rust-raspberrypi-OS-tutorials/06_drivers_gpio_uart

Tutorial 06 - Drivers: GPIO and UART

tl;dr

Now that we enabled safe globals in the previous tutorial, the infrastructure is laid for adding the first real device drivers. We throw out the magic QEMU console and use a real UART now. Like serious embedded hackers do!

• For the first time, we will be able to run the code on the real hardware.
  • Therefore, building is now differentiated between the RPi 3 and the RPi4.
  • By default, all Makefile targets will build for the RPi 3.
  • In order to build for the the RPi4, prepend BSP=rpi4 to each target. For example:
    • BSP=rpi4 make
    • BSP=rpi4 make doc
  • Unfortunately, QEMU does not yet support the RPi4, so BSP=rpi4 make qemu won't work.
• A DeviceDriver trait is added for abstracting BSP driver implementations from kernel code.
• Drivers are stored in bsp/driver, and can be reused between BSPs.
  • Introducing the GPIO driver, which pinmuxes the RPi's PL011 UART.
  • Most importantly, the PL011Uart driver: It implements the Console traits and is from now on used as the system console output.
• BSPs now contain amemory_map.rs. In the specific case, they contain the RPi's MMIO addresses which are used to instantiate compatible device drivers from bsp/driver.
• We also modify the panic! handler, so that it does not anymore rely on println!, which uses the globally-shared instance of the UART that might be locked when an error is encountered (for now this can't happen due to the NullLock, but with a real lock it becomes an issue).
  • Instead, it creates a new UART driver instance, re-initializes the device and uses that one to print. This increases the chances that the system is able to print a final important message before it suspends itself.

Boot it from SD card

Some steps for preparing the SD card differ between RPi3 and RPi4, so be careful.

Common for both

1. Make a single FAT32 partition named boot.
2. On the card, generate a file named config.txt with the following contents:

init_uart_clock=48000000

Pi 3

3. Copy the following files from the Raspberry Pi firmware repo onto the SD card:
   • bootcode.bin
   • fixup.dat
   • start.elf
4. Run make and copy the kernel8.img onto the SD card.

Pi 4

3. Copy the following files from the Raspberry Pi firmware repo onto the SD card:
   • fixup4.dat
   • start4.elf
   • bcm2711-rpi-4-b.dtb
4. Run BSP=rpi4 make and copy the kernel8.img onto the SD card.

Common again

5. Insert the SD card into the RPi and connect the USB serial to your host PC.
   • Wiring diagram at top-level README.
6. Run screen (you might need to install it first):

sudo screen /dev/ttyUSB0 115200

7. Hit Enter to kick off the kernel boot process. Observe the output:

[0] Booting on: Raspberry Pi 3
[1] Drivers loaded:
      1. GPIO
      2. PL011Uart
[2] Chars written: 84
[3] Echoing input now

8. Exit screen by pressing ctrl-a ctrl-d or disconnecting the USB serial.

Diff to previous

diff -uNr 05_safe_globals/Cargo.toml 06_drivers_gpio_uart/Cargo.toml
--- 05_safe_globals/Cargo.toml
+++ 06_drivers_gpio_uart/Cargo.toml
@@ -10,11 +10,12 @@
 # The features section is used to select the target board.
 [features]
 default = []
-bsp_rpi3 = ["cortex-a"]
-bsp_rpi4 = ["cortex-a"]
+bsp_rpi3 = ["cortex-a", "register"]
+bsp_rpi4 = ["cortex-a", "register"]

 [dependencies]
 r0 = "0.2.*"

 # Optional dependencies
 cortex-a = { version = "2.8.x", optional = true }
+register = { version = "0.4.x", optional = true }

diff -uNr 05_safe_globals/src/arch/aarch64.rs 06_drivers_gpio_uart/src/arch/aarch64.rs
--- 05_safe_globals/src/arch/aarch64.rs
+++ 06_drivers_gpio_uart/src/arch/aarch64.rs
@@ -33,6 +33,15 @@
 // Implementation of the kernel's architecture abstraction code
 //--------------------------------------------------------------------------------------------------

