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41 lines
2.0 KiB
Markdown
41 lines
2.0 KiB
Markdown
6 years ago
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Tutorial 15 - Raspbootin64
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==========================
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Because changing SD card is boring and also to avoid potential SD card damage, we create a kernel8.img that will
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load the real kernel8.img over serial.
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This tutorial is a rewrite of the well known serial boot loader, [raspbootin](https://github.com/mrvn/raspbootin) in 64 bit.
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I only provide one part of the loader, the kernel receiver, which runs on the RPi. For the other
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part, the sender, which runs on your PC see the original [raspbootcom](https://github.com/mrvn/raspbootin/raspbootcom) utility.
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If you want to send kernels from a Windows machine, I suggest to take a look at John Cronin's rewrite,
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[raspbootin-server](https://github.com/jncronin/rpi-boot/blob/master/raspbootin-server.c) which can be compiled for the Win32 API.
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In order to load the new kernel to the same address, we have to move ourself out of the way. It's called chain
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loading: one code loads the next code to the same position in memory, therefore the latter thinks it was loaded
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by the firmware. To implement that we use a different linking address this time, and since GPU loads us to 0x80000
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regardless, we have to copy our code to that link address. That's important that we can only use relative addresses
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while doing so. When we're done, the memory at 0x80000 must be free to use. We also should minimize the size of the
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loader, since it will be regarded by the newlt loaded code anyway. By removing `uart_puts()` I've managed to shrink the
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size below 1024 bytes. I've checked that with:
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```sh
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$ aarch64-elf-readelf -s kernel8.elf | grep __bss_end
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21: 000000000007ffd0 0 NOTYPE GLOBAL DEFAULT 4 __bss_end
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```
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Start
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-----
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Added a loop to relocate our code to the address it should have been loaded to.
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Linker
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------
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We use a different linking address this time. Similarly to bss size calculation, we calculate our code's size to
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know how many bytes we have to copy.
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Main
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----
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We print 'RBIN64', receive the new kernel over serial, and jump to it.
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