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# Tutorial 07 - UART Chainloader
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## tl;dr
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Running from an SD card was a nice experience, but it would be extremely tedious
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to do it for every new binary. Let's write a [chainloader] using [position
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independent code]. This will be the last binary you need to put on the SD card
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for quite some time. Each following tutorial will provide a `chainboot` target in
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the `Makefile` that lets you conveniently load the kernel over `UART`.
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Our chainloader is called `MiniLoad` and is inspired by [raspbootin].
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[chainloader]: https://en.wikipedia.org/wiki/Chain_loading
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[position independent code]: https://en.wikipedia.org/wiki/Position-independent_code
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[raspbootin]: https://github.com/mrvn/raspbootin
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You can try it with this tutorial already:
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1. Depending on your target hardware:`make` or `BSP=rpi4 make`.
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2. Copy `kernel8.img` to the SD card.
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3. Execute `make chainboot` or `BSP=rpi4 make chainboot`.
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4. Now plug in the USB Serial.
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5. Observe the loader fetching a kernel over `UART`:
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```console
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» make chainboot
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[...]
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Minipush 1.0
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[MP] ⏳ Waiting for /dev/ttyUSB0
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[MP] ✅ Connected
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__ __ _ _ _ _
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| \/ (_)_ _ (_) | ___ __ _ __| |
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| |\/| | | ' \| | |__/ _ \/ _` / _` |
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|_| |_|_|_||_|_|____\___/\__,_\__,_|
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Raspberry Pi 3
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[ML] Requesting binary
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[MP] ⏩ Pushing 7 KiB ==========================================🦀 100% 0 KiB/s Time: 00:00:00
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[ML] Loaded! Executing the payload now
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[0] Booting on: Raspberry Pi 3
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[1] Drivers loaded:
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1. GPIO
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2. PL011Uart
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[2] Chars written: 84
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[3] Echoing input now
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```
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In this tutorial, a version of the kernel from the previous tutorial is loaded
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for demo purposes. In subsequent tuts, it will be the working directory's
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kernel.
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## Test it
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The `Makefile` in this tutorial has an additional target, `qemuasm`, that lets
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you nicely observe the jump from the loaded address (`0x80_XXX`) to the
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relocated code at (`0x3EFF_0XXX`):
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```console
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make qemuasm
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[...]
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IN:
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0x000809fc: d0000008 adrp x8, #0x82000
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0x00080a00: 52800020 movz w0, #0x1
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0x00080a04: f9408908 ldr x8, [x8, #0x110]
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0x00080a08: d63f0100 blr x8
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----------------
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IN:
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0x3eff0528: d0000008 adrp x8, #0x3eff2000
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0x3eff052c: d0000009 adrp x9, #0x3eff2000
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0x3eff0530: f9411508 ldr x8, [x8, #0x228]
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0x3eff0534: f9411929 ldr x9, [x9, #0x230]
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0x3eff0538: eb08013f cmp x9, x8
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0x3eff053c: 540000c2 b.hs #0x3eff0554
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[...]
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```
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## Diff to previous
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```diff
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Binary files 06_drivers_gpio_uart/demo_payload_rpi3.img and 07_uart_chainloader/demo_payload_rpi3.img differ
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Binary files 06_drivers_gpio_uart/demo_payload_rpi4.img and 07_uart_chainloader/demo_payload_rpi4.img differ
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diff -uNr 06_drivers_gpio_uart/Makefile 07_uart_chainloader/Makefile
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--- 06_drivers_gpio_uart/Makefile
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+++ 07_uart_chainloader/Makefile
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@@ -7,6 +7,11 @@
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BSP = rpi3
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endif
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+# Default to /dev/ttyUSB0
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+ifndef DEV_SERIAL
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+ DEV_SERIAL = /dev/ttyUSB0
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+endif
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+
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# BSP-specific arguments
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ifeq ($(BSP),rpi3)