+pub use asm::nop;
+
+/// Spin for `n` cycles.
+pub fn spin_for_cycles(n: usize) {
+    for _ in 0..n {
+        asm::nop();
+    }
+}
+
 /// Pause execution on the calling CPU core.
 #[inline(always)]
 pub fn wait_forever() -> ! {

diff -uNr 05_safe_globals/src/bsp/driver/bcm/bcm2xxx_gpio.rs 06_drivers_gpio_uart/src/bsp/driver/bcm/bcm2xxx_gpio.rs
--- 05_safe_globals/src/bsp/driver/bcm/bcm2xxx_gpio.rs
+++ 06_drivers_gpio_uart/src/bsp/driver/bcm/bcm2xxx_gpio.rs
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
+
+//! GPIO driver.
+
+use crate::{arch, arch::sync::NullLock, interface};
+use core::ops;
+use register::{mmio::ReadWrite, register_bitfields, register_structs};
+
+// GPIO registers.
+//
+// Descriptions taken from
+// https://github.com/raspberrypi/documentation/files/1888662/BCM2837-ARM-Peripherals.-.Revised.-.V2-1.pdf
+register_bitfields! {
+    u32,
+
+    /// GPIO Function Select 1
+    GPFSEL1 [
+        /// Pin 15
+        FSEL15 OFFSET(15) NUMBITS(3) [
+            Input = 0b000,
+            Output = 0b001,
+            AltFunc0 = 0b100  // PL011 UART RX
+
+        ],
+
+        /// Pin 14
+        FSEL14 OFFSET(12) NUMBITS(3) [
+            Input = 0b000,
+            Output = 0b001,
+            AltFunc0 = 0b100  // PL011 UART TX
+        ]
+    ],
+
+    /// GPIO Pull-up/down Clock Register 0
+    GPPUDCLK0 [
+        /// Pin 15
+        PUDCLK15 OFFSET(15) NUMBITS(1) [
+            NoEffect = 0,
+            AssertClock = 1
+        ],
+
+        /// Pin 14
+        PUDCLK14 OFFSET(14) NUMBITS(1) [
+            NoEffect = 0,
+            AssertClock = 1
+        ]
+    ]
+}
+
+register_structs! {
+    #[allow(non_snake_case)]
+    RegisterBlock {
+        (0x00 => GPFSEL0: ReadWrite<u32>),
+        (0x04 => GPFSEL1: ReadWrite<u32, GPFSEL1::Register>),
+        (0x08 => GPFSEL2: ReadWrite<u32>),
+        (0x0C => GPFSEL3: ReadWrite<u32>),
+        (0x10 => GPFSEL4: ReadWrite<u32>),
+        (0x14 => GPFSEL5: ReadWrite<u32>),
+        (0x18 => _reserved1),
+        (0x94 => GPPUD: ReadWrite<u32>),
+        (0x98 => GPPUDCLK0: ReadWrite<u32, GPPUDCLK0::Register>),
+        (0x9C => GPPUDCLK1: ReadWrite<u32>),
+        (0xA0 => @END),
+    }
+}
+
+/// The driver's private data.
+struct GPIOInner {
+    base_addr: usize,
+}
+
+/// Deref to RegisterBlock.
+impl ops::Deref for GPIOInner {
+    type Target = RegisterBlock;
+
+    fn deref(&self) -> &Self::Target {
+        unsafe { &*self.ptr() }
+    }
+}
+
+impl GPIOInner {
+    const fn new(base_addr: usize) -> GPIOInner {
+        GPIOInner { base_addr }
+    }
+
+    /// Return a pointer to the register block.
+    fn ptr(&self) -> *const RegisterBlock {
+        self.base_addr as *const _
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
+// BSP-public
+//--------------------------------------------------------------------------------------------------
+use interface::sync::Mutex;
+
+/// The driver's main struct.
+pub struct GPIO {
+    inner: NullLock<GPIOInner>,
+}
+
+impl GPIO {
+    pub const unsafe fn new(base_addr: usize) -> GPIO {
+        GPIO {
+            inner: NullLock::new(GPIOInner::new(base_addr)),
+        }
+    }
+
+    /// Map PL011 UART as standard output.
+    ///
+    /// TX to pin 14
+    /// RX to pin 15
+    pub fn map_pl011_uart(&self) {
+        let mut r = &self.inner;
+        r.lock(|inner| {
+            // Map to pins.
+            inner
+                .GPFSEL1
+                .modify(GPFSEL1::FSEL14::AltFunc0 + GPFSEL1::FSEL15::AltFunc0);
+
+            // Enable pins 14 and 15.
+            inner.GPPUD.set(0);
+            arch::spin_for_cycles(150);
+
+            inner
+                .GPPUDCLK0
+                .write(GPPUDCLK0::PUDCLK14::AssertClock + GPPUDCLK0::PUDCLK15::AssertClock);
+            arch::spin_for_cycles(150);
+
+            inner.GPPUDCLK0.set(0);
+        })
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
+// OS interface implementations
+//--------------------------------------------------------------------------------------------------
+
+impl interface::driver::DeviceDriver for GPIO {
+    fn compatible(&self) -> &str {
+        "GPIO"
+    }
+}

