Xv6驱动（二）：UART串口

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阅读材料

Xv6代码：uart.c
教材第5章
课程视频Lecture9-Interrupts
16550 UART手册：ByteRunner.com | TECHNICAL DATA ON 16550

UART寄存器布局

#define RHR 0 // receive holding register (for input bytes)
#define THR 0 // transmit holding register (for output bytes)
#define IER 1 // interrupt enable register
#define IER_RX_ENABLE (1 << 0)
#define IER_TX_ENABLE (1 << 1)
#define FCR 2 // FIFO control register
#define FCR_FIFO_ENABLE (1 << 0)
#define FCR_FIFO_CLEAR (3 << 1) // clear the content of the two FIFOs
#define ISR 2					// interrupt status register
#define LCR 3					// line control register
#define LCR_EIGHT_BITS (3 << 0)
#define LCR_BAUD_LATCH (1 << 7) // special mode to set baud rate
#define LSR 5					// line status register
#define LSR_RX_READY (1 << 0)	// input is waiting to be read from RHR
#define LSR_TX_IDLE (1 << 5)	// THR can accept another character to send

UART每个寄存器栈1byte，寄存器布局可以参考手册

UART驱动

uartinit函数

函数调用链条：main()--->consoleinit()--->uartinit()

UART是整个内核第一个初始化的组件，因为初始化UART以后，就可以调用printf函数进行输出了

关闭UART中断，因为内核初始化UART时不希望被UART的中断打断
设置波特率
设置传输的word的长度为8bits
情况并使能FIFO，注意这个FIFO是UART内部的，对程序员不可见
使能中断
初始化锁

void uartinit(void)
{// disable interrupts.WriteReg(IER, 0x00);// special mode to set baud rate.WriteReg(LCR, LCR_BAUD_LATCH);// LSB for baud rate of 38.4K.WriteReg(0, 0x03);// MSB for baud rate of 38.4K.WriteReg(1, 0x00);// leave set-baud mode,// and set word length to 8 bits, no parity.WriteReg(LCR, LCR_EIGHT_BITS);// reset and enable FIFOs.WriteReg(FCR, FCR_FIFO_ENABLE | FCR_FIFO_CLEAR);// enable transmit and receive interrupts.WriteReg(IER, IER_TX_ENABLE | IER_RX_ENABLE);initlock(&uart_tx_lock, "uart");
}

uartputc_sync函数

这个函数只会被内核panic函数调用，panic函数是当内核发生严重错误时调用，打印出错信息。

当panic函数被调用时，会设置panicked = 1，这样当其他核心调用printf/panic函数时，会陷入死循环，起到了阻塞输出的作用

该函数会等待THR寄存器为空，然后将字符写入该寄存器中。

void uartputc_sync(int c)
{push_off();if (panicked){for (;;);}// wait for Transmit Holding Empty to be set in LSR.while ((ReadReg(LSR) & LSR_TX_IDLE) == 0);WriteReg(THR, c);pop_off();
}

uartputc函数

函数调用链条：write()--->consolewrite()--->uartputc()

判断缓冲区是否已满，如果满了的化调用sleep函数
否则，将字符放入缓冲区中，并更新缓冲区索引
调用uartstart函数开始执行UART字符传输

void uartputc(int c)
{acquire(&uart_tx_lock);if (panicked){for (;;);}while (uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){// buffer is full.// wait for uartstart() to open up space in the buffer.sleep(&uart_tx_r, &uart_tx_lock);}uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;uart_tx_w += 1;uartstart();release(&uart_tx_lock);
}

uartstart函数

首先判断THR是否为满，如果满的化会直接返回
移动uart_tx_r索引，标识已经发送一个字符
唤醒其他因为缓存区满而sleep的进程
发送该字符

void uartstart()
{while (1){if (uart_tx_w == uart_tx_r){// transmit buffer is empty.ReadReg(ISR);return;}if ((ReadReg(LSR) & LSR_TX_IDLE) == 0){// the UART transmit holding register is full,// so we cannot give it another byte.// it will interrupt when it's ready for a new byte.return;}int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];uart_tx_r += 1;// maybe uartputc() is waiting for space in the buffer.wakeup(&uart_tx_r);WriteReg(THR, c);}
}

uartgetc函数

该函数首先判断RHR寄存器是否有字符，有的化读取并返回该字符；否则返回-1

int uartgetc(void)
{if (ReadReg(LSR) & 0x01){// input data is ready.return ReadReg(RHR);}else{return -1;}
}

uartintr函数

有两种情况UART会产生中断

有新的字符到达RHR寄存器
THR发送完毕，现在THR已经空了，可以接受新的字符

该函数会判断这两种情况，并调用相应的函数来处理

void uartintr(void)
{// read and process incoming characters.while (1){int c = uartgetc();if (c == -1)break;consoleintr(c);}// send buffered characters.acquire(&uart_tx_lock);uartstart();release(&uart_tx_lock);
}