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TARGET = aarch64-unknown-none-softfloat
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@@ -15,7 +20,8 @@
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QEMU_MACHINE_TYPE = raspi3
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QEMU_RELEASE_ARGS = -serial stdio -display none
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LINKER_FILE = src/bsp/rpi/link.ld
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- RUSTC_MISC_ARGS = -C target-cpu=cortex-a53
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+ RUSTC_MISC_ARGS = -C target-cpu=cortex-a53 -C relocation-model=pic
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+ CHAINBOOT_DEMO_PAYLOAD = demo_payload_rpi3.img
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else ifeq ($(BSP),rpi4)
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TARGET = aarch64-unknown-none-softfloat
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OUTPUT = kernel8.img
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@@ -23,7 +29,8 @@
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# QEMU_MACHINE_TYPE =
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# QEMU_RELEASE_ARGS = -serial stdio -display none
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LINKER_FILE = src/bsp/rpi/link.ld
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- RUSTC_MISC_ARGS = -C target-cpu=cortex-a72
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+ RUSTC_MISC_ARGS = -C target-cpu=cortex-a72 -C relocation-model=pic
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+ CHAINBOOT_DEMO_PAYLOAD = demo_payload_rpi4.img
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endif
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RUSTFLAGS = -C link-arg=-T$(LINKER_FILE) $(RUSTC_MISC_ARGS)
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@@ -46,9 +53,12 @@
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DOCKER_IMAGE = rustembedded/osdev-utils
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DOCKER_CMD = docker run -it --rm
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DOCKER_ARG_DIR_TUT = -v $(shell pwd):/work -w /work
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+DOCKER_ARG_DIR_UTILS = -v $(shell pwd)/../utils:/utils
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+DOCKER_ARG_TTY = --privileged -v /dev:/dev
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DOCKER_EXEC_QEMU = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
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+DOCKER_EXEC_MINIPUSH = ruby /utils/minipush.rb
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-.PHONY: all doc qemu clippy clean readelf objdump nm
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+.PHONY: all doc qemu qemuasm chainboot clippy clean readelf objdump nm
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all: clean $(OUTPUT)
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@@ -66,13 +76,26 @@
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ifeq ($(QEMU_MACHINE_TYPE),)
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qemu:
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@echo "This board is not yet supported for QEMU."
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+
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+qemuasm:
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+ @echo "This board is not yet supported for QEMU."
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else
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qemu: all
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@$(DOCKER_CMD) $(DOCKER_ARG_DIR_TUT) $(DOCKER_IMAGE) \
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$(DOCKER_EXEC_QEMU) $(QEMU_RELEASE_ARGS) \
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-kernel $(OUTPUT)
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+
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+qemuasm: all
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+ @$(DOCKER_CMD) $(DOCKER_ARG_DIR_TUT) $(DOCKER_IMAGE) \
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+ $(DOCKER_EXEC_QEMU) $(QEMU_RELEASE_ARGS) \
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+ -kernel $(OUTPUT) -d in_asm
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endif
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+chainboot:
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+ @$(DOCKER_CMD) $(DOCKER_ARG_DIR_TUT) $(DOCKER_ARG_DIR_UTILS) $(DOCKER_ARG_TTY) \
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+ $(DOCKER_IMAGE) $(DOCKER_EXEC_MINIPUSH) $(DEV_SERIAL) \
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+ $(CHAINBOOT_DEMO_PAYLOAD)
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+
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clippy:
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RUSTFLAGS="$(RUSTFLAGS_PEDANTIC)" cargo xclippy --target=$(TARGET) --features bsp_$(BSP)
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diff -uNr 06_drivers_gpio_uart/src/arch/aarch64.rs 07_uart_chainloader/src/arch/aarch64.rs
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--- 06_drivers_gpio_uart/src/arch/aarch64.rs
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+++ 07_uart_chainloader/src/arch/aarch64.rs
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@@ -22,7 +22,7 @@
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if bsp::BOOT_CORE_ID == MPIDR_EL1.get() & CORE_MASK {
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SP.set(bsp::BOOT_CORE_STACK_START);
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- crate::runtime_init::runtime_init()
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+ crate::relocate::relocate_self::<u64>()
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} else {
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// If not core0, infinitely wait for events.
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wait_forever()
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diff -uNr 06_drivers_gpio_uart/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs 07_uart_chainloader/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs
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--- 06_drivers_gpio_uart/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs
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+++ 07_uart_chainloader/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs
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@@ -272,6 +272,16 @@
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let mut r = &self.inner;
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r.lock(|inner| fmt::Write::write_fmt(inner, args))
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}
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+
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+ fn flush(&self) {
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+ let mut r = &self.inner;
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+ // Spin until TX FIFO empty is set.
