PC Chipset: Improve serial divisor calculation

This fixes several problems I found in the UART serial implementation.
Now all divisor values are allowed, while before divisor values of zero
and below the base baud rate were rejected. All changes are in reference
to http://www.sci.muni.cz/docs/pc/serport.txt

Signed-off-by: Calvin Lee <[email protected]>
Message-Id: <20180512000545[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

diff --git a/hw/char/serial.c b/hw/char/serial.c
index cd7d747c684a007263fc52949c3a62bb419ff4f3..d3b75f09f6740007a96ca3bc90a4166e74ea096e 100644 (file)
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -150,13 +150,10 @@ static void serial_update_irq(SerialState *s)
 
 static void serial_update_parameters(SerialState *s)
 {
-    int speed, parity, data_bits, stop_bits, frame_size;
+    float speed;
+    int parity, data_bits, stop_bits, frame_size;
     QEMUSerialSetParams ssp;
 
-    if (s->divider == 0 || s->divider > s->baudbase) {
-        return;
-    }
-
     /* Start bit. */
     frame_size = 1;
     if (s->lcr & 0x08) {
@@ -169,14 +166,16 @@ static void serial_update_parameters(SerialState *s)
     } else {
             parity = 'N';
     }
-    if (s->lcr & 0x04)
+    if (s->lcr & 0x04) {
         stop_bits = 2;
-    else
+    } else {
         stop_bits = 1;
+    }
 
     data_bits = (s->lcr & 0x03) + 5;
     frame_size += data_bits + stop_bits;
-    speed = s->baudbase / s->divider;
+    /* Zero divisor should give about 3500 baud */
+    speed = (s->divider == 0) ? 3500 : (float) s->baudbase / s->divider;
     ssp.speed = speed;
     ssp.parity = parity;
     ssp.data_bits = data_bits;
@@ -184,7 +183,7 @@ static void serial_update_parameters(SerialState *s)
     s->char_transmit_time =  (NANOSECONDS_PER_SECOND / speed) * frame_size;
     qemu_chr_fe_ioctl(&s->chr, CHR_IOCTL_SERIAL_SET_PARAMS, &ssp);
 
-    DPRINTF("speed=%d parity=%c data=%d stop=%d\n",
+    DPRINTF("speed=%.2f parity=%c data=%d stop=%d\n",
            speed, parity, data_bits, stop_bits);
 }
 
@@ -341,7 +340,11 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
     default:
     case 0:
         if (s->lcr & UART_LCR_DLAB) {
-            s->divider = (s->divider & 0xff00) | val;
+            if (size == 2) {
+                s->divider = (s->divider & 0xff00) | val;
+            } else if (size == 4) {
+                s->divider = val;
+            }
             serial_update_parameters(s);
         } else {
             s->thr = (uint8_t) val;