[v3,2/2] hw/char: sifive_uart: Print uart characters async

The current approach of using qemu_chr_fe_write() and ignoring the
return values results in dropped characters [1].

Let's update the SiFive UART to use a async sifive_uart_xmit() function
to transmit the characters and apply back pressure to the guest with
the SIFIVE_UART_TXFIFO_FULL status.

This should avoid dropped characters and more realisticly model the
hardware.

1: https://gitlab.com/qemu-project/qemu/-/issues/2114

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Thomas Huth <thuth@redhat.com>
---
 include/hw/char/sifive_uart.h | 17 ++++++-
 hw/char/sifive_uart.c         | 88 +++++++++++++++++++++++++++++++++--
 2 files changed, 99 insertions(+), 6 deletions(-)

On 9/8/24 11:13 PM, Alistair Francis wrote:
> The current approach of using qemu_chr_fe_write() and ignoring the
> return values results in dropped characters [1].
> 
> Let's update the SiFive UART to use a async sifive_uart_xmit() function
> to transmit the characters and apply back pressure to the guest with
> the SIFIVE_UART_TXFIFO_FULL status.
> 
> This should avoid dropped characters and more realisticly model the
> hardware.
> 
> 1: https://gitlab.com/qemu-project/qemu/-/issues/2114
> 
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> Tested-by: Thomas Huth <thuth@redhat.com>

