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rust-raspberrypi-OS-tutorials/07_uart_chainloader
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Andre Richter c3632eeb3c
Refactor tutorial 07 		4 years ago
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README.md

Tutorial 07 - UART Chainloader

tl;dr

Running from an SD card was a nice experience, but it would be extremely tedious to do it for every new binary. Let's write a chainloader using position independent code. This will be the last binary you need to put on the SD card. Each following tutorial will provide a chainboot target in the Makefile that lets you conveniently load the kernel over UART.

Install and test it

Our chainloader is called MiniLoad and is inspired by raspbootin.

You can try it with this tutorial already:

  1. Depending on your target hardware:make or BSP=rpi4 make.
  2. Copy kernel8.img to the SD card.
  3. Execute make chainboot or BSP=rpi4 make chainboot.
  4. Now plug in the USB Serial.
  5. Observe the loader fetching a kernel over UART:
$ make chainboot
[...]
Minipush 1.0

[MP] ⏳ Waiting for /dev/ttyUSB0
[MP] ✅ Connected
 __  __ _      _ _                 _
|  \/  (_)_ _ (_) |   ___  __ _ __| |
| |\/| | | ' \| | |__/ _ \/ _` / _` |
|_|  |_|_|_||_|_|____\___/\__,_\__,_|

           Raspberry Pi 3

[ML] Requesting binary
[MP] ⏩ Pushing 7 KiB ==========================================🦀 100% 0 KiB/s Time: 00:00:00
[ML] Loaded! Executing the payload now

[0] Booting on: Raspberry Pi 3
[1] Drivers loaded:
      1. BCM GPIO
      2. BCM PL011 UART
[2] Chars written: 93
[3] Echoing input now

In this tutorial, a version of the kernel from the previous tutorial is loaded for demo purposes. In subsequent tuts, it will be the working directory's kernel.

Test it

The Makefile in this tutorial has an additional target, qemuasm, that lets you nicely observe the jump from the loaded address (0x80_XXX) to the relocated code at (0x3EFF_0XXX):

$ make qemuasm
[...]
IN:
0x00080990:  d0000008  adrp     x8, #0x82000
0x00080994:  52800020  movz     w0, #0x1
0x00080998:  f9416908  ldr      x8, [x8, #0x2d0]
0x0008099c:  d63f0100  blr      x8

----------------
IN:
0x3eff0b10:  d0000008  adrp     x8, #0x3eff2000
0x3eff0b14:  d0000009  adrp     x9, #0x3eff2000
0x3eff0b18:  f941ad08  ldr      x8, [x8, #0x358]
0x3eff0b1c:  f941b129  ldr      x9, [x9, #0x360]
0x3eff0b20:  eb08013f  cmp      x9, x8
0x3eff0b24:  540000c2  b.hs     #0x3eff0b3c
[...]

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