diff -uNr 05_safe_globals/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs 06_drivers_gpio_uart/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs
--- 05_safe_globals/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs
+++ 06_drivers_gpio_uart/src/bsp/driver/bcm/bcm2xxx_pl011_uart.rs
@@ -0,0 +1,317 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
+
+//! PL011 UART driver.
+
+use crate::{arch, arch::sync::NullLock, interface};
+use core::{fmt, ops};
+use register::{mmio::*, register_bitfields, register_structs};
+
+// PL011 UART registers.
+//
+// Descriptions taken from
+// https://github.com/raspberrypi/documentation/files/1888662/BCM2837-ARM-Peripherals.-.Revised.-.V2-1.pdf
+register_bitfields! {
+    u32,
+
+    /// Flag Register
+    FR [
+        /// Transmit FIFO empty. The meaning of this bit depends on the state of the FEN bit in the
+        /// Line Control Register, UARTLCR_ LCRH.
+        ///
+        /// If the FIFO is disabled, this bit is set when the transmit holding register is empty. If
+        /// the FIFO is enabled, the TXFE bit is set when the transmit FIFO is empty. This bit does
+        /// not indicate if there is data in the transmit shift register.
+        TXFE OFFSET(7) NUMBITS(1) [],
+
+        /// Transmit FIFO full. The meaning of this bit depends on the state of the FEN bit in the
+        /// UARTLCR_ LCRH Register.
+        ///
+        /// If the FIFO is disabled, this bit is set when the transmit holding register is full. If
+        /// the FIFO is enabled, the TXFF bit is set when the transmit FIFO is full.
+        TXFF OFFSET(5) NUMBITS(1) [],
+
+        /// Receive FIFO empty. The meaning of this bit depends on the state of the FEN bit in the
+        /// UARTLCR_H Register.
+        ///
+        /// If the FIFO is disabled, this bit is set when the receive holding register is empty. If
+        /// the FIFO is enabled, the RXFE bit is set when the receive FIFO is empty.
+        RXFE OFFSET(4) NUMBITS(1) []
+    ],
+
+    /// Integer Baud rate divisor
+    IBRD [
+        /// Integer Baud rate divisor
+        IBRD OFFSET(0) NUMBITS(16) []
+    ],
+
+    /// Fractional Baud rate divisor
+    FBRD [
+        /// Fractional Baud rate divisor
+        FBRD OFFSET(0) NUMBITS(6) []
+    ],
+
+    /// Line Control register
+    LCRH [
+        /// Word length. These bits indicate the number of data bits transmitted or received in a
+        /// frame.
+        WLEN OFFSET(5) NUMBITS(2) [
+            FiveBit = 0b00,
+            SixBit = 0b01,
+            SevenBit = 0b10,
+            EightBit = 0b11
+        ],
+
+        /// Enable FIFOs:
+        ///
+        /// 0 = FIFOs are disabled (character mode) that is, the FIFOs become 1-byte-deep holding
+        /// registers
+        ///
+        /// 1 = transmit and receive FIFO buffers are enabled (FIFO mode).
+        FEN  OFFSET(4) NUMBITS(1) [
+            FifosDisabled = 0,
+            FifosEnabled = 1
+        ]
+    ],
+
+    /// Control Register
+    CR [
+        /// Receive enable. If this bit is set to 1, the receive section of the UART is enabled.
+        /// Data reception occurs for UART signals. When the UART is disabled in the middle of
+        /// reception, it completes the current character before stopping.
+        RXE    OFFSET(9) NUMBITS(1) [
+            Disabled = 0,
+            Enabled = 1
+        ],
+
+        /// Transmit enable. If this bit is set to 1, the transmit section of the UART is enabled.
