In Through the Out Door

In the previous episode of this reverse engineering effort, we finally found a good way to get hold of the real hardware register addresses, and we extracted the UART registers to begin with. What’s needed for a minimal booting kernel is first interrupts, meaning information on how to drive the SoC’s interrupt controller, and timers, also a facility supplied by the SoC. We really only need one timer: the system timer, which supplies Linux’s “ticks” – it’s what drives the scheduler.

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Emulate Your Way to Success

In the last episode of unlocking the Goke GK7101 SoC, we found ourselves faced with a big obstacle: a HAL layer in the form of I/O read/write calls that translated on-board peripherals’ register locations to their real addresses. The HAL’s underlying code is convoluted and much too hard to parse – it’s a large maze of twisty little if-then-elses, all alike. And since this SoC has tons of functionality, there are hundreds of register addresses to find.

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Hal of Horrors

In the previous post we found the GK7101 SoC’s UART base address and a few registers by decompiling their version of Linux’s decompressor: UART base address is 0xa0005000 The input/output register is at offset 0x04 Offset 0x14 holds flags: Bit 6 needs to be high before sending. To drain the input buffer, read from offset 0x04 until bit 2 goes low. This all looks suspiciously like a standard 16550 UART, but not quite: offset 0x04 corresponds to the 16550’s RHR/THR register (Read/Transmit Holding Register), except the 16550 has it on offset 0x00.

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Getting Started on a New Board

In part 1 of this series we found an interesting little board, and located the built-in UART pins. Let’s take a look at the output on boot: console init done U-Boot 2012.10 (Dec 07 2016 - 13:48:53) for GK7101 rb imx222 v1.00 (GOKE) HAL: 20151223 DRAM: 128 MiB Flash: 16 MiB 16 MiB NAND: SPINAND MID = 0xff, DID = 0xffff, Data = 0x1ffffff !spinand_board_init[1581]: No support this SPI nand! SF: Detected GD25Q128C with page size 256 B, sector size 64 KiB, total size 16 MiB In: serial Out: serial Err: serial Net: arm_freq(600MHz).

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This Little Camera

Some time ago I came across a strange product on Banggood. It was a small camera, with a base and separate camera head on top. The camera head supports pan and tilt, like a bobblehead. Digoo DG-M1Z The specs aren’t half bad: supports 1080p, has wifi and ethernet on board, and has a microSD card slot. These are intended for home security. The idea is you set these down somewhere and it streams live video over wifi.

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