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88 lines
3.9 KiB
Markdown
88 lines
3.9 KiB
Markdown
# Tutorial 06 - Drivers: GPIO and UART
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## tl;dr
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Now that we enabled safe globals in the previous tutorial, the infrastructure is laid for adding the
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first real device drivers. We throw out the magic QEMU console and use a real UART now. Like serious
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embedded hackers do!
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## Notable additions
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- For the first time, we will be able to run the code on the real hardware.
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- Therefore, building is now differentiated between the **RPi 3** and the **RPi4**.
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- By default, all `Makefile` targets will build for the **RPi 3**.
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- In order to build for the the **RPi4**, prepend `BSP=rpi4` to each target. For example:
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- `BSP=rpi4 make`
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- `BSP=rpi4 make doc`
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- Unfortunately, QEMU does not yet support the **RPi4**, so `BSP=rpi4 make qemu` won't work.
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- A `driver::interface::DeviceDriver` trait is added for abstracting `BSP` driver implementations
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from kernel code.
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- Drivers are stored in `src/bsp/device_driver`, and can be reused between `BSP`s.
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- We introduce the `GPIO` driver, which pinmuxes the RPi's PL011 UART.
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- Most importantly, the `PL011Uart` driver: It implements the `console::interface::*` traits and
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is from now on used as the main system console output.
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- `BSP`s now contain a memory map in `src/bsp/memory.rs`. In the specific case, they contain the
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Raspberry's `MMIO` addresses which are used to instantiate the respectivedevice drivers.
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- We also modify the `panic!` handler, so that it does not anymore rely on `println!`, which uses
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the globally-shared instance of the `UART` that might be locked when an error is encountered (for
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now this can't happen due to the `NullLock`, but with a real lock it becomes an issue).
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- Instead, it creates a new UART driver instance, re-initializes the device and uses that one to
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print. This increases the chances that the system is able to print a final important message
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before it suspends itself.
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## Boot it from SD card
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Some steps for preparing the SD card differ between RPi3 and RPi4, so be careful.
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### Common for both
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1. Make a single `FAT32` partition named `boot`.
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2. On the card, generate a file named `config.txt` with the following contents:
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```txt
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init_uart_clock=48000000
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```
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### Pi 3
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3. Copy the following files from the [Raspberry Pi firmware repo](https://github.com/raspberrypi/firmware/tree/master/boot) onto the SD card:
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- [bootcode.bin](https://github.com/raspberrypi/firmware/raw/master/boot/bootcode.bin)
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- [fixup.dat](https://github.com/raspberrypi/firmware/raw/master/boot/fixup.dat)
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- [start.elf](https://github.com/raspberrypi/firmware/raw/master/boot/start.elf)
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4. Run `make` and copy the [kernel8.img](kernel8.img) onto the SD card.
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### Pi 4
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3. Copy the following files from the [Raspberry Pi firmware repo](https://github.com/raspberrypi/firmware/tree/master/boot) onto the SD card:
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- [fixup4.dat](https://github.com/raspberrypi/firmware/raw/master/boot/fixup4.dat)
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- [start4.elf](https://github.com/raspberrypi/firmware/raw/master/boot/start4.elf)
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- [bcm2711-rpi-4-b.dtb](https://github.com/raspberrypi/firmware/raw/master/boot/bcm2711-rpi-4-b.dtb)
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4. Run `BSP=rpi4 make` and copy the [kernel8.img](kernel8.img) onto the SD card.
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_**Note**: Should it not work on your RPi4, try renaming `start4.elf` to `start.elf` (without the 4)
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on the SD card._
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### Common again
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5. Insert the SD card into the RPi and connect the USB serial to your host PC.
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- Wiring diagram at [top-level README](../README.md#usb-serial).
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6. Run `screen` (you might need to install it first):
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```console
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sudo screen /dev/ttyUSB0 230400
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```
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7. Hit <kbd>Enter</kbd> to kick off the kernel boot process. Observe the output:
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```console
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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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8. Exit screen by pressing <kbd>ctrl-a</kbd> <kbd>ctrl-d</kbd> or disconnecting the USB serial.
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## Diff to previous
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