+        /// Data transmission occurs for UART signals. When the UART is disabled in the middle of
+        /// transmission, it completes the current character before stopping.
+        TXE    OFFSET(8) NUMBITS(1) [
+            Disabled = 0,
+            Enabled = 1
+        ],
+
+        /// UART enable
+        UARTEN OFFSET(0) NUMBITS(1) [
+            /// If the UART is disabled in the middle of transmission or reception, it completes the
+            /// current character before stopping.
+            Disabled = 0,
+            Enabled = 1
+        ]
+    ],
+
+    /// Interrupt Clear Register
+    ICR [
+        /// Meta field for all pending interrupts
+        ALL OFFSET(0) NUMBITS(11) []
+    ]
+}
+
+register_structs! {
+    #[allow(non_snake_case)]
+    RegisterBlock {
+        (0x00 => DR: ReadWrite<u32>),
+        (0x04 => _reserved1),
+        (0x18 => FR: ReadOnly<u32, FR::Register>),
+        (0x1c => _reserved2),
+        (0x24 => IBRD: WriteOnly<u32, IBRD::Register>),
+        (0x28 => FBRD: WriteOnly<u32, FBRD::Register>),
+        (0x2c => LCRH: WriteOnly<u32, LCRH::Register>),
+        (0x30 => CR: WriteOnly<u32, CR::Register>),
+        (0x34 => _reserved3),
+        (0x44 => ICR: WriteOnly<u32, ICR::Register>),
+        (0x48 => @END),
+    }
+}
+
+/// The driver's mutex protected part.
+pub struct PL011UartInner {
+    base_addr: usize,
+    chars_written: usize,
+    chars_read: usize,
+}
+
+/// Deref to RegisterBlock.
+///
+/// Allows writing
+/// ```
+/// self.DR.read()
+/// ```
+/// instead of something along the lines of
+/// ```
+/// unsafe { (*PL011UartInner::ptr()).DR.read() }
+/// ```
+impl ops::Deref for PL011UartInner {
+    type Target = RegisterBlock;
+
+    fn deref(&self) -> &Self::Target {
+        unsafe { &*self.ptr() }
+    }
+}
+
+impl PL011UartInner {
+    pub const unsafe fn new(base_addr: usize) -> PL011UartInner {
+        PL011UartInner {
+            base_addr,
+            chars_written: 0,
+            chars_read: 0,
+        }
+    }
+
+    /// Set up baud rate and characteristics.
+    ///
+    /// Results in 8N1 and 115200 baud (if the clk has been previously set to 4 MHz by the
+    /// firmware).
+    pub fn init(&self) {
+        // Turn it off temporarily.
+        self.CR.set(0);
+
+        self.ICR.write(ICR::ALL::CLEAR);
+        self.IBRD.write(IBRD::IBRD.val(26)); // Results in 115200 baud for UART Clk of 48 MHz.
+        self.FBRD.write(FBRD::FBRD.val(3));
+        self.LCRH
+            .write(LCRH::WLEN::EightBit + LCRH::FEN::FifosEnabled); // 8N1 + Fifo on
+        self.CR
+            .write(CR::UARTEN::Enabled + CR::TXE::Enabled + CR::RXE::Enabled);
+    }
+
+    /// Return a pointer to the register block.
+    fn ptr(&self) -> *const RegisterBlock {
+        self.base_addr as *const _
+    }
+
+    /// Send a character.
+    fn write_char(&mut self, c: char) {
+        // Spin while TX FIFO full is set, waiting for an empty slot.
+        while self.FR.matches_all(FR::TXFF::SET) {
+            arch::nop();
+        }
+
+        // Write the character to the buffer.
+        self.DR.set(c as u32);
+    }
+}
+
+/// Implementing `core::fmt::Write` enables usage of the `format_args!` macros, which in turn are
+/// used to implement the `kernel`'s `print!` and `println!` macros. By implementing `write_str()`,
+/// we get `write_fmt()` automatically.
+///
