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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 to do it for every
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new binary. Let's write a [chainloader] using [position independent code]. This will be the last
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binary you need to put on the SD card. 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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[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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## Install and test it
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Our chainloader is called `MiniLoad` and is inspired by [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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> ❗ **NOTE**: By default, `make chainboot` tries to connect to `/dev/ttyUSB0`.
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> Should the USB serial on your system have a different name, you have to provide it explicitly. For
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> example:
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>
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> `DEV_SERIAL=/dev/tty.usbserial-0001 make chainboot`
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[raspbootin]: https://github.com/mrvn/raspbootin
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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. BCM GPIO
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2. BCM PL011 UART
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[2] Chars written: 93
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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 (`0x0200_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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0x0008098c: b0000008 adrp x8, #0x81000
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0x00080990: b0000000 adrp x0, #0x81000
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0x00080994: 912a8000 add x0, x0, #0xaa0
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0x00080998: f9471908 ldr x8, [x8, #0xe30]
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0x0008099c: d63f0100 blr x8
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----------------
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IN:
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0x02000b1c: b0000008 adrp x8, #0x2001000
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0x02000b20: b0000009 adrp x9, #0x2001000
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0x02000b24: f9475d08 ldr x8, [x8, #0xeb8]
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0x02000b28: f9476129 ldr x9, [x9, #0xec0]
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0x02000b2c: eb08013f cmp x9, x8
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0x02000b30: 540000c2 b.hs #0x2000b48
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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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@@ -5,6 +5,12 @@
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# Default to the RPi3
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BSP ?= rpi3
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+# Default to a serial device name that is common in Linux.
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+DEV_SERIAL ?= /dev/ttyUSB0
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+
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+# Query the host system's kernel name
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+UNAME_S = $(shell uname -s)
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+
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# BSP-specific arguments
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ifeq ($(BSP),rpi3)
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TARGET = aarch64-unknown-none-softfloat
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@@ -13,7 +19,8 @@
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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/raspberrypi/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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KERNEL_BIN = kernel8.img
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@@ -21,7 +28,8 @@
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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/raspberrypi/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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# Export for build.rs
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@@ -46,12 +54,23 @@
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DOCKER_IMAGE = rustembedded/osdev-utils
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DOCKER_CMD = docker run -it --rm -v $(shell pwd):/work/tutorial -w /work/tutorial
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+DOCKER_ARG_DIR_UTILS = -v $(shell pwd)/../utils:/work/utils
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+DOCKER_ARG_DEV = --privileged -v /dev:/dev
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DOCKER_QEMU = $(DOCKER_CMD) $(DOCKER_IMAGE)
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-EXEC_QEMU = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
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+# Dockerize commands that require USB device passthrough only on Linux
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+ifeq ($(UNAME_S),Linux)
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+ DOCKER_CMD_DEV = $(DOCKER_CMD) $(DOCKER_ARG_DEV)
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+
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+ DOCKER_CHAINBOOT = $(DOCKER_CMD_DEV) $(DOCKER_ARG_DIR_UTILS) $(DOCKER_IMAGE)
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+endif
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-.PHONY: all $(KERNEL_ELF) $(KERNEL_BIN) doc qemu clippy clean readelf objdump nm check
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+EXEC_QEMU = $(QEMU_BINARY) -M $(QEMU_MACHINE_TYPE)
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+EXEC_MINIPUSH = ruby ../utils/minipush.rb
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+
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+.PHONY: all $(KERNEL_ELF) $(KERNEL_BIN) doc qemu qemuasm chainboot clippy clean readelf objdump nm \
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+ check
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all: $(KERNEL_BIN)
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@@ -65,13 +84,19 @@
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$(DOC_CMD) --document-private-items --open
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ifeq ($(QEMU_MACHINE_TYPE),)
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-qemu:
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+qemu qemuasm:
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@echo "This board is not yet supported for QEMU."
