Thursday, August 27, 2009

Standing on the shoulders of giants - Fixing OpenOCD for the Beagleboard

Ever since fixing a little bug in the breakpoint code for the s3c2442 I'm subscribed to the OpenOCD mailinglist. One mail catched my eye, the mail was from a former Openmoko colleague Matt Hsu. He is working with most of the Taiwan Openmoko Team at the newly founded 0xlab and he was trying OpenOCD on the OMAP3 beagleboard.

Yesterday I decided to go to their lab to hack with Matt on the Cortex-A8 support in OpenOCD. After downloading the datasheets, we started to poke the code and tried to get a picture of how things are supposed to work. We have fixed various candidates for failures, like not waiting for the previous instruction to be finished when executing, not waiting for data to be there before reading.

Our symptoms were that after typing "halt" a lot of "invalid mode" messages got printed. After some tracing we ended at the execution of one instruction. The Cortex-A8 supports a mode were an ARM instruction can be written to a register and this instruction will be executed when being in debug mode. Now for a debugger the instruction is copying registers and other data to a special channel that can be read from the debugger. After looking further every register dumped liked this had the value 0x0...After this we were sure to have found the place were things go wrong. All we needed to find out why this instruction is not properly executed.

After studying the datasheet some more, going through the bits of DSCR and DCCR, and looking at the value of DSCR we have, it was clear that we just need to enable execution of instructions from the ITR (Instruction Transfer Register).

After this debugging session I still have little knowledge about ADI and the related things and this is why Free Software is so great, I can spend one afternoon, and can make a big change, and I can do it because I build on support from previous developers!

1 comment:

Methril said...

It's true that the FOSS is wonderfull.
I'm also interested in Cotex-A8 development, and maybe some time soon i'm going to spent some time on it, as i'm going to have a i.MX515 board to give support to OpenOCD.
Hope to see your developments soon.

Best Regards,