+/// The function takes an `&mut self`, so it must be implemented for the inner struct.
+///
+/// See [`src/print.rs`].
+///
+/// [`src/print.rs`]: ../../print/index.html
+impl fmt::Write for PL011UartInner {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        for c in s.chars() {
+            // Convert newline to carrige return + newline.
+            if c == '\n' {
+                self.write_char('\r')
+            }
+
+            self.write_char(c);
+        }
+
+        self.chars_written += s.len();
+
+        Ok(())
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
+// Export the inner struct so that BSPs can use it for the panic handler
+//--------------------------------------------------------------------------------------------------
+pub use PL011UartInner as PanicUart;
+
+//--------------------------------------------------------------------------------------------------
+// BSP-public
+//--------------------------------------------------------------------------------------------------
+
+/// The driver's main struct.
+pub struct PL011Uart {
+    inner: NullLock<PL011UartInner>,
+}
+
+impl PL011Uart {
+    /// # Safety
+    ///
+    /// The user must ensure to provide the correct `base_addr`.
+    pub const unsafe fn new(base_addr: usize) -> PL011Uart {
+        PL011Uart {
+            inner: NullLock::new(PL011UartInner::new(base_addr)),
+        }
+    }
+}
+
+//--------------------------------------------------------------------------------------------------
+// OS interface implementations
+//--------------------------------------------------------------------------------------------------
+use interface::sync::Mutex;
+
+impl interface::driver::DeviceDriver for PL011Uart {
+    fn compatible(&self) -> &str {
+        "PL011Uart"
+    }
+
+    fn init(&self) -> interface::driver::Result {
+        let mut r = &self.inner;
+        r.lock(|inner| inner.init());
+
+        Ok(())
+    }
+}
+
+impl interface::console::Write for PL011Uart {
+    /// Passthrough of `args` to the `core::fmt::Write` implementation, but guarded by a Mutex to
+    /// serialize access.
+    fn write_char(&self, c: char) {
+        let mut r = &self.inner;
+        r.lock(|inner| inner.write_char(c));
+    }
+
+    fn write_fmt(&self, args: core::fmt::Arguments) -> fmt::Result {
+        // Fully qualified syntax for the call to `core::fmt::Write::write:fmt()` to increase
+        // readability.
+        let mut r = &self.inner;
+        r.lock(|inner| fmt::Write::write_fmt(inner, args))
+    }
+}
+
+impl interface::console::Read for PL011Uart {
+    fn read_char(&self) -> char {
+        let mut r = &self.inner;
+        r.lock(|inner| {
+            // Spin while RX FIFO empty is set.
+            while inner.FR.matches_all(FR::RXFE::SET) {
+                arch::nop();
+            }
+
+            // Read one character.
+            let mut ret = inner.DR.get() as u8 as char;
+
+            // Convert carrige return to newline.
+            if ret == '\r' {
+                ret = '\n'
+            }
+
+            // Update statistics.
+            inner.chars_read += 1;
+
+            ret
+        })
+    }
+}
+
+impl interface::console::Statistics for PL011Uart {
+    fn chars_written(&self) -> usize {
+        let mut r = &self.inner;
+        r.lock(|inner| inner.chars_written)
+    }
+
+    fn chars_read(&self) -> usize {
+        let mut r = &self.inner;
+        r.lock(|inner| inner.chars_read)
+    }
+}