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+ r.lock(|inner| {
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+ while !inner.FR.matches_all(FR::TXFE::SET) {
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+ arch::nop();
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+ }
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+ });
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+ }
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}
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impl interface::console::Read for PL011Uart {
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@@ -283,18 +293,21 @@
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arch::nop();
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}
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- // Read one character.
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- let mut ret = inner.DR.get() as u8 as char;
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-
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- // Convert carrige return to newline.
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- if ret == '\r' {
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- ret = '\n'
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- }
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-
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// Update statistics.
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inner.chars_read += 1;
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- ret
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+ // Read one character.
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+ inner.DR.get() as u8 as char
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+ })
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+ }
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+
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+ fn clear(&self) {
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+ let mut r = &self.inner;
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+ r.lock(|inner| {
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+ // Read from the RX FIFO until it is indicating empty.
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+ while !inner.FR.matches_all(FR::RXFE::SET) {
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+ inner.DR.get();
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+ }
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})
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}
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}
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diff -uNr 06_drivers_gpio_uart/src/bsp/rpi/link.ld 07_uart_chainloader/src/bsp/rpi/link.ld
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--- 06_drivers_gpio_uart/src/bsp/rpi/link.ld
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+++ 07_uart_chainloader/src/bsp/rpi/link.ld
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@@ -5,9 +5,10 @@
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SECTIONS
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{
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- /* Set current address to the value from which the RPi starts execution */
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- . = 0x80000;
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+ /* Set the link address to the top-most 40 KiB of DRAM (assuming 1GiB) */
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+ . = 0x3F000000 - 0x10000;
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+ __binary_start = .;
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.text :
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{
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*(.text._start) *(.text*)
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@@ -32,5 +33,14 @@
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__bss_end = .;
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}
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+ .got :
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+ {
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+ *(.got*)
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+ }
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+
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+ /* Fill up to 8 byte, b/c relocating the binary is done in u64 chunks */
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+ . = ALIGN(8);
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+ __binary_end = .;
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+
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/DISCARD/ : { *(.comment*) }
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}
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diff -uNr 06_drivers_gpio_uart/src/bsp/rpi.rs 07_uart_chainloader/src/bsp/rpi.rs
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--- 06_drivers_gpio_uart/src/bsp/rpi.rs
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+++ 07_uart_chainloader/src/bsp/rpi.rs
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@@ -16,6 +16,9 @@
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/// The early boot core's stack address.
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pub const BOOT_CORE_STACK_START: u64 = 0x80_000;
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+/// The address on which the RPi3 firmware loads every binary by default.
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+pub const BOARD_DEFAULT_LOAD_ADDRESS: usize = 0x80_000;
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+
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//--------------------------------------------------------------------------------------------------
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// Global BSP driver instances
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//--------------------------------------------------------------------------------------------------
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diff -uNr 06_drivers_gpio_uart/src/interface.rs 07_uart_chainloader/src/interface.rs
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--- 06_drivers_gpio_uart/src/interface.rs
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+++ 07_uart_chainloader/src/interface.rs
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@@ -29,6 +29,10 @@
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/// Write a Rust format string.
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fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
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+
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+ /// Block execution until the last character has been physically put on the TX wire
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+ /// (draining TX buffers/FIFOs, if any).
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+ fn flush(&self);
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}
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/// Console read functions.
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@@ -37,6 +41,9 @@
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fn read_char(&self) -> char {
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' '
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}
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+
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+ /// Clear RX buffers, if any.
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+ fn clear(&self);
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}
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/// Console statistics.
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diff -uNr 06_drivers_gpio_uart/src/main.rs 07_uart_chainloader/src/main.rs
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--- 06_drivers_gpio_uart/src/main.rs
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+++ 07_uart_chainloader/src/main.rs
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@@ -29,7 +29,11 @@
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// the first function to run.
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mod arch;
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-// `_start()` then calls `runtime_init()`, which on completion, jumps to `kernel_init()`.
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+// `_start()` then calls `relocate::relocate_self()`.
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+mod relocate;
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+
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+// `relocate::relocate_self()` calls `runtime_init()`, which on completion, jumps to
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+// `kernel_init()`.
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mod runtime_init;
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// Conditionally includes the selected `BSP` code.
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@@ -65,25 +69,49 @@
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fn kernel_main() -> ! {
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use interface::console::All;
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- // UART should be functional now. Wait for user to hit Enter.