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else
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qemu: $(KERNEL_BIN)
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@$(DOCKER_QEMU) $(EXEC_QEMU) $(QEMU_RELEASE_ARGS) -kernel $(KERNEL_BIN)
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+
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+qemuasm: $(KERNEL_BIN)
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+ @$(DOCKER_QEMU) $(EXEC_QEMU) $(QEMU_RELEASE_ARGS) -kernel $(KERNEL_BIN) -d in_asm
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endif
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+chainboot:
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+ @$(DOCKER_CHAINBOOT) $(EXEC_MINIPUSH) $(DEV_SERIAL) $(CHAINBOOT_DEMO_PAYLOAD)
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+
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clippy:
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RUSTFLAGS="$(RUSTFLAGS_PEDANTIC)" $(CLIPPY_CMD)
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diff -uNr 06_drivers_gpio_uart/src/_arch/aarch64/cpu.rs 07_uart_chainloader/src/_arch/aarch64/cpu.rs
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--- 06_drivers_gpio_uart/src/_arch/aarch64/cpu.rs
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+++ 07_uart_chainloader/src/_arch/aarch64/cpu.rs
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@@ -21,12 +21,12 @@
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#[naked]
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#[no_mangle]
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pub unsafe extern "C" fn _start() -> ! {
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- use crate::runtime_init;
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+ use crate::relocate;
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// Expect the boot core to start in EL2.
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if bsp::cpu::BOOT_CORE_ID == cpu::smp::core_id() {
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SP.set(bsp::memory::boot_core_stack_end() as u64);
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- runtime_init::runtime_init()
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+ 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/device_driver/bcm/bcm2xxx_pl011_uart.rs 07_uart_chainloader/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs
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--- 06_drivers_gpio_uart/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs
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+++ 07_uart_chainloader/src/bsp/device_driver/bcm/bcm2xxx_pl011_uart.rs
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@@ -271,6 +271,16 @@
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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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+ // Spin until TX FIFO empty is set.
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+ let mut r = &self.inner;
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+ r.lock(|inner| {
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+ while !inner.registers.FR.matches_all(FR::TXFE::SET) {
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+ cpu::nop();
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+ }
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+ });
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+ }
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}
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impl console::interface::Read for PL011Uart {
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@@ -282,18 +292,21 @@
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cpu::nop();
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}
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- // Read one character.
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- let mut ret = inner.registers.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.registers.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.registers.FR.matches_all(FR::RXFE::SET) {
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+ inner.registers.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/raspberrypi/link.ld 07_uart_chainloader/src/bsp/raspberrypi/link.ld
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--- 06_drivers_gpio_uart/src/bsp/raspberrypi/link.ld
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+++ 07_uart_chainloader/src/bsp/raspberrypi/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 32 MiB */
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+ . = 0x2000000;
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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/raspberrypi/memory.rs 07_uart_chainloader/src/bsp/raspberrypi/memory.rs
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--- 06_drivers_gpio_uart/src/bsp/raspberrypi/memory.rs
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+++ 07_uart_chainloader/src/bsp/raspberrypi/memory.rs
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@@ -23,10 +23,12 @@
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/// The board's memory map.
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#[rustfmt::skip]
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pub(super) mod map {
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- pub const BOOT_CORE_STACK_END: usize = 0x8_0000;
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+ pub const BOOT_CORE_STACK_END: usize = 0x8_0000;
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- pub const GPIO_OFFSET: usize = 0x0020_0000;
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- pub const UART_OFFSET: usize = 0x0020_1000;
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+ pub const BOARD_DEFAULT_LOAD_ADDRESS: usize = 0x8_0000;
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+
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+ pub const GPIO_OFFSET: usize = 0x0020_0000;
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+ pub const UART_OFFSET: usize = 0x0020_1000;
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/// Physical devices.
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#[cfg(feature = "bsp_rpi3")]
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@@ -59,6 +61,12 @@
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map::BOOT_CORE_STACK_END
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}
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+/// The address on which the Raspberry firmware loads every binary by default.
