[参考译文] BEAGL-BONE-BLACK：GPIO 引脚多路复用器上的下拉电阻器不工作

const CONTROL_MODULE_BASE: u32 = 0x44E10000;

enum Offset {
    Lock = 0x20C,
    GpmcBen1 = 0x878, // GPIO1_28
    GpmcAd6 = 0x818,  // GPIO1_6
    GpmcA0 = 0x840,   // GPIO1_16
}

pub fn configure() {
    set_pin_mode(Offset::GpmcBen1, 7, true, PullResistor::PullUp);
    set_pin_mode(Offset::GpmcAd6, 7, true, PullResistor::PullDown);
    set_pin_mode(Offset::GpmcA0, 7, true, PullResistor::PullDown);
}

fn set_pin_mode(offset: Offset, mode: u32, input_enable: bool, pull_resistor: PullResistor) {
    write_addr(CONTROL_MODULE_BASE + Offset::Lock as u32, 0xAAAA);
    let control_module = CONTROL_MODULE_BASE + offset as u32;

    write_addr(
        control_module,
        (mode & 7) | pull_resistor.to_mask() | ((input_enable as u32) << 5),
    );
}

pub enum PullResistor {
    None,
    PullDown,
    PullUp,
}

impl PullResistor {
    pub fn to_mask(self) -> u32 {
        match self {
            PullResistor::PullDown => 0b00 << 3,
            PullResistor::PullUp => 0b10 << 3,
            PullResistor::None => 0b01 << 3,
        }
    }
}