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Sheer luck, or lack of?

Okay, so today I spent the whole day following a bug for what I’m handling for my job, and when evening came, I was tired and in need of something that relaxed me.

Missing still the information to actually start taking seriously the idea of building model ships, and missing a jigsaw (I looked for some the other day at the supermarket, but I can’t find a subject that I like, most of them are paintings of the Renaisance, but I’m not really into that kind of art, I never liked studying it when I was at school, but I admit that graphic arts are not really something I’m into in general, but if it had to be a painting, I would have preferred finding something from Magritte.. I still remember seeing one of his works at the Guggenheim Musem here in Venice some time ago and being quite impressed; surrealism can be a kind of graphic art that I make myself like, it’s actually interesting), my best choice to get a relaxing night was to start looking for the USB serial converter driver.

I installed the only Windows I still have a license of (Windows 98) on a virtual machine, and set up usbmon in the kernel so that I could use it (it would be nice if the usbmon module depended on the debugfs support, as the latter is needed to use the former), and then started fighting to get the device up. After some usual Win98 driver setup (install driver, insert peripheral, remove driver, remove peripheral, re-insert peripheral, install driver, reboot, curse, remove peripheral, remove driver and so on), I’ve started having a working COM3 port with the adapter. Too bad that anything I tried to type was sent two or more times over the wires, but I knew VMware was far from perfect with USB.

Anyway, I started looking at the output of the usbmon tracing, and I was able to isolate some of the control messages used to set the parameters up (that are basically the only thing I need, as the actual I/O seems to happen just like PL2303 code (or usbserial) handles it); I’ve got the commit sequence and the okay string that I should expect from the driver, but I still have to make sense of all of them.

Unfortunately the USBmon (note the case) utility is written in Java. I say unfortunately simply because I still can’t get Sun’s JRE to work with XCB-enabled libX11, so I can’t run the utility on my box.

After some tries, which finally resulted in an hard lockup of Enterprise, which is now shut down waiting for me resuming it tomorrow, I finally decided that the next days I’ll start by writing some tools that might help me during the development of the driver, and maybe someone else in the future.

The first tool will be a serial signal generator: basically it would be a simple app that repeated the same string over and over and over on a serial port, using the configured baud rate, bits, parity and stop bits. This is useful because I can connect the adapter in look with my real serial port, leave the generator open on the serial port, and try sending different commands to the adapter till I get the signal as I need it, then I can move to try another parameter, for instance changing the stop bits, and try the commands again until I get the same parameters as configured in the generator. Right now I’ve used for this a simple C source file that sent “Flameeyes” string with the hardcoded parameters, I’d like to be able to set the parameters at runtime so you don’t have to stop and start it every time, but I’m not yet sure of which interface I could use for that.

The second tool is the one that most likely will be helpful if I can get it as good as I hope, and it would be a Ruby-based ncurses frontend for reading the data out of usbmon. First it would show the data in a more structured fashion, with the list of read packets that can be actually inspected without having to copy them around, with a ringbuffer for the read packets, and a different buffer for particular packets you want to inspect while continuing recording, and if I can with some kind of filters so that I can pinpoint the request for the single device, and then start discarding stuff like the I/O operations I don’t care about while trying to set the control stuff.

It’s far from being an easy task (especially since I never used ncurses), and I know I probably won’t be able to complete it as I want to, but at least I can try to provide a start if someone else will need to read that usbmon data in a more comprehensible shape. Right now SnoopyPro on Windows has a simpler view of the data, although it can really improve, especially to get a decent output to print and study.

But now, to actually make some sense out of the title I used for this blog, I have to tell you what happened while I was debugging. When I was ready to start messing with the parameters of the device, I wanted to be able to get no serial connection, and just start with the pristine adapter status, so I took one of the three remaining adapters (I ordered four from the same place) and connected it to Enterprise, launched picocom (that I started using as a test – you can find it in my overlay if you want it) on the ttyUSB0 device and.. found it not being there. An lsusb run later I seen that the adapter I just took was actually a PL2303-based device, and after looking at the other two adapters, I discovered that out of the four adapters I bought, only one has actually the chip on itself, and that is the one I chosen when I wanted to try them out, luck or lack of it?

But anyway I won’t stop working on the driver, I’ll probably take it more easy though, as three serial ports are enough for now, I ordered one more “just in case”, and that proven a good idea. I have my job to complete, so I’ll dedicate to the driver only my free time, compatibly with the other projects I’m working on. Not only I can’t really try to get the adapter replaced (I ordered it from an Hong Kong reseller on e-bay, the price to sending it back would probably be higher than I paid for it, besides I can’t really complain, the adapters were cheap and they never told me which chip they used), but I have no intention to: writing a driver for this adapter is an interesting challenge, it can be useful to other people, might be useful to me again in the future (I might need more serial adapters, and I’ll probably just order them as I ordered these ones, so I might find more devices), and it’s a nice way to start kernel hacking, as it’s a simple enough device.

Comments 3
  1. Nice, I didn’t know of wireshark, thanks!Now I just need a libpcap snapshot 😛

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