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- loop {
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- if bsp::console().read_char() == '\n' {
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- break;
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- }
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+ println!(" __ __ _ _ _ _ ");
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+ println!("| \\/ (_)_ _ (_) | ___ __ _ __| |");
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+ println!("| |\\/| | | ' \\| | |__/ _ \\/ _` / _` |");
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+ println!("|_| |_|_|_||_|_|____\\___/\\__,_\\__,_|");
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+ println!();
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+ println!("{:^37}", bsp::board_name());
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+ println!();
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+ println!("[ML] Requesting binary");
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+ bsp::console().flush();
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+
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+ // Clear the RX FIFOs, if any, of spurious received characters before starting with the loader
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+ // protocol.
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+ bsp::console().clear();
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+
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+ // Notify `Minipush` to send the binary.
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+ for _ in 0..3 {
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+ bsp::console().write_char(3 as char);
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}
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- println!("[0] Booting on: {}", bsp::board_name());
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-
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|
- println!("[1] Drivers loaded:");
|
|
|
|
- for (i, driver) in bsp::device_drivers().iter().enumerate() {
|
|
|
|
- println!(" {}. {}", i + 1, driver.compatible());
|
|
|
|
+ // Read the binary's size.
|
|
|
|
+ let mut size: u32 = u32::from(bsp::console().read_char() as u8);
|
|
|
|
+ size |= u32::from(bsp::console().read_char() as u8) << 8;
|
|
|
|
+ size |= u32::from(bsp::console().read_char() as u8) << 16;
|
|
|
|
+ size |= u32::from(bsp::console().read_char() as u8) << 24;
|
|
|
|
+
|
|
|
|
+ // Trust it's not too big.
|
|
|
|
+ bsp::console().write_char('O');
|
|
|
|
+ bsp::console().write_char('K');
|
|
|
|
+
|
|
|
|
+ let kernel_addr: *mut u8 = bsp::BOARD_DEFAULT_LOAD_ADDRESS as *mut u8;
|
|
|
|
+ unsafe {
|
|
|
|
+ // Read the kernel byte by byte.
|
|
|
|
+ for i in 0..size {
|
|
|
|
+ *kernel_addr.offset(i as isize) = bsp::console().read_char() as u8;
|
|
|
|
+ }
|
|
|
|
}
|
|
|
|
|
|
|
|
- println!("[2] Chars written: {}", bsp::console().chars_written());
|
|
|
|
- println!("[3] Echoing input now");
|
|
|
|
+ println!("[ML] Loaded! Executing the payload now\n");
|
|
|
|
+ bsp::console().flush();
|
|
|
|
|
|
|
|
- loop {
|
|
|
|
- let c = bsp::console().read_char();
|
|
|
|
- bsp::console().write_char(c);
|
|
|
|
- }
|
|
|
|
+ // Use black magic to get a function pointer.
|
|
|
|
+ let kernel: extern "C" fn() -> ! = unsafe { core::mem::transmute(kernel_addr as *const ()) };
|
|
|
|
+
|
|
|
|
+ // Jump to loaded kernel!
|
|
|
|
+ kernel()
|
|
|
|
}
|
|
|
|
|
|
|
|
diff -uNr 06_drivers_gpio_uart/src/relocate.rs 07_uart_chainloader/src/relocate.rs
|
|
|
|
--- 06_drivers_gpio_uart/src/relocate.rs
|
|
|
|
+++ 07_uart_chainloader/src/relocate.rs
|
|
|
|
@@ -0,0 +1,46 @@
|
|
|
|
+// SPDX-License-Identifier: MIT OR Apache-2.0
|
|
|
|
+//
|
|
|
|
+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
|
|
|
|
+
|
|
|
|
+//! Relocation code.
|
|
|
|
+
|
|
|
|
+/// Relocates the own binary from `bsp::BOARD_DEFAULT_LOAD_ADDRESS` to the `__binary_start` address
|
|
|
|
+/// from the linker script.
|
|
|
|
+///
|
|
|
|
+/// # Safety
|
|
|
|
+///
|
|
|
|
+/// - Only a single core must be active and running this function.
|
|
|
|
+/// - Function must not use the `bss` section.
|
|
|
|
+pub unsafe fn relocate_self<T>() -> ! {
|
|
|
|
+ extern "C" {
|
|
|
|
+ static __binary_start: usize;
|
|
|
|
+ static __binary_end: usize;
|
|
|
|
+ }
|
|
|
|
+
|
|
|
|
+ let binary_start_addr: usize = &__binary_start as *const _ as _;
|
|
|
|
+ let binary_end_addr: usize = &__binary_end as *const _ as _;
|
|
|
|
+ let binary_size_in_byte: usize = binary_end_addr - binary_start_addr;
|
|
|
|
+
|
|
|
|
+ // Get the relocation destination address from the linker symbol.
