interrupt-driven uart output, hopefully a nice example for teaching.

kernel/console.c

@@ -27,6 +27,8 @@
 
 //
 // send one character to the uart.
+// called by printf, and to echo input characters,
+// but not from write().
 //
 void
 consputc(int c)
@@ -40,9 +42,9 @@ consputc(int c)
 
   if(c == BACKSPACE){
     // if the user typed backspace, overwrite with a space.
-    uartputc('\b'); uartputc(' '); uartputc('\b');
+    uartputc('\b', 0); uartputc(' ', 0); uartputc('\b', 0);
   } else {
-    uartputc(c);
+    uartputc(c, 0);
   }
 }
 
@@ -70,7 +72,7 @@ consolewrite(int user_src, uint64 src, int n)
     char c;
     if(either_copyin(&c, user_src, src+i, 1) == -1)
       break;
-    consputc(c);
+    uartputc(c, 1);
   }
   release(&cons.lock);
 

kernel/defs.h

@@ -149,7 +149,7 @@ void            usertrapret(void);
 // uart.c
 void            uartinit(void);
 void            uartintr(void);
-void            uartputc(int);
+void            uartputc(int, int);
 int             uartgetc(void);
 
 // vm.c

kernel/plic.c

@@ -33,7 +33,6 @@ int
 plic_claim(void)
 {
   int hart = cpuid();
-  //int irq = *(uint32*)(PLIC + 0x201004);
   int irq = *(uint32*)PLIC_SCLAIM(hart);
   return irq;
 }
@@ -43,6 +42,5 @@ void
 plic_complete(int irq)
 {
   int hart = cpuid();
-  //*(uint32*)(PLIC + 0x201004) = irq;
   *(uint32*)PLIC_SCLAIM(hart) = irq;
 }

kernel/trap.c

@@ -91,8 +91,9 @@ usertrapret(void)
 {
   struct proc *p = myproc();
 
-  // turn off interrupts, since we're switching
-  // now from kerneltrap() to usertrap().
+  // we're about to switch the destination of traps from
+  // kerneltrap() to usertrap(), so turn off interrupts until
+  // we're back in user space, where usertrap() is correct.
   intr_off();
 
   // send syscalls, interrupts, and exceptions to trampoline.S
@@ -192,6 +193,9 @@ devintr()
       printf("unexpected interrupt irq=%d\n", irq);
     }
 
+    // the PLIC allows each device to raise at most one
+    // interrupt at a time; tell the PLIC the device is
+    // now allowed to interrupt again.
     if(irq)
       plic_complete(irq);
 

kernel/uart.c

@@ -18,7 +18,7 @@
 // the UART control registers.
 // some have different meanings for
 // read vs write.
-// http://byterunner.com/16550.html
+// see http://byterunner.com/16550.html
 #define RHR 0 // receive holding register (for input bytes)
 #define THR 0 // transmit holding register (for output bytes)
 #define IER 1 // interrupt enable register
@@ -30,6 +30,15 @@
 #define ReadReg(reg) (*(Reg(reg)))
 #define WriteReg(reg, v) (*(Reg(reg)) = (v))
 
+// the transmit output buffer.
+struct spinlock uart_tx_lock;
+#define UART_TX_BUF_SIZE 32
+char uart_tx_buf[UART_TX_BUF_SIZE];
+int uart_tx_w; // write next to uart_tx_buf[uart_tx_w++]
+int uart_tx_r; // read next from uart_tx_buf[uar_tx_r++]
+
+void uartstart();
+
 void
 uartinit(void)
 {
@@ -52,18 +61,79 @@ uartinit(void)
   // reset and enable FIFOs.
   WriteReg(FCR, 0x07);
 
-  // enable receive interrupts.
-  WriteReg(IER, 0x01);
+  // enable transmit and receive interrupts.
+  WriteReg(IER, 0x02 | 0x01);
+
+  initlock(&uart_tx_lock, "uart");
 }
 
-// write one output character to the UART.
+// add a character to the output buffer and tell the
+// UART to start sending if it isn't already.
+//
+// usually called from the top-half -- by a process
+// calling write(). can also be called from a uart
+// interrupt to echo a received character, or by printf
+// or panic from anywhere in the kernel.
+//
+// the block argument controls what happens if the
+// buffer is full. for write(), block is 1, and the
+// process waits. for kernel printf's and echoed
+// characters, block is 0, and the character is
+// discarded; this is necessary since sleep() is
+// not possible in interrupts.
 void
-uartputc(int c)
+uartputc(int c, int block)
 {
-  // wait for Transmit Holding Empty to be set in LSR.
-  while((ReadReg(LSR) & (1 << 5)) == 0)
-    ;
-  WriteReg(THR, c);
+  acquire(&uart_tx_lock);
+  while(1){
+    if(((uart_tx_w + 1) % UART_TX_BUF_SIZE) == uart_tx_r){
+      // buffer is full.
+      if(block){
+        // wait for uartstart() to open up space in the buffer.
+        sleep(&uart_tx_r, &uart_tx_lock);
+      } else {
+        // caller does not want us to wait.
+        release(&uart_tx_lock);
+        return;
+      }
+    } else {
+      uart_tx_buf[uart_tx_w] = c;
+      uart_tx_w = (uart_tx_w + 1) % UART_TX_BUF_SIZE;
+      uartstart();
+      release(&uart_tx_lock);
+      return;
+    }
+  }
+}
+
+// if the UART is idle, and a character is waiting
+// in the transmit buffer, send it.
+// caller must hold uart_tx_lock.
+// called from both the top- and bottom-half.
+void
+uartstart()
+{
+  while(1){
+    if(uart_tx_w == uart_tx_r){
+      // transmit buffer is empty.
+      return;
+    }
+    
+    if((ReadReg(LSR) & (1 << 5)) == 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_r = (uart_tx_r + 1) % UART_TX_BUF_SIZE;
+    
+    // maybe uartputc() is waiting for space in the buffer.
+    wakeup(&uart_tx_r);
+    
+    WriteReg(THR, c);
+  }
 }
 
 // read one input character from the UART.
@@ -79,14 +149,22 @@ uartgetc(void)
   }
 }
 
-// trap.c calls here when the uart interrupts.
+// handle a uart interrupt, raised because input has
+// arrived, or the uart is ready for more output, or
+// both. called from trap.c.
 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);
 }