Development: How to develop drivers for USB based devices
This article is meant to serve as an introduction to the task of developing a driver for a usb based dvb device. Currently, in terms of this subject, there are a number of scattered resources available that, when organized together, could form the basis of a howto suitable for the noice developer. Hence, it might be very worthwile documenting the process.
The very first thing you would want to do is to identify the components used in your device as well as determine the device's subsystem ID; refer to the section entitled "Gathering Information About Your Unidentified/Unsupported Device" for details.
The next logical step would be to try to obtain technical datasheets on the component ICs. Many chip manufacturers make this documentation directly readily available. In other cases, the datasheets are available, but it will take the likes of a google search to find (i.e. using the chip's part or model number can quickly track down multiple sources for such documentation ... Note: some "datasheet archive" websites ask you to log in or pay in order to obtain the information, however, be advised that the documentation will almost always be available elsewhere completely free of such restrictions). When no information whatsoever exists, or has been freely released by the chip vendor, if you are still serious about developing driver support for your device, you may wish to contact the vendor directly to see if they will agree to releasing such information to you (which most likely will come in the form of being under a NDA). In the very worst instances, particularly those cases of complex chips that also contain DRM (digital rights management) type engines for conditional access purposes, it is unlikely you, as an individual, will be able to obtain help or information from the vendor.
Familiarizing yourself with a USB driver
To start with, there are some great Linux USB tutorials on Linux Journal:
- start here: How to Write a Linux USB Device Driver
- then here: Writing a Simple USB Driver
- here: Hot Plug .... Note: hot plug etc have been replaced by udev
- and then here: Snooping the USB Data Stream
In addition, get the source code for the LinuxTV V4L-DVB driver set. You will find that USB based DVB drivers are contained within the ./v4l-dvb/linux/drivers/media/dvb/dvb-usb directory. Have a bit of a browse through them while you're reading through the first article listed above, and try to get a feel for how the driver is put together (note: there is also a procedure about this that is described in a thread found here). Sometimes you can get a good head start in your own development efforts by attempting to leverage parts of earlier released code -- that which may have been written specifically for the exact same chip as contained in your own device or via code for a near similar chip, such as say from a previous production generation. Simply, modifying existing code to suite your own endeavour can greatly expediate the process of driver development.
Some useful tools
- usbsnoop- a Windows USB sniffer utility
- parser.pl - a script for parsing the huge usbsniff log file
- USB Replay - allows one to replay parsed usbsnoop logfiles on a Linux system
- also see this M920x specific script for some ideas
- Usbmon2usbsnoop - a script that converts the output from usbmon to a format that is compatible with USB Replay