diff -uNr 05_safe_globals/src/bsp/driver/bcm.rs 06_drivers_gpio_uart/src/bsp/driver/bcm.rs
--- 05_safe_globals/src/bsp/driver/bcm.rs
+++ 06_drivers_gpio_uart/src/bsp/driver/bcm.rs
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
+
+//! BCM driver top level.
+
+mod bcm2xxx_gpio;
+mod bcm2xxx_pl011_uart;
+
+pub use bcm2xxx_gpio::GPIO;
+pub use bcm2xxx_pl011_uart::{PL011Uart, PanicUart};

diff -uNr 05_safe_globals/src/bsp/driver.rs 06_drivers_gpio_uart/src/bsp/driver.rs
--- 05_safe_globals/src/bsp/driver.rs
+++ 06_drivers_gpio_uart/src/bsp/driver.rs
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
+
+//! Drivers.
+
+#[cfg(any(feature = "bsp_rpi3", feature = "bsp_rpi4"))]
+mod bcm;
+
+#[cfg(any(feature = "bsp_rpi3", feature = "bsp_rpi4"))]
+pub use bcm::*;

diff -uNr 05_safe_globals/src/bsp/rpi/memory_map.rs 06_drivers_gpio_uart/src/bsp/rpi/memory_map.rs
--- 05_safe_globals/src/bsp/rpi/memory_map.rs
+++ 06_drivers_gpio_uart/src/bsp/rpi/memory_map.rs
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
+
+//! The board's memory map.
+
+/// Physical devices.
+#[rustfmt::skip]
+pub mod mmio {
+    #[cfg(feature = "bsp_rpi3")]
+    pub const BASE:            usize =        0x3F00_0000;
+
+    #[cfg(feature = "bsp_rpi4")]
+    pub const BASE:            usize =        0xFE00_0000;
+
+    pub const GPIO_BASE:       usize = BASE + 0x0020_0000;
+    pub const PL011_UART_BASE: usize = BASE + 0x0020_1000;
+}

diff -uNr 05_safe_globals/src/bsp/rpi.rs 06_drivers_gpio_uart/src/bsp/rpi.rs
--- 05_safe_globals/src/bsp/rpi.rs
+++ 06_drivers_gpio_uart/src/bsp/rpi.rs
@@ -4,7 +4,10 @@

 //! Board Support Package for the Raspberry Pi.

-use crate::{arch::sync::NullLock, interface};
+mod memory_map;
+
+use super::driver;
+use crate::interface;
 use core::fmt;

 /// Used by `arch` code to find the early boot core.
@@ -13,108 +16,59 @@
 /// The early boot core's stack address.
 pub const BOOT_CORE_STACK_START: u64 = 0x80_000;

-/// A mystical, magical device for generating QEMU output out of the void.
-///
-/// The mutex protected part.
-struct QEMUOutputInner {
-    chars_written: usize,
-}
-
-impl QEMUOutputInner {
-    const fn new() -> QEMUOutputInner {
-        QEMUOutputInner { chars_written: 0 }
-    }
-
-    /// Send a character.
-    fn write_char(&mut self, c: char) {
-        unsafe {
-            core::ptr::write_volatile(0x3F20_1000 as *mut u8, c as u8);
-        }
-    }
-}
-
-/// Implementing `core::fmt::Write` enables usage of the `format_args!` macros, which in turn are
-/// used to implement the `kernel`'s `print!` and `println!` macros. By implementing `write_str()`,
-/// we get `write_fmt()` automatically.
-///
-/// The function takes an `&mut self`, so it must be implemented for the inner struct.
-///
-/// See [`src/print.rs`].
-///
-/// [`src/print.rs`]: ../../print/index.html
-impl fmt::Write for QEMUOutputInner {
-    fn write_str(&mut self, s: &str) -> fmt::Result {
-        for c in s.chars() {
-            // Convert newline to carrige return + newline.
-            if c == '\n' {
-                self.write_char('\r')
-            }
-
-            self.write_char(c);
-        }
-
-        self.chars_written += s.len();
-
-        Ok(())
-    }
-}
-
 //--------------------------------------------------------------------------------------------------
-// BSP-public
+// Global BSP driver instances
 //--------------------------------------------------------------------------------------------------