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+#[inline(always)]
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+pub fn board_default_load_addr() -> usize {
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+ map::BOARD_DEFAULT_LOAD_ADDRESS
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+}
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+
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/// Return the range spanning the .bss section.
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///
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/// # Safety
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diff -uNr 06_drivers_gpio_uart/src/console.rs 07_uart_chainloader/src/console.rs
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--- 06_drivers_gpio_uart/src/console.rs
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+++ 07_uart_chainloader/src/console.rs
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@@ -19,6 +19,10 @@
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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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@@ -27,6 +31,9 @@
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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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@@ -108,7 +108,8 @@
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#![no_std]
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// `mod cpu` provides the `_start()` function, the first function to run. `_start()` then calls
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-// `runtime_init()`, which jumps to `kernel_init()`.
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+// `relocate::relocate_self()`. `relocate::relocate_self()` calls `runtime_init()`, which jumps to
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+// `kernel_init()`.
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mod bsp;
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mod console;
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@@ -117,6 +118,7 @@
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mod memory;
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mod panic_wait;
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mod print;
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+mod relocate;
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mod runtime_init;
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mod synchronization;
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@@ -143,35 +145,52 @@
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/// The main function running after the early init.
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fn kernel_main() -> ! {
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+ use bsp::console::console;
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use console::interface::All;
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- use driver::interface::DriverManager;
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- // Wait for user to hit Enter.
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- loop {
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- if bsp::console::console().read_char() == '\n' {
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- break;
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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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+ 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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+ 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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+ console().write_char(3 as char);
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+ }
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+
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+ // Read the binary's size.
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+ let mut size: u32 = u32::from(console().read_char() as u8);
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+ size |= u32::from(console().read_char() as u8) << 8;
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+ size |= u32::from(console().read_char() as u8) << 16;
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+ size |= u32::from(console().read_char() as u8) << 24;
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+
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+ // Trust it's not too big.
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+ console().write_char('O');
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+ console().write_char('K');
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+
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+ let kernel_addr: *mut u8 = bsp::memory::board_default_load_addr() as *mut u8;
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+ unsafe {
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+ // Read the kernel byte by byte.
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+ for i in 0..size {
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+ *kernel_addr.offset(i as isize) = console().read_char() as u8;
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}
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}
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- println!("[0] Booting on: {}", bsp::board_name());
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+ println!("[ML] Loaded! Executing the payload now\n");
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+ console().flush();
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- println!("[1] Drivers loaded:");
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- for (i, driver) in bsp::driver::driver_manager()
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- .all_device_drivers()
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- .iter()
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- .enumerate()
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- {
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- println!(" {}. {}", i + 1, driver.compatible());
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- }
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+ // Use black magic to get a function pointer.
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+ let kernel: extern "C" fn() -> ! = unsafe { core::mem::transmute(kernel_addr as *const ()) };
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- println!(
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- "[2] Chars written: {}",
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- bsp::console::console().chars_written()
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- );
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- println!("[3] Echoing input now");
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-
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- loop {
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- let c = bsp::console::console().read_char();
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- bsp::console::console().write_char(c);
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- }
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+ // Jump to loaded kernel!
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+ kernel()
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}
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diff -uNr 06_drivers_gpio_uart/src/relocate.rs 07_uart_chainloader/src/relocate.rs
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--- 06_drivers_gpio_uart/src/relocate.rs
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+++ 07_uart_chainloader/src/relocate.rs
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@@ -0,0 +1,52 @@
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+// SPDX-License-Identifier: MIT OR Apache-2.0
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+//
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+// Copyright (c) 2018-2020 Andre Richter <andre.o.richter@gmail.com>
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+
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+//! Relocation code.
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+
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+use crate::{bsp, runtime_init};
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Code
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+//--------------------------------------------------------------------------------------------------
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+
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+/// Relocates the own binary from `bsp::cpu::BOARD_DEFAULT_LOAD_ADDRESS` to the `__binary_start`
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+/// address from the linker script.
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+///
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+/// # Safety
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+///
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+/// - Only a single core must be active and running this function.