|
|
|
|
+ let mut reloc_dst_addr: *mut T = binary_start_addr as *mut T;
|
|
|
|
+
|
|
|
|
+ // The address of where the previous firmware loaded us.
|
|
|
|
+ let mut src_addr: *const T = crate::bsp::BOARD_DEFAULT_LOAD_ADDRESS as *const _;
|
|
|
|
+
|
|
|
|
+ // Copy the whole binary.
|
|
|
|
+ //
|
|
|
|
+ // This is essentially a `memcpy()` optimized for throughput by transferring in chunks of T.
|
|
|
|
+ let n = binary_size_in_byte / core::mem::size_of::<T>();
|
|
|
|
+ for _ in 0..n {
|
|
|
|
+ use core::ptr;
|
|
|
|
+
|
|
|
|
+ ptr::write_volatile::<T>(reloc_dst_addr, ptr::read_volatile::<T>(src_addr));
|
|
|
|
+ reloc_dst_addr = reloc_dst_addr.offset(1);
|
|
|
|
+ src_addr = src_addr.offset(1);
|
|
|
|
+ }
|
|
|
|
+
|
|
|
|
+ // Call `init()` through a trait object, causing the jump to use an absolute address to reach
|
|
|
|
+ // the relocated binary. An elaborate explanation can be found in the runtime_init.rs source
|
|
|
|
+ // comments.
|
|
|
|
+ crate::runtime_init::get().runtime_init()
|
|
|
|
+}
|
|
|
|
|
|
|
|
diff -uNr 06_drivers_gpio_uart/src/runtime_init.rs 07_uart_chainloader/src/runtime_init.rs
|
|
|
|
--- 06_drivers_gpio_uart/src/runtime_init.rs
|
|
|
|
+++ 07_uart_chainloader/src/runtime_init.rs
|
|
|
|
@@ -36,14 +36,32 @@
|
|
|
|
memory::zero_volatile(bss_range());
|
|
|
|
}
|
|
|
|
|
|
|
|
-/// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to kernel
|
|
|
|
-/// init code.
|
|
|
|
+/// We are outsmarting the compiler here by using a trait as a layer of indirection. Because we are
|
|
|
|
+/// generating PIC code, a static dispatch to `init()` would generate a relative jump from the
|
|
|
|
+/// callee to `init()`. However, when calling `init()`, code just finished copying the binary to the
|
|
|
|
+/// actual link-time address, and hence is still running at whatever location the previous loader
|
|
|
|
+/// has put it. So we do not want a relative jump, because it would not jump to the relocated code.
|
|
|
|
///
|
|
|
|
-/// # Safety
|
|
|
|
-///
|
|
|
|
-/// - Only a single core must be active and running this function.
|
|
|
|
-pub unsafe fn runtime_init() -> ! {
|
|
|
|
- zero_bss();
|
|
|
|
+/// By indirecting through a trait object, we can make use of the property that vtables store
|
|
|
|
+/// absolute addresses. So calling `init()` this way will kick execution to the relocated binary.
|
|
|
|
+pub trait RunTimeInit {
|
|
|
|
+ /// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to
|
|
|
|
+ /// kernel init code.
|
|
|
|
+ ///
|
|
|
|
+ /// # Safety
|
|
|
|
+ ///
|
|
|
|
+ /// - Only a single core must be active and running this function.
|
|
|
|
+ unsafe fn runtime_init(&self) -> ! {
|
|
|
|
+ zero_bss();
|
|
|
|
+
|
|
|
|
+ crate::kernel_init()
|
|
|
|
+ }
|
|
|
|
+}
|
|
|
|
+
|
|
|
|
+struct Traitor;
|
|
|
|
+impl RunTimeInit for Traitor {}
|
|
|
|
|
|
|
|
- crate::kernel_init()
|
|
|
|
+/// Give the callee a `RunTimeInit` trait object.
|
|
|
|
+pub fn get() -> &'static dyn RunTimeInit {
|
|
|
|
+ &Traitor {}
|
|
|
|
}
|
|
|
|
|
|
|
|
```
|