-/// The main struct.
-pub struct QEMUOutput {
-    inner: NullLock<QEMUOutputInner>,
-}
-
-impl QEMUOutput {
-    pub const fn new() -> QEMUOutput {
-        QEMUOutput {
-            inner: NullLock::new(QEMUOutputInner::new()),
-        }
-    }
-}
+static GPIO: driver::GPIO = unsafe { driver::GPIO::new(memory_map::mmio::GPIO_BASE) };
+static PL011_UART: driver::PL011Uart =
+    unsafe { driver::PL011Uart::new(memory_map::mmio::PL011_UART_BASE) };

 //--------------------------------------------------------------------------------------------------
-// OS interface implementations
+// Implementation of the kernel's BSP calls
 //--------------------------------------------------------------------------------------------------

-/// Passthrough of `args` to the `core::fmt::Write` implementation, but guarded by a Mutex to
-/// serialize access.
-impl interface::console::Write for QEMUOutput {
-    fn write_fmt(&self, args: core::fmt::Arguments) -> fmt::Result {
-        use interface::sync::Mutex;
-
-        // Fully qualified syntax for the call to `core::fmt::Write::write:fmt()` to increase
-        // readability.
-        let mut r = &self.inner;
-        r.lock(|inner| fmt::Write::write_fmt(inner, args))
+/// Board identification.
+pub fn board_name() -> &'static str {
+    #[cfg(feature = "bsp_rpi3")]
+    {
+        "Raspberry Pi 3"
     }
-}

-impl interface::console::Read for QEMUOutput {}
-
-impl interface::console::Statistics for QEMUOutput {
-    fn chars_written(&self) -> usize {
-        use interface::sync::Mutex;
-
-        let mut r = &self.inner;
-        r.lock(|inner| inner.chars_written)
+    #[cfg(feature = "bsp_rpi4")]
+    {
+        "Raspberry Pi 4"
     }
 }

-//--------------------------------------------------------------------------------------------------
-// Global instances
-//--------------------------------------------------------------------------------------------------
+/// Return a reference to a `console::All` implementation.
+pub fn console() -> &'static impl interface::console::All {
+    &PL011_UART
+}

-static QEMU_OUTPUT: QEMUOutput = QEMUOutput::new();
+/// In case of a panic, the panic handler uses this function to take a last shot at printing
+/// something before the system is halted.
+///
+/// # Safety
+///
+/// - Use only for printing during a panic.
+pub unsafe fn panic_console_out() -> impl fmt::Write {
+    let uart = driver::PanicUart::new(memory_map::mmio::PL011_UART_BASE);
+    uart.init();
+    uart
+}

-//--------------------------------------------------------------------------------------------------
-// Implementation of the kernel's BSP calls
-//--------------------------------------------------------------------------------------------------
+/// Return an array of references to all `DeviceDriver` compatible `BSP` drivers.
+///
+/// # Safety
+///
+/// The order of devices is the order in which `DeviceDriver::init()` is called.
+pub fn device_drivers() -> [&'static dyn interface::driver::DeviceDriver; 2] {
+    [&GPIO, &PL011_UART]
+}

-/// Return a reference to a `console::All` implementation.
-pub fn console() -> &'static impl interface::console::All {
-    &QEMU_OUTPUT
+/// BSP initialization code that runs after driver init.
+pub fn post_driver_init() {
+    // Configure PL011Uart's output pins.
+    GPIO.map_pl011_uart();
 }

diff -uNr 05_safe_globals/src/bsp.rs 06_drivers_gpio_uart/src/bsp.rs
--- 05_safe_globals/src/bsp.rs
+++ 06_drivers_gpio_uart/src/bsp.rs
@@ -4,6 +4,8 @@

 //! Conditional exporting of Board Support Packages.