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+/// - Function must not use the `bss` section.
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+pub unsafe fn relocate_self<T>() -> ! {
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+ extern "C" {
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+ static __binary_start: usize;
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+ static __binary_end: usize;
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+ }
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+
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+ let binary_start_addr: usize = &__binary_start as *const _ as _;
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+ let binary_end_addr: usize = &__binary_end as *const _ as _;
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+ let binary_size_in_byte: usize = binary_end_addr - binary_start_addr;
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+
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+ // Get the relocation destination address from the linker symbol.
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+ let mut reloc_dst_addr: *mut T = binary_start_addr as *mut T;
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+
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+ // The address of where the previous firmware loaded us.
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+ let mut src_addr: *const T = bsp::memory::board_default_load_addr() as *const _;
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+
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+ // Copy the whole binary.
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+ //
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+ // This is essentially a `memcpy()` optimized for throughput by transferring in chunks of T.
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+ let n = binary_size_in_byte / core::mem::size_of::<T>();
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+ for _ in 0..n {
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+ use core::ptr;
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+
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+ ptr::write_volatile::<T>(reloc_dst_addr, ptr::read_volatile::<T>(src_addr));
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+ reloc_dst_addr = reloc_dst_addr.offset(1);
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+ src_addr = src_addr.offset(1);
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+ }
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+
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+ // Call `runtime_init()` through a trait object, causing the jump to use an absolute address to
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+ // reach the relocated binary. An elaborate explanation can be found in the `runtime_init.rs`
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+ // source comments.
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+ runtime_init::get().runtime_init()
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+}
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diff -uNr 06_drivers_gpio_uart/src/runtime_init.rs 07_uart_chainloader/src/runtime_init.rs
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--- 06_drivers_gpio_uart/src/runtime_init.rs
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+++ 07_uart_chainloader/src/runtime_init.rs
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@@ -7,9 +7,43 @@
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use crate::{bsp, memory};
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//--------------------------------------------------------------------------------------------------
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+// Private Definitions
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+//--------------------------------------------------------------------------------------------------
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+
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+struct Traitor;
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+
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+//--------------------------------------------------------------------------------------------------
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+// Public Definitions
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+//--------------------------------------------------------------------------------------------------
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+
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+/// We are outsmarting the compiler here by using a trait as a layer of indirection. Because we are
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+/// generating PIC code, a static dispatch to `init()` would generate a relative jump from the
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+/// callee to `init()`. However, when calling `init()`, code just finished copying the binary to the
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+/// actual link-time address, and hence is still running at whatever location the previous loader
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+/// has put it. So we do not want a relative jump, because it would not jump to the relocated code.
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+///
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+/// By indirecting through a trait object, we can make use of the property that vtables store
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+/// absolute addresses. So calling `init()` this way will kick execution to the relocated binary.
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+pub trait RunTimeInit {
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+ /// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to
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+ /// kernel init code.
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+ ///
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+ /// # Safety
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+ ///
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+ /// - Only a single core must be active and running this function.
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+ unsafe fn runtime_init(&self) -> ! {
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+ zero_bss();
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+
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+ crate::kernel_init()
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+ }
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+}
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+
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+//--------------------------------------------------------------------------------------------------
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// Private Code
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//--------------------------------------------------------------------------------------------------
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+impl RunTimeInit for Traitor {}
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+
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/// Zero out the .bss section.
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///
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/// # Safety
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@@ -24,14 +58,7 @@
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// Public Code
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//--------------------------------------------------------------------------------------------------
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-/// Equivalent to `crt0` or `c0` code in C/C++ world. Clears the `bss` section, then jumps to kernel
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-/// init code.
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-///
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-/// # Safety
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-///
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-/// - Only a single core must be active and running this function.
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-pub unsafe fn runtime_init() -> ! {
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- zero_bss();
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-
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- crate::kernel_init()
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+/// Give the callee a `RunTimeInit` trait object.
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+pub fn get() -> &'static dyn RunTimeInit {
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+ &Traitor {}
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}
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```
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