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/*  In this file, we ported codes from RustSBI.
    Thus we can handle serial in S mode.
*/
use core::convert::Infallible;
use core::ptr::{read_volatile, write_volatile};
use embedded_hal::serial::nb::{Read, Write};

pub struct Ns16550a {
    base: usize,
    shift: usize,
}

impl Ns16550a {
    pub fn new(base: usize, shift: usize /* , clk: u64, baud: u64*/) -> Self {
        // already init in RustSBI
        Self { base, shift }
    }
}

impl embedded_hal::serial::ErrorType for Ns16550a {
    type Error = Infallible;
}

impl Read<u8> for Ns16550a {
    fn read(&mut self) -> nb::Result<u8, Self::Error> {
        let pending =
            unsafe { read_volatile((self.base + (offsets::LSR << self.shift)) as *const u8) }
                & masks::DR;
        if pending != 0 {
            let word =
                unsafe { read_volatile((self.base + (offsets::RBR << self.shift)) as *const u8) };
            Ok(word)
        } else {
            Err(nb::Error::WouldBlock)
        }
    }
}

impl Write<u8> for Ns16550a {
    fn write(&mut self, word: u8) -> nb::Result<(), Self::Error> {
        // 写,但是不刷新
        unsafe { write_volatile((self.base + (offsets::THR << self.shift)) as *mut u8, word) };
        Ok(())
    }

    fn flush(&mut self) -> nb::Result<(), Self::Error> {
        let pending =
            unsafe { read_volatile((self.base + (offsets::LSR << self.shift)) as *const u8) }
                & masks::THRE;
        if pending != 0 {
            // 发送已经结束了
            Ok(())
        } else {
            // 发送还没有结束,继续等
            Err(nb::Error::WouldBlock)
        }
    }
}

mod offsets {
    pub const RBR: usize = 0x0;
    pub const THR: usize = 0x0;

    pub const IER: usize = 0x1;
    pub const FCR: usize = 0x2;
    pub const LCR: usize = 0x3;
    pub const MCR: usize = 0x4;
    pub const LSR: usize = 0x5;

    pub const DLL: usize = 0x0;
    pub const DLH: usize = 0x1;
}

mod masks {
    pub const THRE: u8 = 1 << 5;
    pub const DR: u8 = 1;
}