An LCD driver for the Weebox

Yesterday I mostly spent learning about Linux kernel module development. I decided to bite the bullet and modify lcdmod to support the TS7200-series LCD panel. It turned out to be fairly simple in the end.

I firstly created a new configuration define ‘TS7200_CONFIG’ which switched between the default parallel port behaviour of lcdmod and the memory-mapped hardware approach the Weebox needs. Then I refactored the LCD sample code from Embedded ARM to have a rational API, replacing the ARM-specific code with kernel equivalents (mostly ndelay() calls). With appropriate #ifdefs in the lcd.c to call the TS7200 support routines, and a patched Makefile, it built fine. A bit of debugging ensued (and a fair number of reboots!) until I had everything working — the stupidest bug being me getting the parameters to writel() the wrong way around at first, so instead of writing 0x38 to address 0x8084004c, I was writing 0x8084004c to address 0x38 — boom!

In modifying the driver I discovered a crash bug in the implementation which probably doesn’t manifest itself on the PC (well, I guess not as seemingly nobody’s noticed it before!) Upon reprogramming the user-defined graphics, the virtual cursor was left at ‘-1, -1’ (to invalidate the current cursor position). This is all sensible and good…except the next character write reads and updates a virtual display array at array[-1][-1] — CRASH. The UDG reprogram code really should have moved the hardware cursor position — then in the character write code the driver spots the actual and virtual cursors are out of synch and resets the hardware cursor position. I’ve submitted a patch to the author, though with no activity on the driver since August 2004 there’s a chance the author may have moved on.

While I was at it I put support for devfs in so I didn’t need to make a /dev/lcd node myself, I let the OS do it. To be complete I need to put support for udev in too, but as my kernel’s 2.4.26 that’s not something I can test just yet. Once I’m happy I’ve got all the bugs out, and pending a reply from the author, I’ll either submit my patch to him or make a new lcdmod distribution with my changes in. For now I’ve made a snapshot of the current code available.

With my nice new shiny LCD module installed and working, I made the modifications to my prototype Python Weebox code to talk to /dev/lcd. During this process I had a thought: Currently on the Windows development version I’m using TCP to talk to a Tkinter window emulating an LCD display. Until now this has meant implementing an LCD emulator, which I’ve only cursorily attempted. However, now I have a /dev/lcd device on the Weebox, I realised I could use netcat to make the Weebox’s LCD display accesible from the network! By running ‘nc -l -p 4000 > /dev/lcd’ on the Weebox, and modifying the display driver on the Windows machine to send LCD commands to port 4000 of the Weebox I could get the Windows machine to easily control the actual LCD device! Much easier; and means the difference between the Weebox version and the Windows version is kept to the bare minimum.

This morning I’ve been tinkering with input. I’ve drawn out a diagram of the Xbox DVD remote control that I’m planning on using and I’m sketching out ideas for menus and similar. I’ve put in a very quick ‘Input’ layer which on Windows maps keyboard presses to remote control buttons, and I’ve got the simple stuff (play, pause, stop, next, previous, seek) wired in, so I can actually control the Weebox now. In doing so I spotted a few simple optimisations in the Python code, and now ‘only’ takes 5-10% CPU on the Weebox.

I’m getting dangerously close to a working prototype — all that’s missing now is the infra-red decoder logic, which I’m hoping will turn up on Monday. However, I expect development on the Weebox to slow down a bit now as I’m off to a wedding this weekend and I’m back to work next week so I’ll only be Wee- developing in the evenings. I can’t wait though, I haven’t had so much fun with computers in a very long time!

Filed under: WeeBox Project
Posted at 11:48:46 BST on 8th July 2006.

About Matt Godbolt

Matt Godbolt is a C++ developer working in Chicago in the finance industry.