From: Ben Pfaff <blp@cs.stanford.edu>
Date: Sat, 11 Sep 2004 23:16:03 +0000 (+0000)
Subject: Only poll on output if the txq is full and interrupts are off, and
X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?p=pintos-anon;a=commitdiff_plain;h=f8c3387bc9d7f7280c3b6f3626826e21ca584af0

Only poll on output if the txq is full and interrupts are off, and
only one byte's worth.
---

diff --git a/src/devices/serial.c b/src/devices/serial.c
index 230a320..d1a4eca 100644
--- a/src/devices/serial.c
+++ b/src/devices/serial.c
@@ -52,18 +52,26 @@ serial_putc (uint8_t byte)
 {
   enum intr_level old_level = intr_disable ();
 
-  if (mode == POLL || old_level == INTR_OFF)
+  if (mode == POLL)
     {
       /* If we're not set up for interrupt-driven I/O yet,
-         or if interrupts are off,
-         use dumb polling for serial I/O. */
-      serial_flush ();
+         use dumb polling to transmit a byte. */
       putc_poll (byte); 
     }
-  else
+  else 
     {
       /* Otherwise, queue a byte and update the interrupt enable
          register. */
+      if (old_level == INTR_OFF && intq_full (&txq)) 
+        {
+          /* Interrupts are off and the transmit queue is full.
+             If we wanted to wait for the queue to empty,
+             we'd have to reenable interrupts.
+             That's impolite, so we'll send a character via
+             polling instead. */
+          putc_poll (intq_getc (&txq)); 
+        }
+
       intq_putc (&txq, byte); 
       write_ier ();
     }