At this point in my Chumby kernel upgrade project (parts 1, 2, 3, and 4 here), I had made a ton of progress but there wasn’t really much to show for it because I didn’t have the LCD working. Even though I had put a ton of work into the project, the display was still black. I knew it was time to get it working.
I started out with U-Boot. As a very basic overview of the LCD controller in the PXA168, basically you just set aside some of your RAM for a framebuffer, copy image data into it, tell the controller the format and address of the framebuffer, set up the clocking and timing, and turn it on. Then it just handles everything in the background for you.
The steps I listed above are overly simplified — there is more stuff going on with the PXA168’s display controller. But it’s enough to get a splash screen working in U-Boot. I booted into the old kernel and dumped the LCD registers using devmem. Here’s an example of this process. The LCD_SPU_DMA_CTRL0 register contains a bunch of format configuration bits for the framebuffer, such as which bits are red/green/blue. It’s at offset 0x190 in the LCD controller, and the LCD controller is located at an offset of 0xD420B000, so I could dump the 32-bit register value with this command:
devmem 0xD420B190 32
This resulted in a printout of the value of the register:
0x8441111
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