From 0f732c946ec2f0fb401f890ec4a9d49efcb2f77b Mon Sep 17 00:00:00 2001
From: "kaf24@firebug.cl.cam.ac.uk" <kaf24@firebug.cl.cam.ac.uk>
Date: Mon, 30 May 2005 16:04:00 +0000
Subject: [PATCH] bitkeeper revision 1.1595 (429b3970iSiBE6JNLmeO-KDY_v3Hdg)

Better locking strategy in serial rx interrupt handler and getc. Release
the spinlock after each loop iteration to prevent starvation of other
cpus and of interrupt handling. Also neatly folds byte_matches() into
serial_getc().
Signed-off-by: Keir Fraser <keir@xensource.com>
---
 xen/drivers/char/serial.c | 67 ++++++++++++++++++---------------------
 1 file changed, 30 insertions(+), 37 deletions(-)

diff --git a/xen/drivers/char/serial.c b/xen/drivers/char/serial.c
index c578de152c..f3828716bc 100644
--- a/xen/drivers/char/serial.c
+++ b/xen/drivers/char/serial.c
@@ -28,12 +28,14 @@ void serial_rx_interrupt(struct serial_port *port, struct cpu_user_regs *regs)
     BUG_ON(!port->driver);
     BUG_ON(!port->driver->getc);
 
-    spin_lock_irqsave(&port->lock, flags);
-
-    while ( port->driver->getc(port, &c) )
+    for ( ; ; )
     {
-        fn = NULL;
+        spin_lock_irqsave(&port->lock, flags);
+
+        if ( !port->driver->getc(port, &c) )
+            break;
 
+        fn = NULL;
         if ( port->rx != NULL )
             fn = port->rx;
         else if ( (c & 0x80) && (port->rx_hi != NULL) )
@@ -43,12 +45,12 @@ void serial_rx_interrupt(struct serial_port *port, struct cpu_user_regs *regs)
         else if ( (port->rxbufp - port->rxbufc) != RXBUFSZ )
             port->rxbuf[MASK_RXBUF_IDX(port->rxbufp++)] = c;            
 
+        spin_unlock_irqrestore(&port->lock, flags);
+
         if ( fn != NULL )
-        {
-            spin_unlock_irqrestore(&port->lock, flags);
             (*fn)(c & 0x7f, regs);
-            spin_lock_irqsave(&port->lock, flags);
-        }
+
+        cpu_relax();
     }
 
     spin_unlock_irqrestore(&port->lock, flags);
@@ -83,22 +85,6 @@ void serial_puts(int handle, const char *s)
         serial_putc(handle, *s++);
 }
 
-/* Returns TRUE if given character (*pc) matches the serial handle. */
-static int byte_matches(int handle, unsigned char *pc)
-{
-    if ( !(handle & SERHND_HI) )
-    {
-        if ( !(handle & SERHND_LO) || !(*pc & 0x80) )
-            return 1;
-    }
-    else if ( *pc & 0x80 )
-    {
-        *pc &= 0x7f;
-        return 1;
-    }
-    return 0;
-}
-
 char serial_getc(int handle)
 {
     struct serial_port *port = &com[handle & SERHND_IDX];
@@ -108,21 +94,28 @@ char serial_getc(int handle)
     if ( (handle == -1) || !port->driver || !port->driver->getc )
         return '\0';
 
-    spin_lock_irqsave(&port->lock, flags);
+    do {        
+        for ( ; ; )
+        {
+            spin_lock_irqsave(&port->lock, flags);
+            
+            if ( port->rxbufp != port->rxbufc )
+            {
+                c = port->rxbuf[MASK_RXBUF_IDX(port->rxbufc++)];
+                break;
+            }
+            
+            if ( port->driver->getc(port, &c) )
+                break;
 
-    while ( port->rxbufp != port->rxbufc )
-    {
-        c = port->rxbuf[MASK_RXBUF_IDX(port->rxbufc++)];
-        if ( byte_matches(handle, &c) )
-            goto out;
-    }
-    
-    while ( !port->driver->getc(port, &c) && !byte_matches(handle, &c) )
-        continue;
+            spin_unlock_irqrestore(&port->lock, flags);
 
- out:
-    spin_unlock_irqrestore(&port->lock, flags);
-    return c;
+            cpu_relax();
+        }
+    } while ( ((handle & SERHND_LO) &&  (c & 0x80)) ||
+              ((handle & SERHND_HI) && !(c & 0x80)) );
+    
+    return c & 0x7f;
 }
 
 int serial_parse_handle(char *conf)
-- 
2.30.2