+mod driver;
+
 #[cfg(any(feature = "bsp_rpi3", feature = "bsp_rpi4"))]
 mod rpi;


diff -uNr 05_safe_globals/src/interface.rs 06_drivers_gpio_uart/src/interface.rs
--- 05_safe_globals/src/interface.rs
+++ 06_drivers_gpio_uart/src/interface.rs
@@ -24,6 +24,9 @@

     /// Console write functions.
     pub trait Write {
+        /// Write a single character.
+        fn write_char(&self, c: char);
+
         /// Write a Rust format string.
         fn write_fmt(&self, args: fmt::Arguments) -> fmt::Result;
     }
@@ -85,3 +88,20 @@
         fn lock<R>(&mut self, f: impl FnOnce(&mut Self::Data) -> R) -> R;
     }
 }
+
+/// Driver interfaces.
+pub mod driver {
+    /// Driver result type, e.g. for indicating successful driver init.
+    pub type Result = core::result::Result<(), ()>;
+
+    /// Device Driver functions.
+    pub trait DeviceDriver {
+        /// Return a compatibility string for identifying the driver.
+        fn compatible(&self) -> &str;
+
+        /// Called by the kernel to bring up the device.
+        fn init(&self) -> Result {
+            Ok(())
+        }
+    }
+}

diff -uNr 05_safe_globals/src/main.rs 06_drivers_gpio_uart/src/main.rs
--- 05_safe_globals/src/main.rs
+++ 06_drivers_gpio_uart/src/main.rs
@@ -41,16 +41,48 @@

 /// Early init code.
 ///
+/// Concerned with with initializing `BSP` and `arch` parts.
+///
 /// # Safety
 ///
 /// - Only a single core must be active and running this function.
+/// - The init calls in this function must appear in the correct order.
 unsafe fn kernel_init() -> ! {
-    use interface::console::Statistics;
+    for i in bsp::device_drivers().iter() {
+        if let Err(()) = i.init() {
+            panic!("Error loading driver: {}", i.compatible())
+        }
+    }
+    bsp::post_driver_init();
+    // println! is usable from here on.
+
+    // Transition from unsafe to safe.
+    kernel_main()
+}
+
+/// The main function running after the early init.
+fn kernel_main() -> ! {
+    use interface::console::All;
+
+    // UART should be functional now. Wait for user to hit Enter.
+    loop {
+        if bsp::console().read_char() == '\n' {
+            break;
+        }
+    }
+
+    println!("[0] Booting on: {}", bsp::board_name());

-    println!("[0] Hello from pure Rust!");
+    println!("[1] Drivers loaded:");
+    for (i, driver) in bsp::device_drivers().iter().enumerate() {
+        println!("      {}. {}", i + 1, driver.compatible());
+    }

-    println!("[1] Chars written: {}", bsp::console().chars_written());
+    println!("[2] Chars written: {}", bsp::console().chars_written());
+    println!("[3] Echoing input now");

-    println!("[2] Stopping here.");
-    arch::wait_forever()
+    loop {
+        let c = bsp::console().read_char();
+        bsp::console().write_char(c);
+    }
 }

diff -uNr 05_safe_globals/src/panic_wait.rs 06_drivers_gpio_uart/src/panic_wait.rs
--- 05_safe_globals/src/panic_wait.rs
+++ 06_drivers_gpio_uart/src/panic_wait.rs
@@ -4,15 +4,31 @@

 //! A panic handler that infinitely waits.

-use crate::{arch, println};
-use core::panic::PanicInfo;
+use crate::{arch, bsp};
+use core::{fmt, panic::PanicInfo};
+
+fn _panic_print(args: fmt::Arguments) {
+    use fmt::Write;
+
+    unsafe { bsp::panic_console_out().write_fmt(args).unwrap() };
+}
+
+/// Prints with a newline - only use from the panic handler.
+///
+/// Carbon copy from https://doc.rust-lang.org/src/std/macros.rs.html
+#[macro_export]
+macro_rules! panic_println {
+    ($($arg:tt)*) => ({
+        _panic_print(format_args_nl!($($arg)*));
+    })
+}

 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
     if let Some(args) = info.message() {
-        println!("Kernel panic: {}", args);
+        panic_println!("Kernel panic: {}", args);
     } else {
-        println!("Kernel panic!");
+        panic_println!("Kernel panic!");
     }

     arch::wait_forever()