[Xen-changelog] [xen-unstable] qemu 16550 uart: Fast-drop bursts of transmitted characters to avoid

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1197128764 0
# Node ID 180e4a77e805c0a0b280e3b30d960f2feac2926d
# Parent  d4a3479e68ce0e7775759bd2dc94ced4ffe84eb6
qemu 16550 uart: Fast-drop bursts of transmitted characters to avoid
stalling due to a disconnected pipe or pty.
Signed-off-by: Trolle Selander <trolle.selander@xxxxxxxxx>
---
 tools/ioemu/hw/serial.c |   22 ++++++++++++++--------
 1 files changed, 14 insertions(+), 8 deletions(-)

diff -r d4a3479e68ce -r 180e4a77e805 tools/ioemu/hw/serial.c
--- a/tools/ioemu/hw/serial.c   Fri Dec 07 18:24:33 2007 +0000
+++ b/tools/ioemu/hw/serial.c   Sat Dec 08 15:46:04 2007 +0000
@@ -359,7 +359,7 @@ static void serial_xmit(void *opaque) {
     SerialState *s = opaque;
     uint64_t new_xmit_ts = qemu_get_clock(vm_clock);
     
-    if ( s->tsr_retry == 0 ) {
+    if ( s->tsr_retry <= 0 ) {
         if (s->fcr & UART_FCR_FE) {
             s->tsr = fifo_get(s,XMIT_FIFO);
             if ( !s->xmit_fifo.count )
@@ -371,16 +371,22 @@ static void serial_xmit(void *opaque) {
     }
 
     if ( qemu_chr_write(s->chr, &s->tsr, 1) != 1 ) {
-        s->tsr_retry++;
-        if ( s->tsr_retry <= MAX_XMIT_RETRY ) {
+        if ( ( s->tsr_retry > 0 ) && ( s->tsr_retry <= MAX_XMIT_RETRY ) ) {
+            s->tsr_retry++;
             qemu_mod_timer(s->transmit_timer,  new_xmit_ts + 
s->char_transmit_time );
             return;
-        }
-    }
- 
-    s->tsr_retry = 0;
+        } else if ( s->poll_msl < 0 ) {
+            /* If we exceed MAX_XMIT_RETRY and the backend is not a real 
serial port, then
+            drop any further failed writes instantly, until we get one that 
goes through.
+            This is to prevent guests that log to unconnected pipes or pty's 
from stalling. */
+            s->tsr_retry = -1;
+        }
+    }
+    else {
+        s->tsr_retry = 0;
+    }
+
     s->last_xmit_ts = qemu_get_clock(vm_clock);
-
     if ( !(s->lsr & UART_LSR_THRE) )
         qemu_mod_timer(s->transmit_timer, s->last_xmit_ts + 
s->char_transmit_time );
 

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