// GPIO driver for Raspberry Pi 4

const LINEAR_MAP_BASE u64 = 0xFFFF000000000000
const DEVICE_BASE u64 = 0xFE000000
const GPIO_BASE u64 = DEVICE_BASE + 0x00200000 + LINEAR_MAP_BASE

const GpioPinData = extern struct {
    reserved u32,
    data [2]u32
}

const GpioRegs = extern struct {
    func_select [6]u32,
    output_set GpioPinData,
    output_clear GpioPinData,
    level GpioPinData,
    ev_detect_status GpioPinData,
    re_detect_enable GpioPinData,
    fe_detect_enable GpioPinData,
    hi_detect_enable GpioPinData,
    lo_detect_enable GpioPinData,
    async_re_detect GpioPinData,
    async_fe_detect GpioPinData,
    reserved u32,
    pupd_enable u32,
    pupd_enable_clocks [2]u32,
    reserved2 [18]u32,
    pullup_pulldown [4]u32
}

fn getGpioRegs() *mut volatile GpioRegs {
    return #as(*mut volatile GpioRegs, #ptrFromInt(GPIO_BASE))
}

/// Set the alternate function for a GPIO pin
export fn gpio_pin_set_func(pin_number u8, func u8) void {
    const regs = getGpioRegs()
    const bit_start u5 = #intCast((pin_number % 10) * 3)
    const reg usize = pin_number / 10

    var selector = regs.func_select[reg]
    selector &= ~(#as(u32, 7) << bit_start)
    selector |= (#as(u32, func) << bit_start)
    regs.func_select[reg] = selector
}

/// Disable pull-up/pull-down for a GPIO pin
export fn gpio_pin_enable(pin_number u8) void {
    const regs = getGpioRegs()
    const idx usize = pin_number / 16
    const bit_start u5 = #intCast((pin_number % 16) * 2)

    var reg = regs.pullup_pulldown[idx]
    reg &= ~(#as(u32, 3) << bit_start)
    regs.pullup_pulldown[idx] = reg
}