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89 lines
3.5 KiB
Markdown
89 lines
3.5 KiB
Markdown
# Tutorial 02 - Multicore Rust
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Now let's try something more complex, shall we? By complex I mean stopping the
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CPU cores just like in the first tutorial, but this time stop one of them from
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**Rust**!
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## Boot code
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In order to conveniently incorporate Rust code, we are restructuring our crate a
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bit.
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We reuse a lot of steps that are explained in great detail in [The
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Embedonomicon][nom], so please take your time and read up on it. Afterwards, you
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can compare to the files in this crate and see what we actually kept to get our
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Raspberry Pi 3 tutorial going. Here's a short summary of the new structure of
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the crate:
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- `raspi3_boot/`: The extern crate containing boot code as presented in the
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Embedonomicon.
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- In a small deviation to the Embedonomicon, `lib.rs` also includes
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`boot_cores.S` from the previous tutorial, still with the
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[global_asm!][gasm] macro.
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- Therefore, `boot_cores.S` has been moved into `raspi3_boot/src/`.
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- `src`: Source code of our actual Rust code, currently only containing
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`main.rs` executing an endless loop.
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[nom]: https://rust-embedded.github.io/embedonomicon/
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[gasm]: https://doc.rust-lang.org/unstable-book/language-features/global-asm.html
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### Changes to `boot_cores.S`
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In contrast to the previous tutorial, we are now [distinguishing the
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cores][dist]. To do so, we read the [mpidr_el1][mpdir] system register. If it is
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not zero, we enter the former infinite waiting loop, aka stopping the respective
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CPU core.
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If the result of the read from `mpidr_el1` is zero, which means we are executing
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on core0, we set up the stack for that core, and afterwards call the Rust
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`reset()` function of the boot code in `raspi3_boot/src/lib.rs`. In case the
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Rust code returns (which it never should), we also jump to the same infinite
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loop the other CPU cores are running.
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The Rust `reset()`, in turn, will then zero-out the `bss section` (the next
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section explains what that is) and finally call our `main()` function from
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`main.rs`.
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[dist]: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0024a/CFHCIDCH.html
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[mpdir]: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500g/BABHBJCI.html
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## Changes to `link.ld`
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Since we are using a high-level language now in the form of Rust, we also take
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precautions to have eventual space reserved in memory for the [bss
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segment][bss], which is needed in case zero-initialized static variables are
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allocated in the Rust code.
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[bss]: https://en.wikipedia.org/wiki/.bss
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Therefore, we added the `bss` segment to the linker script and export its
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properties via `__bss_start` and `__bss_size`, which will be picked up and
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zeroed out by the boot code in `raspi3_boot/src/lib.rs`.
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Additionally, there is a [data segment][data] now.
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[data]: https://en.wikipedia.org/wiki/Data_segment
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Finally, we need to take care that we still start the text segment with the
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assembly code and not the newly added Rust code. This is taken care of by
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placing the `.text.boot` section before all other new text sections
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`KEEP(*(.text.boot)) *(.text .text.* ...`.
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This way, the assembly stays at the `0x80_000` address, which is the entry point
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of the RPi3 CPU.
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## Changes to `Makefile`
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We've added one more target:
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- [clippy] is Rust's linter, and can give you useful advise to improve your
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code. Invoke with `make clippy`.
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[clippy]: https://github.com/rust-lang-nursery/rust-clippy
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From now on, we can use the same Makefile for every tutorial, regardless of the
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number of Rust sources, and we won't discuss it any further.
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## main.rs
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Finally, our first Rust code. Just an empty loop, but still! :-)
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