Webcams

I got a Creative Labs NX Pro webcam, but unfortunately there aren’t linux drivers for it yet. I did find a project which is developing a driver for zc030x-based webcams. They’ve managed to grab a still image from a webcam, so that’s hopeful. However, the driver only builds under linux 2.4 at the moment and I’m using 2.6.

First of all, I did a bit of reading about the kernel usb support.

• There’s a skeleton usb driver (in drivers/usb/usb-skeleton.c). Under Documentation/DocBook there’s an article about the USB system.
• drivers/usb/media/dabusb.c is quite a clean looking driver
• USB programmers guide
• Loads of great articles about porting drivers to the 2.6 kernel

I read through the existing code for the zc030x driver, and used Freemind to organise my notes about it.

Since then, I’ve crashed my computer more times that I can remember. Writing kernel code is very unforgiving. If you make a mistake, the whole computer will probably hang. I’ve ended up adopting a very different, even-more-paranoid-than-usual coding style, and have aliased insmod/rmmod to automatically run sync beforehand! But I’ve been making progress – the driver compiles, loads, creates the /dev file correctly, and responds to incoming data .. but I don’t think the webcam is being initialised properly yet. I need to do more snooping of the datastream under Windows to understand what needs to be sent when.

USB itself has been really nicely designed. The designers have obviously considered real use-cases and designed for these. For example, there are four different modes of data transfer, including isochronous (regular time interval, eg. for a webcam running at 15fps) and bulk transfer (eg. for a usb disk, which wants to use as much bandwidth as is available). When there are multiple devices plugged in to your PC, the USB system itself is responsible for arbitrating who is allowed to use how much bandwidth. The usb system knows that it’s better to drop a webcam frame entirely than to deliver it really late. On the other hand, it knows that it should stop files being copied to my usb storage stick if the webcam wants to send a frame. All of this happens behind the scenes, as far as a programmer is concerned. USB is not just an end-to-end pipe, like rs232 was. There’s a whole bandwidth/device management layer on top of that.

Oh, finally .. learning a new code always takes a bit of effort, but it can be made easier by using decent tools. Xref is a refactoring/navigation tool for C. If you’re working on a C program with emacs, and you’re not using this already then you must enjoy pain. Note that you don’t want to be browsing the kernel headers in /usr/include – you need to use the ones under /lib/modules/version/build. Here’s the options for xref to set this up.

[webcam]
  //  input files and directories (processed recursively)
  ~/Projects/zc0302/
  -D__KERNEL__
  -resetIncludeDirs
  -I /lib/modules/2.6.5-gentoo/build/include/linux
  -I /lib/modules/2.6.5-gentoo/build/include
  -I /lib/modules/2.6.5-gentoo/build/
  //  directory where tag files are stored
  -refs ~/Xrefs/webcam
  //  number of tag files
  -refnum=10