From: Peter H. Froehlich <peter.hans.froehlich@gmail.com>
Date: Tue, 20 Oct 2015 15:23:33 +0000 (-0400)
Subject: A second serial port for debugging output.
X-Git-Url: https://hydra-www.ietfng.org/gitweb/?a=commitdiff_plain;h=378d9a5d5724d7d2effc3d270b27cbdc36f0aaba;p=xv6-public

A second serial port for debugging output.
---

diff --git a/Makefile b/Makefile
index 0a1e31a..38f6692 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,7 @@
 OBJS = \
 	bio.o\
 	console.o\
+	debug.o\
 	exec.o\
 	file.o\
 	fs.o\
@@ -213,7 +214,7 @@ endif
 QEMUOPTS = -drive file=fs.img,index=1,media=disk,format=raw -drive file=xv6.img,index=0,media=disk,format=raw -smp $(CPUS) -m 512 $(QEMUEXTRA)
 
 qemu: fs.img xv6.img
-	$(QEMU) -serial mon:stdio $(QEMUOPTS)
+	$(QEMU) -serial mon:stdio -serial file:DEBUGLOG $(QEMUOPTS)
 
 qemu-memfs: xv6memfs.img
 	$(QEMU) -drive file=xv6memfs.img,index=0,media=disk,format=raw -smp $(CPUS) -m 256
diff --git a/debug.c b/debug.c
new file mode 100644
index 0000000..d62c5f1
--- /dev/null
+++ b/debug.c
@@ -0,0 +1,151 @@
+// A second Intel 8250 serial port (UART) for early-boot debug output.
+// Requires that QEMU was started with a second serial port, otherwise
+// debug output is suppressed.
+//
+// We want to allow debug output as early as possible and from anywhere,
+// including the spinlock code. This requires a custom lock to make sure
+// output is complete and consistent. Our custom lock cannot possibly be
+// acquired from an interrupt context so it doesn't disable interrupts.
+
+#include "types.h"
+#include "defs.h"
+#include "x86.h"
+#include "uart.h"
+
+static int uart;  // do we have a second uart?
+static volatile uint locked;  // custom lock
+
+static void
+lock(void)
+{
+  while(xchg(&locked, 1) != 0)
+    ;
+}
+
+static void
+unlock(void)
+{
+  xchg(&locked, 0);
+}
+
+static int
+cantransmit(void)
+{
+  return inb(COM2+UART_LINE_STATUS) & UART_TRANSMIT_READY;
+}
+
+static void
+debugputc(int c)
+{
+  int i;
+
+  if(!uart)
+    return;
+  for(i = 0; i < 128 && !cantransmit(); i++)
+    microdelay(10);
+  outb(COM2+UART_TRANSMIT_BUFFER, c);
+}
+
+void
+debuginit(void)
+{
+  // Turn off the FIFO
+  outb(COM2+UART_FIFO_CONTROL, 0);
+
+  // 9600 baud, 8 data bits, 1 stop bit, parity off.
+  outb(COM2+UART_LINE_CONTROL, UART_DIVISOR_LATCH);
+  outb(COM2+UART_DIVISOR_LOW, 115200/9600);
+  outb(COM2+UART_DIVISOR_HIGH, 0);
+  outb(COM2+UART_LINE_CONTROL, 0x03);   // 8 data bits.
+  outb(COM2+UART_MODEM_CONTROL, 0);     // No RTS/DTR handshake.
+  outb(COM2+UART_INTERRUPT_ENABLE, 0);  // Disable interrupts.
+
+  // If status is 0xFF, no serial port.
+  if(inb(COM2+UART_LINE_STATUS) == 0xFF)
+    return;
+  uart = 1;
+
+  // Acknowledge pre-existing interrupt conditions, if any.
+  // We don't use any of the interrupt stuff ourselves.
+  inb(COM2+UART_INTERRUPT_ID);
+  inb(COM2+UART_RECEIVE_BUFFER);
+}
+
+// It's certainly sad that we have to replicate the printf code *again* for
+// this module. Alas there doesn't seem to be a nice way to modularize.
+
+static void
+printint(int xx, int base, int sign)
+{
+  static char digits[] = "0123456789abcdef";
+  char buf[16];
+  int i;
+  uint x;
+
+  if(sign && (sign = (xx < 0)))
+    x = -xx;
+  else
+    x = xx;
+
+  i = 0;
+  do{
+    buf[i++] = digits[x % base];
+  }while((x /= base) != 0);
+
+  if(sign)
+    buf[i++] = '-';
+
+  while(--i >= 0)
+    debugputc(buf[i]);
+}
+
+void
+debugf(char *fmt, ...)
+{
+  int i, c;
+  uint *argp;
+  char *s;
+
+  if(fmt == 0){
+    debugf("debugf: null fmt\n");  // no panic(), no console.c dependency
+    return;
+  }
+
+  lock();
+
+  argp = (uint*)(void*)(&fmt + 1);
+  for(i = 0; (c = fmt[i] & 0xff) != 0; i++){
+    if(c != '%'){
+      debugputc(c);
+      continue;
+    }
+    c = fmt[++i] & 0xff;
+    if(c == 0)
+      break;
+    switch(c){
+    case 'd':
+      printint(*argp++, 10, 1);
+      break;
+    case 'x':
+    case 'p':
+      printint(*argp++, 16, 0);
+      break;
+    case 's':
+      if((s = (char*)*argp++) == 0)
+        s = "(null)";
+      for(; *s; s++)
+        debugputc(*s);
+      break;
+    case '%':
+      debugputc('%');
+      break;
+    default:
+      // Print unknown % sequence to draw attention.
+      debugputc('%');
+      debugputc(c);
+      break;
+    }
+  }
+
+  unlock();
+}
diff --git a/defs.h b/defs.h
index 3758232..8c3568c 100644
--- a/defs.h
+++ b/defs.h
@@ -22,6 +22,10 @@ void            cprintf(char*, ...);
 void            consoleintr(int(*)(void));
 void            panic(char*) __attribute__((noreturn));
 
+// debug.c
+void            debuginit(void);
+void            debugf(char*, ...);
+
 // exec.c
 int             exec(char*, char**);
 
diff --git a/main.c b/main.c
index 88a3867..d25e719 100644
--- a/main.c
+++ b/main.c
@@ -16,6 +16,7 @@ extern char end[]; // first address after kernel loaded from ELF file
 int
 main(void)
 {
+  debuginit();     // enable debug output first
   kinit1(end, P2V(4*1024*1024)); // phys page allocator
   kvmalloc();      // kernel page table
   mpinit();        // collect info about this machine