1 //
2 // low-level driver routines for 16550a UART.
3 //
4
5 #include "types.h"
6 #include "param.h"
7 #include "memlayout.h"
8 #include "riscv.h"
9 #include "spinlock.h"
10 #include "proc.h"
11 #include "defs.h"
12
13 // the UART control registers are memory-mapped
14 // at address UART0. this macro returns the
15 // address of one of the registers.
16 #define Reg(reg) ((volatile unsigned char *)(UART0 + reg))
17
18 // the UART control registers.
19 // some have different meanings for
20 // read vs write.
21 // see http://byterunner.com/16550.html
22 #define RHR 0 // receive holding register (for input bytes)
23 #define THR 0 // transmit holding register (for output bytes)
24 #define IER 1 // interrupt enable register
25 #define IER_RX_ENABLE (1<<0)
26 #define IER_TX_ENABLE (1<<1)
27 #define FCR 2 // FIFO control register
28 #define FCR_FIFO_ENABLE (1<<0)
29 #define FCR_FIFO_CLEAR (3<<1) // clear the content of the two FIFOs
30 #define ISR 2 // interrupt status register
31 #define LCR 3 // line control register
32 #define LCR_EIGHT_BITS (3<<0)
33 #define LCR_BAUD_LATCH (1<<7) // special mode to set baud rate
34 #define LSR 5 // line status register
35 #define LSR_RX_READY (1<<0) // input is waiting to be read from RHR
36 #define LSR_TX_IDLE (1<<5) // THR can accept another character to send
37
38 #define ReadReg(reg) (*(Reg(reg)))
39 #define WriteReg(reg, v) (*(Reg(reg)) = (v))
40
41 // the transmit output buffer.
42 struct spinlock uart_tx_lock;
43 #define UART_TX_BUF_SIZE 32
44 char uart_tx_buf[UART_TX_BUF_SIZE];
45 uint64 uart_tx_w; // write next to uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE]
46 uint64 uart_tx_r; // read next from uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE]
47
48 extern volatile int panicked; // from printf.c
49
50 void uartstart();
51
52 void
53 uartinit(void)
54 {
55 // disable interrupts.
56 WriteReg(IER, 0x00);
57
58 // special mode to set baud rate.
59 WriteReg(LCR, LCR_BAUD_LATCH);
60
61 // LSB for baud rate of 38.4K.
62 WriteReg(0, 0x03);
63
64 // MSB for baud rate of 38.4K.
65 WriteReg(1, 0x00);
66
67 // leave set-baud mode,
68 // and set word length to 8 bits, no parity.
69 WriteReg(LCR, LCR_EIGHT_BITS);
70
71 // reset and enable FIFOs.
72 WriteReg(FCR, FCR_FIFO_ENABLE | FCR_FIFO_CLEAR);
73
74 // enable transmit and receive interrupts.
75 WriteReg(IER, IER_TX_ENABLE | IER_RX_ENABLE);
76
77 initlock(&uart_tx_lock, "uart");
78 }
79
80 // add a character to the output buffer and tell the
81 // UART to start sending if it isn't already.
82 // blocks if the output buffer is full.
83 // because it may block, it can't be called
84 // from interrupts; it's only suitable for use
85 // by write().
86 void
87 uartputc(int c)
88 {
89 acquire(&uart_tx_lock);
90
91 if(panicked){
92 for(;;)
93 ;
94 }
95 while(uart_tx_w == uart_tx_r + UART_TX_BUF_SIZE){
96 // buffer is full.
97 // wait for uartstart() to open up space in the buffer.
98 sleep(&uart_tx_r, &uart_tx_lock);
99 }
100 uart_tx_buf[uart_tx_w % UART_TX_BUF_SIZE] = c;
101 uart_tx_w += 1;
102 uartstart();
103 release(&uart_tx_lock);
104 }
105
106
107 // alternate version of uartputc() that doesn't
108 // use interrupts, for use by kernel printf() and
109 // to echo characters. it spins waiting for the uart's
110 // output register to be empty.
111 void
112 uartputc_sync(int c)
113 {
114 push_off();
115
116 if(panicked){
117 for(;;)
118 ;
119 }
120
121 // wait for Transmit Holding Empty to be set in LSR.
122 while((ReadReg(LSR) & LSR_TX_IDLE) == 0)
123 ;
124 WriteReg(THR, c);
125
126 pop_off();
127 }
128
129 // if the UART is idle, and a character is waiting
130 // in the transmit buffer, send it.
131 // caller must hold uart_tx_lock.
132 // called from both the top- and bottom-half.
133 void
134 uartstart()
135 {
136 while(1){
137 if(uart_tx_w == uart_tx_r){
138 // transmit buffer is empty.
139 return;
140 }
141
142 if((ReadReg(LSR) & LSR_TX_IDLE) == 0){
143 // the UART transmit holding register is full,
144 // so we cannot give it another byte.
145 // it will interrupt when it's ready for a new byte.
146 return;
147 }
148
149 int c = uart_tx_buf[uart_tx_r % UART_TX_BUF_SIZE];
150 uart_tx_r += 1;
151
152 // maybe uartputc() is waiting for space in the buffer.
153 wakeup(&uart_tx_r);
154
155 WriteReg(THR, c);
156 }
157 }
158
159 // read one input character from the UART.
160 // return -1 if none is waiting.
161 int
162 uartgetc(void)
163 {
164 if(ReadReg(LSR) & 0x01){
165 // input data is ready.
166 return ReadReg(RHR);
167 } else {
168 return -1;
169 }
170 }
171
172 // handle a uart interrupt, raised because input has
173 // arrived, or the uart is ready for more output, or
174 // both. called from devintr().
175 void
176 uartintr(void)
177 {
178 // read and process incoming characters.
179 while(1){
180 int c = uartgetc();
181 if(c == -1)
182 break;
183 consoleintr(c);
184 }
185
186 // send buffered characters.
187 acquire(&uart_tx_lock);
188 uartstart();
189 release(&uart_tx_lock);
190 }