of help, and without them I certainly wouldn't have gotten my little light to go green.
Also, thanks to my supervisor, Dr Paul O'Leary, at WIT, for his encouragement and analytical skills. It always is good to have an experienced pair of eyes to guide me in the right direction.
libhid uses the HIDParser framework made available by MGE.
Resources for this article: /article/8275.
Eoin Verling (firstname.lastname@example.org) qualified in 1998 and has been a sysadmin since. He currently is undertaking a research Master's in parallel computing at Waterford Institute of Technology, Ireland. There's nothing he likes better than a bit of ceol agus craic!
|Making Linux and Android Get Along (It's Not as Hard as It Sounds)||May 16, 2013|
|Drupal Is a Framework: Why Everyone Needs to Understand This||May 15, 2013|
|Home, My Backup Data Center||May 13, 2013|
|Non-Linux FOSS: Seashore||May 10, 2013|
|Trying to Tame the Tablet||May 08, 2013|
|Dart: a New Web Programming Experience||May 07, 2013|
- RSS Feeds
- Making Linux and Android Get Along (It's Not as Hard as It Sounds)
- New Products
- Drupal Is a Framework: Why Everyone Needs to Understand This
- A Topic for Discussion - Open Source Feature-Richness?
- Home, My Backup Data Center
- Validate an E-Mail Address with PHP, the Right Way
- New Products
- Developer Poll
- Tech Tip: Really Simple HTTP Server with Python
- git-annex assistant
11 min 21 sec ago
- direct cable connection
33 min 51 sec ago
- Agreed on AirDroid. With my
44 min 7 sec ago
- I just learned this
48 min 17 sec ago
1 hour 18 min ago
- not living upto the mobile revolution
4 hours 9 min ago
- Deceptive Advertising and
4 hours 45 min ago
- Let\'s declare that you have
4 hours 46 min ago
- Alterations in Contest Due
4 hours 47 min ago
- At a numbers mindset, your
4 hours 48 min ago
Enter to Win an Adafruit Prototyping Pi Plate Kit for Raspberry Pi
It's Raspberry Pi month at Linux Journal. Each week in May, Adafruit will be giving away a Pi-related prize to a lucky, randomly drawn LJ reader. Winners will be announced weekly.
Fill out the fields below to enter to win this week's prize-- a Prototyping Pi Plate Kit for Raspberry Pi.
Congratulations to our winners so far:
- 5-8-13, Pi Starter Pack: Jack Davis
- 5-15-13, Pi Model B 512MB RAM: Patrick Dunn
- Next winner announced on 5-21-13!
Free Webinar: Linux Backup and Recovery
Most companies incorporate backup procedures for critical data, which can be restored quickly if a loss occurs. However, fewer companies are prepared for catastrophic system failures, in which they lose all data, the entire operating system, applications, settings, patches and more, reducing their system(s) to “bare metal.” After all, before data can be restored to a system, there must be a system to restore it to.
In this one hour webinar, learn how to enhance your existing backup strategies for better disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible bare-metal recovery solution for UNIX and Linux systems.