Development of a User-Space Application for an HID Device, Using libhid
The Matrix is a USB bill validator, sometimes known as a note reader or bill acceptor, made by Validation Technologies International. The bundled software was developed for Microsoft Windows, but fortunately the device comes with low-level technical documentation that defines device-specific aspects, such as flow control, status bytes and local status LEDs.
The device is a Human Interface Device (HID), as identified by an enumeration process upon connection. The Windows device manager reports the device as such, as does usbfs on Linux. This article is specific to this particular HID device, so including all of its code probably is unnecessary, but it should provide help for developing for other HID-class devices.
After some initial research, I decided to develop user-space code using an in-development library called libhid, which provides a cross-platform way to access and interact with USB HID devices. libhid is implemented on top of libusb, so it does not depend directly on the kernel's USB support.
Another option for driving the Matrix is to use libusb directly, but doing so would be re-inventing the libhid wheel. A third option is to implement the Matrix as a kernel module, but it would incur the large overhead of learning kernel particulars. This option also would render the code platform-specific.
USB devices are categorized into device classes. A modem is in the communications class, and a speaker falls into the audio class. The HID class mainly consists of devices that people use to control computers. Examples of HID devices are mice, joysticks and force-feedback game controllers. Also included in the HID class are devices that may not require human interaction but do provide data in a similar format to HID-class devices, such as bar-code readers and, in my case, the Matrix note reader.
Information about a USB device is stored in segments of its ROM called descriptors. A diagram of the descriptor structure is provided in Figure 1, where an overall view of the hierarchy can be seen. When a USB device is attached to a USB bus, an enumeration process takes place that equates to the descriptors on the device being read into memory. Information about an HID-class device is contained in its HID report descriptors.
I plugged the device in to the Linux box in order to read the descriptors and monitor the device, the machine and the communications. I did this to try to get as much information as possible so I could have a better understanding of how to write code for the device.
A key component of these report descriptors is the usage information, which is defined in the USB HID Usage Tables (see the on-line Resources). Usage values describe three basic types of information about the device:
Controls—information about the state of the device such as on/off or enable/disable.
Data—all other information that passes between the device and the host.
Collections—groups of related controls and data.
Taken together, the usage page and usage number define a unique constant that describes a particular type of device or part of that device. For example, on the Generic Desktop usage page (page number 0x01), usage number 0x05 is a game pad, and usage number 0x39 is a hat switch.
Because my device is unique—it isn't a mouse, joystick or something commonly found in the examples of HID-class devices—the usage page is set to 65,440, which is a vendor-defined value. In comparing outputs of lsusb for other HID-class devices, they all had a defined usage page, such as Generic Desktop Controls or Game Controls. Because libhid still is in development, few previous examples of code are available to browse for reference. My work was much like an exploratory investigation.
On Linux, with a standard Debian 2.6.9 kernel and usbutils, I was able to see that Linux recognises the device as a USB HID device, bInterfaceClass = HID, and loads the hiddev kernel module. This module, or piece of kernel code, is a generic driver for HID devices. It is not specific to our needs—it mainly is used for mice, joysticks and the like—so it needs to be detached from the device or disabled (see the Communicating with the Device section).
The device, like all USB devices, is enumerated upon connection to the USB bus. So looking at the output of lsusb -vvv, run as root, for more information is helpful in determining what the device capabilities are. lsusb parses the usbfs filesystem into a more readable format:
[sample lsusb -vvv] Bus 001 Device 004: ID 0ce5:0003 Device Descriptor: ... idVendor 0x0ce5 idProduct 0x0003 ... Configuration Descriptor: ... Interface Descriptor: ... bNumEndpoints 1 bInterfaceClass 3 Human Interface Devices bInterfaceSubClass 0 No Subclass bInterfaceProtocol 0 None ... HID Device Descriptor: ... Report Descriptor: (length is 32) Item(Global):Usage Page,data=[0xa0 0xff]65440 (null) Item(Local ):Usage, data= [ 0x01 ] 1 (null) Item(Main ):Collection, data= [ 0x01 ] 1 Application Item(Local ):Usage, data= [ 0x03 ] 3 (null) Item(Global):Logical Minimum,data=[ 0x00 ] 0 Item(Global):Logical Maximum,data=[ 0xff ]255 Item(Global): Report Size, data= [ 0x08 ] 8 Item(Global): Report Count, data= [ 0x05 ] 5 Item(Main ): Input, data= [ 0x02 ] 2 Data Variable Absolute No_Wrap Linear Preferred_State No_Null_Position Non_Volatile Bitfield Item(Local ): Usage, data= [ 0x05 ] 5 (null) Item(Global):Logical Minimum,data=[ 0x00 ]0 Item(Global):Logical Maximum,data=[ 0xff ]255 Item(Global): Report Size, data= [ 0x08 ] 8 Item(Global): Report Count, data= [ 0x05 ] 5 Item(Main ): Output, data= [ 0x02 ] 2 Data Variable Absolute No_Wrap Linear Preferred_State No_Null_Position Non_Volatile Bitfield Item(Main ): End Collection, data=none
The above output—some of the information has been omitted—follows the hierarchy depicted in Figure 1. Some values of note are:
idVendor and idProduct—unique identifiers for all USB devices, used for identifying and accessing the device in code.
bNumEndpoints—lists the number of endpoints available in a device. This value actually means the number of endpoints in addition to the default endpoint, endpoint 0, available in every USB device.
bInterfaceClass—the value that determines that a device is an HID-class device.
bInterfaceSubClass—the subclass of a device, in this case, HID. For example, the boot interface subclass of the device must be bootable or available to the BIOS, such as a mouse or keyboard.
bInterfaceProtocol—the protocol used. Possible values are 0 for none, 1 for keyboard or 2 for mouse; additional information is available in the HID spec.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
Free to Linux Journal readers.Register Now!
- Google's SwiftShader Released
- SUSE LLC's SUSE Manager
- My +1 Sword of Productivity
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Managing Linux Using Puppet
- Non-Linux FOSS: Caffeine!
- Interview with Patrick Volkerding
- SuperTuxKart 0.9.2 Released
- Parsing an RSS News Feed with a Bash Script
- Doing for User Space What We Did for Kernel Space
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide