[Qemu-devel] [PATCH] upgrading emulated UART to 16550A

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

This patch upgrades the emulated UART to 16550A, the code comes from
xen-unstable.
The main improvement was introduced with the following patch and
subsequent email thread:

http://lists.xensource.com/archives/html/xen-devel/2007-12/msg00129.html

I only made few changes to use qemu_get_clock instead of
clock_gettime, as Anthony suggested a little while ago.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

---


diff --git a/hw/serial.c b/hw/serial.c
index ffd6ac9..f63e62a 100644
--- a/hw/serial.c
+++ b/hw/serial.c
@@ -1,5 +1,5 @@
 /*
- * QEMU 16450 UART emulation
+ * QEMU 16550A UART emulation
  *
  * Copyright (c) 2003-2004 Fabrice Bellard
  *
@@ -21,6 +21,13 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
+
+#include <sys/time.h>
+#include <time.h>
+#include <assert.h>
+#include <termios.h>
+#include <sys/ioctl.h>
+
 #include "hw.h"
 #include "qemu-char.h"
 #include "isa.h"
@@ -43,6 +50,10 @@
 #define UART_IIR_THRI	0x02	/* Transmitter holding register empty */
 #define UART_IIR_RDI	0x04	/* Receiver data interrupt */
 #define UART_IIR_RLSI	0x06	/* Receiver line status interrupt */
+#define UART_IIR_CTI    0x0C    /* Character Timeout Indication */
+
+#define UART_IIR_FENF   0x80    /* Fifo enabled, but not functionning */
+#define UART_IIR_FE     0xC0    /* Fifo enabled */
 
 /*
  * These are the definitions for the Modem Control Register
@@ -73,17 +84,39 @@
 #define UART_LSR_PE	0x04	/* Parity error indicator */
 #define UART_LSR_OE	0x02	/* Overrun error indicator */
 #define UART_LSR_DR	0x01	/* Receiver data ready */
-
-/*
- * Delay TX IRQ after sending as much characters as the given interval would
- * contain on real hardware. This avoids overloading the guest if it processes
- * its output buffer in a loop inside the TX IRQ handler.
- */
-#define THROTTLE_TX_INTERVAL	10 /* ms */
+#define UART_LSR_INT_ANY 0x1E	/* Any of the lsr-interrupt-triggering status bits */
+
+/* Interrupt trigger levels. The byte-counts are for 16550A - in newer UARTs the byte-count for each ITL is higher. */
+
+#define UART_FCR_ITL_1      0x00 /* 1 byte ITL */
+#define UART_FCR_ITL_2      0x40 /* 4 bytes ITL */
+#define UART_FCR_ITL_3      0x80 /* 8 bytes ITL */
+#define UART_FCR_ITL_4      0xC0 /* 14 bytes ITL */
+
+#define UART_FCR_DMS        0x08    /* DMA Mode Select */
+#define UART_FCR_XFR        0x04    /* XMIT Fifo Reset */
+#define UART_FCR_RFR        0x02    /* RCVR Fifo Reset */
+#define UART_FCR_FE         0x01    /* FIFO Enable */
+
+#define UART_FIFO_LENGTH    16      /* 16550A Fifo Length */
+
+#define XMIT_FIFO           0
+#define RECV_FIFO           1
+#define MAX_XMIT_RETRY      4
+ 
+struct SerialFIFO {
+    uint8_t data[UART_FIFO_LENGTH];
+    uint8_t count;
+    uint8_t itl;                        /* Interrupt Trigger Level */
+    uint8_t tail;
+    uint8_t head;
+} typedef SerialFIFO;
 
 struct SerialState {
     uint16_t divider;
     uint8_t rbr; /* receive register */
+    uint8_t thr; /* transmit holding register */
+    uint8_t tsr; /* transmit shift register */
     uint8_t ier;
     uint8_t iir; /* read only */
     uint8_t lcr;
@@ -91,6 +124,7 @@ struct SerialState {
     uint8_t lsr; /* read only */
     uint8_t msr; /* read only */
     uint8_t scr;
+    uint8_t fcr;
     /* NOTE: this hidden state is necessary for tx irq generation as
        it can be reset while reading iir */
     int thr_ipending;
@@ -100,55 +134,120 @@ struct SerialState {
     target_phys_addr_t base;
     int it_shift;
     int baudbase;
-    QEMUTimer *tx_timer;
-    int tx_burst;
+    int tsr_retry;
+
+    uint64_t last_xmit_ts;              /* Time when the last byte was successfully sent out of the tsr */
+    SerialFIFO recv_fifo;
+    SerialFIFO xmit_fifo;
+
+    struct QEMUTimer *fifo_timeout_timer;
+    int timeout_ipending;                   /* timeout interrupt pending state */
+    struct QEMUTimer *transmit_timer;
+
+
+    uint64_t char_transmit_time;               /* time to transmit a char in ticks*/
+    int poll_msl;
+
+    struct QEMUTimer *modem_status_poll;
 };
 
+/* Rate limit serial requests so that e.g. grub on a serial console
+   doesn't kill dom0.  Simple token bucket.  If we get some actual
+   data from the user, instantly refil the bucket. */
+
+/* How long it takes to generate a token, in nanoseconds. */
+#define TOKEN_PERIOD 1000000
+/* Maximum and initial size of token bucket */
+#define TOKENS_MAX 100000
+
+static int64_t tokens_avail;
+
+static void serial_receive1(void *opaque, const uint8_t *buf, int size);
+
+static void fifo_clear(SerialState *s, int fifo) {
+    SerialFIFO *f = ( fifo ) ? &s->recv_fifo : &s->xmit_fifo;
+    memset(f->data, 0, UART_FIFO_LENGTH);   
+    f->count = 0;
+    f->head = 0;
+    f->tail = 0;
+}
+
+static int fifo_put(SerialState *s, int fifo, uint8_t chr) {
+    SerialFIFO *f = ( fifo ) ? &s->recv_fifo : &s->xmit_fifo;
+
+    f->data[f->head++] = chr;
+
+    if (f->head == UART_FIFO_LENGTH)
+        f->head = 0;
+    f->count++;
+
+    tokens_avail = TOKENS_MAX;
+
+    return 1;
+}
+
+uint8_t fifo_get(SerialState *s, int fifo) {
+    SerialFIFO *f = ( fifo ) ? &s->recv_fifo : &s->xmit_fifo;
+    uint8_t c;
+
+    if( f->count == 0 )
+        return 0;
+
+    c = f->data[f->tail++];
+    if (f->tail == UART_FIFO_LENGTH)
+        f->tail = 0;
+    f->count--;
+
+    tokens_avail = TOKENS_MAX;
+
+    return c; 
+}
+
 static void serial_receive_byte(SerialState *s, int ch);
 
 static void serial_update_irq(SerialState *s)
 {
-    if ((s->lsr & UART_LSR_DR) && (s->ier & UART_IER_RDI)) {
-        s->iir = UART_IIR_RDI;
-    } else if (s->thr_ipending && (s->ier & UART_IER_THRI)) {
-        s->iir = UART_IIR_THRI;
-    } else {
-        s->iir = UART_IIR_NO_INT;
-    }
-    if (s->iir != UART_IIR_NO_INT) {
-        qemu_irq_raise(s->irq);
-    } else {
+    uint8_t tmp_iir = UART_IIR_NO_INT;
+
+    if (!s->ier) {
         qemu_irq_lower(s->irq);
+        return;
     }
-}
 
-static void serial_tx_done(void *opaque)
-{
-    SerialState *s = opaque;
-
-    if (s->tx_burst < 0) {
-        uint16_t divider;
+    if ( ( s->ier & UART_IER_RLSI ) && (s->lsr & UART_LSR_INT_ANY ) ) {
+        tmp_iir = UART_IIR_RLSI;
+    } else if ( s->timeout_ipending ) {
+        tmp_iir = UART_IIR_CTI;
+    } else if ( ( s->ier & UART_IER_RDI ) && (s->lsr & UART_LSR_DR ) ) {
+        if ( !(s->iir & UART_FCR_FE) ) {
+           tmp_iir = UART_IIR_RDI;
+        } else if ( s->recv_fifo.count >= s->recv_fifo.itl ) {
+           tmp_iir = UART_IIR_RDI;
+        }
+    } else if ( (s->ier & UART_IER_THRI) && s->thr_ipending ) {
+        tmp_iir = UART_IIR_THRI;
+    } else if ( (s->ier & UART_IER_MSI) && (s->msr & UART_MSR_ANY_DELTA) ) {
+        tmp_iir = UART_IIR_MSI;
+    }
 
-        if (s->divider)
-          divider = s->divider;
-        else
-          divider = 1;
+    s->iir = tmp_iir | ( s->iir & 0xF0 );
 
-        /* We assume 10 bits/char, OK for this purpose. */
-        s->tx_burst = THROTTLE_TX_INTERVAL * 1000 /
-            (1000000 * 10 / (s->baudbase / divider));
+    if ( tmp_iir != UART_IIR_NO_INT ) {
+        qemu_irq_raise(s->irq);
+    } else {
+        qemu_irq_lower(s->irq);
     }
-    s->thr_ipending = 1;
-    s->lsr |= UART_LSR_THRE;
-    s->lsr |= UART_LSR_TEMT;
-    serial_update_irq(s);
 }
 
 static void serial_update_parameters(SerialState *s)
 {
-    int speed, parity, data_bits, stop_bits;
+    int speed, parity, data_bits, stop_bits, frame_size;
     QEMUSerialSetParams ssp;
 
+    if (s->divider == 0)
+        return;
+
+    frame_size = 1;
     if (s->lcr & 0x08) {
         if (s->lcr & 0x10)
             parity = 'E';
@@ -156,19 +255,21 @@ static void serial_update_parameters(SerialState *s)
             parity = 'O';
     } else {
             parity = 'N';
+            frame_size = 0;
     }
     if (s->lcr & 0x04)
         stop_bits = 2;
     else
         stop_bits = 1;
+
     data_bits = (s->lcr & 0x03) + 5;
-    if (s->divider == 0)
-        return;
+    frame_size += data_bits + stop_bits;
     speed = s->baudbase / s->divider;
     ssp.speed = speed;
     ssp.parity = parity;
     ssp.data_bits = data_bits;
     ssp.stop_bits = stop_bits;
+    s->char_transmit_time =  ( ticks_per_sec / speed ) * frame_size;
     qemu_chr_ioctl(s->chr, CHR_IOCTL_SERIAL_SET_PARAMS, &ssp);
 #if 0
     printf("speed=%d parity=%c data=%d stop=%d\n",
@@ -176,6 +277,128 @@ static void serial_update_parameters(SerialState *s)
 #endif
 }
 
+static void serial_get_token(void)
+{
+    static int64_t last_refil_time;
+    static int started;
+
+    assert(tokens_avail >= 0);
+    if (!tokens_avail) {
+	int64_t delta, now;
+	int64_t generated;
+
+	if (!started) {
+	    last_refil_time = qemu_get_clock (vm_clock);
+	    tokens_avail = TOKENS_MAX;
+	    started = 1;
+	    return;
+	}
+    retry:
+	now = qemu_get_clock (vm_clock);
+	delta = now - last_refil_time;
+	assert(delta >= 0);
+	if (delta % 1000000000 < TOKEN_PERIOD) {
+	    struct timespec ts;
+	    /* Wait until at least one token is available. */
+	    ts.tv_sec = TOKEN_PERIOD / 1000000000;
+	    ts.tv_nsec = TOKEN_PERIOD % 1000000000;
+	    while (nanosleep(&ts, &ts) < 0 && errno == EINTR)
+	        ;
+	    goto retry;
+	}
+	generated = delta / TOKEN_PERIOD;
+	assert(generated > 0);
+
+	last_refil_time += delta;
+	if (generated > TOKENS_MAX)
+	    generated = TOKENS_MAX;
+	tokens_avail = generated;
+    }
+    tokens_avail--;
+}
+
+static void serial_update_msl( SerialState *s )
+{
+    uint8_t omsr;
+    int flags;
+
+    qemu_del_timer(s->modem_status_poll);
+
+    if ( qemu_chr_ioctl(s->chr,CHR_IOCTL_SERIAL_GET_TIOCM, &flags) == -ENOTSUP ) {
+        s->poll_msl = -1;
+        return;
+    }
+
+    omsr = s->msr;
+
+    s->msr = ( flags & TIOCM_CTS ) ? s->msr | UART_MSR_CTS : s->msr & ~UART_MSR_CTS;
+    s->msr = ( flags & TIOCM_DSR ) ? s->msr | UART_MSR_DSR : s->msr & ~UART_MSR_DSR;
+    s->msr = ( flags & TIOCM_CAR ) ? s->msr | UART_MSR_DCD : s->msr & ~UART_MSR_DCD;
+    s->msr = ( flags & TIOCM_RI ) ? s->msr | UART_MSR_RI : s->msr & ~UART_MSR_RI;
+
+    if ( s->msr != omsr ) {
+         /* Set delta bits */
+         s->msr = s->msr | ( ( s->msr >> 4 ) ^ ( omsr >> 4 ) );
+         /* UART_MSR_TERI only if change was from 1 -> 0 */
+         if ( ( s->msr & UART_MSR_TERI ) && !( omsr & UART_MSR_RI ) )
+             s->msr &= ~UART_MSR_TERI;
+         serial_update_irq(s);
+    }
+
+    /* The real 16550A apparently has a 250ns response latency to line status changes.
+       We'll be lazy and poll only every 10ms, and only poll it at all if MSI interrupts are turned on */
+
+    if ( s->poll_msl )
+        qemu_mod_timer(s->modem_status_poll, qemu_get_clock(vm_clock) + ticks_per_sec / 100);
+}
+
+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->fcr & UART_FCR_FE) {
+            s->tsr = fifo_get(s,XMIT_FIFO);
+            if ( !s->xmit_fifo.count )
+                s->lsr |= UART_LSR_THRE;
+        } else {
+            s->tsr = s->thr;
+            s->lsr |= UART_LSR_THRE;
+        }
+    }
+
+    if ( (s->mcr & UART_MCR_LOOP
+	  /* in loopback mode, say that we just received a char */
+	  ? (serial_receive1(s, &s->tsr, 1), 1)
+	  : qemu_chr_write(s->chr, &s->tsr, 1))
+	 != 1 ) {
+        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;
+        } 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 );
+
+    if ( s->lsr & UART_LSR_THRE ) {
+        s->lsr |= UART_LSR_TEMT;
+        s->thr_ipending = 1;
+        serial_update_irq(s);
+    }
+}
+
+
 static void serial_ioport_write(void *opaque, uint32_t addr, uint32_t val)
 {
     SerialState *s = opaque;
@@ -192,25 +415,19 @@ static void serial_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             s->divider = (s->divider & 0xff00) | val;
             serial_update_parameters(s);
         } else {
+            s->thr = (uint8_t) val;
+            if(s->fcr & UART_FCR_FE) {
+                  fifo_put(s, XMIT_FIFO, s->thr);
             s->thr_ipending = 0;
+                  s->lsr &= ~UART_LSR_TEMT;
             s->lsr &= ~UART_LSR_THRE;
             serial_update_irq(s);
-            ch = val;
-            if (!(s->mcr & UART_MCR_LOOP)) {
-                /* when not in loopback mode, send the char */
-                qemu_chr_write(s->chr, &ch, 1);
             } else {
-                /* in loopback mode, say that we just received a char */
-                serial_receive_byte(s, ch);
-            }
-            if (s->tx_burst > 0) {
-                s->tx_burst--;
-                serial_tx_done(s);
-            } else if (s->tx_burst == 0) {
-                s->tx_burst--;
-                qemu_mod_timer(s->tx_timer, qemu_get_clock(vm_clock) +
-                               ticks_per_sec * THROTTLE_TX_INTERVAL / 1000);
+                  s->thr_ipending = 0;
+                  s->lsr &= ~UART_LSR_THRE;
+                  serial_update_irq(s);
             }
+            serial_xmit(s);
         }
         break;
     case 1:
@@ -219,13 +436,68 @@ static void serial_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             serial_update_parameters(s);
         } else {
             s->ier = val & 0x0f;
+            /* If the backend device is a real serial port, turn polling of the modem
+               status lines on physical port on or off depending on UART_IER_MSI state */
+            if ( s->poll_msl >= 0 ) { 
+                if ( s->ier & UART_IER_MSI ) {
+                     s->poll_msl = 1;
+                     serial_update_msl(s);
+                } else {
+                     qemu_del_timer(s->modem_status_poll);
+                     s->poll_msl = 0;
+                }
+            }
             if (s->lsr & UART_LSR_THRE) {
                 s->thr_ipending = 1;
+                serial_update_irq(s);
             }
-            serial_update_irq(s);
         }
         break;
     case 2:
+        val = val & 0xFF;
+
+        if ( s->fcr == val)
+            break;
+
+        /* Did the enable/disable flag change? If so, make sure FIFOs get flushed */
+        if ( (val ^ s->fcr) & UART_FCR_FE )
+            val |= UART_FCR_XFR | UART_FCR_RFR;
+
+        /* FIFO clear */
+
+        if ( val & UART_FCR_RFR ) {
+            qemu_del_timer(s->fifo_timeout_timer);
+            s->timeout_ipending=0;
+            fifo_clear(s,RECV_FIFO);
+        }
+
+        if ( val & UART_FCR_XFR ) {
+            fifo_clear(s,XMIT_FIFO);
+        }
+
+        if ( val & UART_FCR_FE ) {
+            s->iir |= UART_IIR_FE;
+            /* Set RECV_FIFO trigger Level */
+            switch ( val & 0xC0 ) {
+            case UART_FCR_ITL_1: 
+                s->recv_fifo.itl = 1;
+                break;
+            case UART_FCR_ITL_2:
+                s->recv_fifo.itl = 4;
+                break;
+            case UART_FCR_ITL_3:
+                s->recv_fifo.itl = 8;
+                break;
+            case UART_FCR_ITL_4:
+                s->recv_fifo.itl = 14;
+                break;
+            }
+        } else
+            s->iir &= ~UART_IIR_FE;
+
+        /* Set fcr - or at least the bits in it that are supposed to "stick" */
+        s->fcr = val & 0xC9;
+        serial_update_irq(s);
         break;
     case 3:
         {
@@ -241,7 +513,30 @@ static void serial_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         }
         break;
     case 4:
-        s->mcr = val & 0x1f;
+        {
+            int flags;
+            int old_mcr = s->mcr;
+            s->mcr = val & 0x1f;
+            if ( val & UART_MCR_LOOP )
+                break;
+
+            if ( s->poll_msl >= 0 && old_mcr != s->mcr ) {
+
+                qemu_chr_ioctl(s->chr,CHR_IOCTL_SERIAL_GET_TIOCM, &flags);
+
+                flags &= ~( TIOCM_RTS | TIOCM_DTR );
+
+                if ( val & UART_MCR_RTS )
+                    flags |= TIOCM_RTS;
+                if ( val & UART_MCR_DTR )
+                    flags |= TIOCM_DTR;
+
+                qemu_chr_ioctl(s->chr,CHR_IOCTL_SERIAL_SET_TIOCM, &flags);
+                /* Update the modem status after a one-character-send wait-time, since there may be a response
+                   from the device/computer at the other end of the serial line */
+                qemu_mod_timer(s->modem_status_poll, qemu_get_clock(vm_clock) + s->char_transmit_time );
+            }
+        }
         break;
     case 5:
         break;
@@ -265,8 +560,17 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t addr)
         if (s->lcr & UART_LCR_DLAB) {
             ret = s->divider & 0xff;
         } else {
-            ret = s->rbr;
-            s->lsr &= ~(UART_LSR_DR | UART_LSR_BI);
+            if(s->fcr & UART_FCR_FE) {
+                ret = fifo_get(s,RECV_FIFO);
+                if ( s->recv_fifo.count == 0 )
+                    s->lsr &= ~(UART_LSR_DR | UART_LSR_BI);
+                else
+                    qemu_mod_timer(s->fifo_timeout_timer, qemu_get_clock (vm_clock) + s->char_transmit_time * 4 );
+                s->timeout_ipending = 0;
+            } else {
+                ret = s->rbr;
+                s->lsr &= ~(UART_LSR_DR | UART_LSR_BI);
+            }
             serial_update_irq(s);
             if (!(s->mcr & UART_MCR_LOOP)) {
                 /* in loopback mode, don't receive any data */
@@ -283,8 +587,6 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t addr)
         break;
     case 2:
         ret = s->iir;
-        /* reset THR pending bit */
-        if ((ret & 0x7) == UART_IIR_THRI)
             s->thr_ipending = 0;
         serial_update_irq(s);
         break;
@@ -295,9 +597,16 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t addr)
         ret = s->mcr;
         break;
     case 5:
+        serial_get_token();
         ret = s->lsr;
+        /* Clear break interrupt */
+        if ( s->lsr & UART_LSR_BI ) {
+            s->lsr &= ~UART_LSR_BI;
+            serial_update_irq(s);
+        }
         break;
     case 6:
+        serial_get_token();
         if (s->mcr & UART_MCR_LOOP) {
             /* in loopback, the modem output pins are connected to the
                inputs */
@@ -305,7 +614,14 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t addr)
             ret |= (s->mcr & 0x02) << 3;
             ret |= (s->mcr & 0x01) << 5;
         } else {
+            if ( s->poll_msl >= 0 )
+                serial_update_msl(s);
             ret = s->msr;
+            /* Clear delta bits & msr int after read, if they were set */
+            if ( s->msr & UART_MSR_ANY_DELTA ) {
+                s->msr &= 0xF0;
+                serial_update_irq(s);
+            }
         }
         break;
     case 7:
@@ -320,14 +636,17 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t addr)
 
 static int serial_can_receive(SerialState *s)
 {
+    if(s->fcr & UART_FCR_FE) {
+        if(s->recv_fifo.count < UART_FIFO_LENGTH)
+        /* Advertise (fifo.itl - fifo.count) bytes when count < ITL, and 1 if above. If UART_FIFO_LENGTH - fifo.count is
+        advertised the effect will be to almost always fill the fifo completely before the guest has a chance to respond,
+        effectively overriding the ITL that the guest has set. */
+             return ( s->recv_fifo.count <= s->recv_fifo.itl ) ? s->recv_fifo.itl - s->recv_fifo.count : 1;
+        else
+             return 0;
+    } else {
     return !(s->lsr & UART_LSR_DR);
-}
-
-static void serial_receive_byte(SerialState *s, int ch)
-{
-    s->rbr = ch;
-    s->lsr |= UART_LSR_DR;
-    serial_update_irq(s);
+    }
 }
 
 static void serial_receive_break(SerialState *s)
@@ -337,6 +656,15 @@ static void serial_receive_break(SerialState *s)
     serial_update_irq(s);
 }
 
+/* There's data in recv_fifo and s->rbr has not been read for 4 char transmit times */
+static void fifo_timeout_int (void *opaque) {
+    SerialState *s = opaque;
+    if ( s->recv_fifo.count ) {
+        s->timeout_ipending = 1;
+        serial_update_irq(s);
+    }
+}
+
 static int serial_can_receive1(void *opaque)
 {
     SerialState *s = opaque;
@@ -346,12 +674,26 @@ static int serial_can_receive1(void *opaque)
 static void serial_receive1(void *opaque, const uint8_t *buf, int size)
 {
     SerialState *s = opaque;
-    serial_receive_byte(s, buf[0]);
+    tokens_avail = TOKENS_MAX;
+    if(s->fcr & UART_FCR_FE) {
+        int i;
+        for (i = 0; i < size; i++) {
+            fifo_put(s, RECV_FIFO, buf[i]);
+        }
+        s->lsr |= UART_LSR_DR;
+        /* call the timeout receive callback in 4 char transmit time */
+        qemu_mod_timer(s->fifo_timeout_timer, qemu_get_clock (vm_clock) + s->char_transmit_time * 4);
+    } else {
+        s->rbr = buf[0];
+        s->lsr |= UART_LSR_DR;
+    }
+    serial_update_irq(s);
 }
 
 static void serial_event(void *opaque, int event)
 {
     SerialState *s = opaque;
+    tokens_avail = TOKENS_MAX;
     if (event == CHR_EVENT_BREAK)
         serial_receive_break(s);
 }
@@ -369,11 +711,13 @@ static void serial_save(QEMUFile *f, void *opaque)
     qemu_put_8s(f,&s->lsr);
     qemu_put_8s(f,&s->msr);
     qemu_put_8s(f,&s->scr);
+    qemu_put_8s(f,&s->fcr);
 }
 
 static int serial_load(QEMUFile *f, void *opaque, int version_id)
 {
     SerialState *s = opaque;
+    uint8_t fcr = 0;
 
     if(version_id > 2)
         return -EINVAL;
@@ -391,6 +735,11 @@ static int serial_load(QEMUFile *f, void *opaque, int version_id)
     qemu_get_8s(f,&s->msr);
     qemu_get_8s(f,&s->scr);
 
+    if (version_id >= 2)
+        qemu_get_8s(f,&fcr);
+
+    /* Initialize fcr via setter to perform essential side-effects */
+    serial_ioport_write(s, 0x02, fcr);
     return 0;
 }
 
@@ -398,21 +747,47 @@ static void serial_reset(void *opaque)
 {
     SerialState *s = opaque;
 
-    s->divider = 0;
     s->rbr = 0;
     s->ier = 0;
     s->iir = UART_IIR_NO_INT;
     s->lcr = 0;
-    s->mcr = 0;
     s->lsr = UART_LSR_TEMT | UART_LSR_THRE;
     s->msr = UART_MSR_DCD | UART_MSR_DSR | UART_MSR_CTS;
+    /* Default to 9600 baud, no parity, one stop bit */
+    s->divider = 0x0C;
+    s->mcr = UART_MCR_OUT2;
     s->scr = 0;
+    s->tsr_retry = 0;
+    s->char_transmit_time = ( ticks_per_sec / 9600 ) * 9;
+    s->poll_msl = 0;
+
+    fifo_clear(s,RECV_FIFO);
+    fifo_clear(s,XMIT_FIFO);
+
+    s->last_xmit_ts = qemu_get_clock(vm_clock);
 
     s->thr_ipending = 0;
     s->last_break_enable = 0;
     qemu_irq_lower(s->irq);
 }
 
+static void serial_init_core(SerialState *s, qemu_irq irq, int baudbase,
+			     CharDriverState *chr)
+{
+    s->irq = irq;
+    s->baudbase = baudbase;
+    s->chr = chr;
+
+    s->modem_status_poll = qemu_new_timer(vm_clock, ( QEMUTimerCB *) serial_update_msl, s); 
+ 
+    s->fifo_timeout_timer = qemu_new_timer(vm_clock, ( QEMUTimerCB *) fifo_timeout_int, s);
+    s->transmit_timer = qemu_new_timer(vm_clock, ( QEMUTimerCB *) serial_xmit, s);
+
+    qemu_register_reset(serial_reset, s);
+    serial_reset(s);
+
+}
+
 /* If fd is zero, it means that the serial device uses the console */
 SerialState *serial_init(int base, qemu_irq irq, int baudbase,
                          CharDriverState *chr)
@@ -422,21 +797,13 @@ SerialState *serial_init(int base, qemu_irq irq, int baudbase,
     s = qemu_mallocz(sizeof(SerialState));
     if (!s)
         return NULL;
-    s->irq = irq;
-    s->baudbase = baudbase;
-
-    s->tx_timer = qemu_new_timer(vm_clock, serial_tx_done, s);
-    if (!s->tx_timer)
-        return NULL;
 
-    qemu_register_reset(serial_reset, s);
-    serial_reset(s);
+    serial_init_core(s, irq, baudbase, chr);
 
     register_savevm("serial", base, 2, serial_save, serial_load, s);
 
     register_ioport_write(base, 8, 1, serial_ioport_write, s);
     register_ioport_read(base, 8, 1, serial_ioport_read, s);
-    s->chr = chr;
     qemu_chr_add_handlers(chr, serial_can_receive1, serial_receive1,
                           serial_event, s);
     return s;
@@ -524,18 +891,11 @@ SerialState *serial_mm_init (target_phys_addr_t base, int it_shift,
     s = qemu_mallocz(sizeof(SerialState));
     if (!s)
         return NULL;
-    s->irq = irq;
+
     s->base = base;
     s->it_shift = it_shift;
-    s->baudbase= baudbase;
-
-    s->tx_timer = qemu_new_timer(vm_clock, serial_tx_done, s);
-    if (!s->tx_timer)
-        return NULL;
-
-    qemu_register_reset(serial_reset, s);
-    serial_reset(s);
 
+    serial_init_core(s, irq, baudbase, chr);
     register_savevm("serial", base, 2, serial_save, serial_load, s);
 
     if (ioregister) {
@@ -543,8 +903,8 @@ SerialState *serial_mm_init (target_phys_addr_t base, int it_shift,
                                              serial_mm_write, s);
         cpu_register_physical_memory(base, 8 << it_shift, s_io_memory);
     }
-    s->chr = chr;
     qemu_chr_add_handlers(chr, serial_can_receive1, serial_receive1,
                           serial_event, s);
+    serial_update_msl(s);
     return s;
 }
diff --git a/qemu-char.h b/qemu-char.h
index 29de03d..41936bc 100644
--- a/qemu-char.h
+++ b/qemu-char.h
@@ -28,6 +28,9 @@ typedef struct {
 #define CHR_IOCTL_PP_EPP_WRITE_ADDR  10
 #define CHR_IOCTL_PP_EPP_WRITE       11
 
+#define CHR_IOCTL_SERIAL_SET_TIOCM   12
+#define CHR_IOCTL_SERIAL_GET_TIOCM   13
+
 typedef void IOEventHandler(void *opaque, int event);
 
 struct CharDriverState {
diff --git a/vl.c b/vl.c
index 03cd386..e9177b1 100644
--- a/vl.c
+++ b/vl.c
@@ -2721,6 +2721,17 @@ static int tty_serial_ioctl(CharDriverState *chr, int cmd, void *arg)
                 tcsendbreak(s->fd_in, 1);
         }
         break;
+    case CHR_IOCTL_SERIAL_GET_TIOCM:
+        {
+            ioctl(s->fd_in, TIOCMGET, arg);
+        }
+        break;
+    case CHR_IOCTL_SERIAL_SET_TIOCM:
+        {
+            ioctl(s->fd_in, TIOCMSET, arg);
+        }
+        break;
+
     default:
         return -ENOTSUP;
     }