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>

> ---
>   include/hw/char/sifive_uart.h | 17 ++++++-
>   hw/char/sifive_uart.c         | 88 +++++++++++++++++++++++++++++++++--
>   2 files changed, 99 insertions(+), 6 deletions(-)
> 
> diff --git a/include/hw/char/sifive_uart.h b/include/hw/char/sifive_uart.h
> index 7f6c79f8bd..b43109bb8b 100644
> --- a/include/hw/char/sifive_uart.h
> +++ b/include/hw/char/sifive_uart.h
> @@ -24,6 +24,7 @@
>   #include "hw/qdev-properties.h"
>   #include "hw/sysbus.h"
>   #include "qom/object.h"
> +#include "qemu/fifo8.h"
>   
>   enum {
>       SIFIVE_UART_TXFIFO        = 0,
> @@ -48,9 +49,13 @@ enum {
>       SIFIVE_UART_IP_RXWM       = 2  /* Receive watermark interrupt pending */
>   };
>   
> +#define SIFIVE_UART_TXFIFO_FULL    0x80000000
> +
>   #define SIFIVE_UART_GET_TXCNT(txctrl)   ((txctrl >> 16) & 0x7)
>   #define SIFIVE_UART_GET_RXCNT(rxctrl)   ((rxctrl >> 16) & 0x7)
> +
>   #define SIFIVE_UART_RX_FIFO_SIZE 8
> +#define SIFIVE_UART_TX_FIFO_SIZE 8
>   
>   #define TYPE_SIFIVE_UART "riscv.sifive.uart"
>   OBJECT_DECLARE_SIMPLE_TYPE(SiFiveUARTState, SIFIVE_UART)
> @@ -63,13 +68,21 @@ struct SiFiveUARTState {
>       qemu_irq irq;
>       MemoryRegion mmio;
>       CharBackend chr;
> -    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> -    uint8_t rx_fifo_len;
> +
> +    uint32_t txfifo;
>       uint32_t ie;
>       uint32_t ip;
>       uint32_t txctrl;
>       uint32_t rxctrl;
>       uint32_t div;
> +
> +    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> +    uint8_t rx_fifo_len;
> +
> +    Fifo8 tx_fifo;
> +
> +    QEMUTimer *fifo_trigger_handle;
> +    uint64_t char_tx_time;
>   };
>   
>   SiFiveUARTState *sifive_uart_create(MemoryRegion *address_space, hwaddr base,
> diff --git a/hw/char/sifive_uart.c b/hw/char/sifive_uart.c
> index 7fc6787f69..ab899b60d6 100644
> --- a/hw/char/sifive_uart.c
> +++ b/hw/char/sifive_uart.c
> @@ -64,6 +64,72 @@ static void sifive_uart_update_irq(SiFiveUARTState *s)
>       }
>   }
>   
> +static gboolean sifive_uart_xmit(void *do_not_use, GIOCondition cond,
> +                                 void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +    int ret;
> +    const uint8_t *charecters;
> +    uint32_t numptr = 0;
> +
> +    /* instant drain the fifo when there's no back-end */
> +    if (!qemu_chr_fe_backend_connected(&s->chr)) {
> +        fifo8_reset(&s->tx_fifo);
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    if (fifo8_is_empty(&s->tx_fifo)) {
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    /* Don't pop the FIFO in case the write fails */
> +    charecters = fifo8_peek_bufptr(&s->tx_fifo,
> +                                   fifo8_num_used(&s->tx_fifo), &numptr);
> +    ret = qemu_chr_fe_write(&s->chr, charecters, numptr);
> +
> +    if (ret >= 0) {
> +        /* We wrote the data, actually pop the fifo */
> +        fifo8_pop_bufptr(&s->tx_fifo, ret, NULL);
> +    }
> +
> +    if (!fifo8_is_empty(&s->tx_fifo)) {
> +        guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,
> +                                        sifive_uart_xmit, s);
> +        if (!r) {
> +            fifo8_reset(&s->tx_fifo);
> +            return G_SOURCE_REMOVE;
> +        }
> +    }
> +
> +    /* Clear the TX Full bit */
> +    if (!fifo8_is_full(&s->tx_fifo)) {
> +        s->txfifo &= ~SIFIVE_UART_TXFIFO_FULL;
> +    }
> +
> +    sifive_uart_update_irq(s);
> +    return G_SOURCE_REMOVE;
> +}
> +
> +static void sifive_uart_write_tx_fifo(SiFiveUARTState *s, const uint8_t *buf,
> +                                      int size)
> +{
> +    uint64_t current_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +
> +    if (size > fifo8_num_free(&s->tx_fifo)) {
> +        size = fifo8_num_free(&s->tx_fifo);
> +        qemu_log_mask(LOG_GUEST_ERROR, "sifive_uart: TX FIFO overflow");
> +    }
> +
> +    fifo8_push_all(&s->tx_fifo, buf, size);
> +
> +    if (fifo8_is_full(&s->tx_fifo)) {
> +        s->txfifo |= SIFIVE_UART_TXFIFO_FULL;
> +    }
> +
> +    timer_mod(s->fifo_trigger_handle, current_time +
> +              (s->char_tx_time * 4));
> +}
> +
>   static uint64_t
>   sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
>   {
> @@ -82,7 +148,7 @@ sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
>           return 0x80000000;
>   
>       case SIFIVE_UART_TXFIFO:
> -        return 0; /* Should check tx fifo */
> +        return s->txfifo;
>       case SIFIVE_UART_IE:
>           return s->ie;
>       case SIFIVE_UART_IP:
> @@ -106,12 +172,10 @@ sifive_uart_write(void *opaque, hwaddr addr,
>   {
>       SiFiveUARTState *s = opaque;
>       uint32_t value = val64;
> -    unsigned char ch = value;
>   
>       switch (addr) {
>       case SIFIVE_UART_TXFIFO:
> -        qemu_chr_fe_write(&s->chr, &ch, 1);
> -        sifive_uart_update_irq(s);
> +        sifive_uart_write_tx_fifo(s, (uint8_t *) &value, 1);
>           return;
>       case SIFIVE_UART_IE:
>           s->ie = val64;
> @@ -131,6 +195,13 @@ sifive_uart_write(void *opaque, hwaddr addr,
>                     __func__, (int)addr, (int)value);
>   }
>   
> +static void fifo_trigger_update(void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +
> +    sifive_uart_xmit(NULL, G_IO_OUT, s);
> +}
> +
>   static const MemoryRegionOps sifive_uart_ops = {
>       .read = sifive_uart_read,
>       .write = sifive_uart_write,
> @@ -197,6 +268,9 @@ static void sifive_uart_realize(DeviceState *dev, Error **errp)
>   {
>       SiFiveUARTState *s = SIFIVE_UART(dev);
>   
> +    s->fifo_trigger_handle = timer_new_ns(QEMU_CLOCK_VIRTUAL,
> +                                          fifo_trigger_update, s);
> +
>       qemu_chr_fe_set_handlers(&s->chr, sifive_uart_can_rx, sifive_uart_rx,
>                                sifive_uart_event, sifive_uart_be_change, s,
>                                NULL, true);
> @@ -206,12 +280,18 @@ static void sifive_uart_realize(DeviceState *dev, Error **errp)
>   static void sifive_uart_reset_enter(Object *obj, ResetType type)
>   {
>       SiFiveUARTState *s = SIFIVE_UART(obj);
> +
> +    s->txfifo = 0;
>       s->ie = 0;
>       s->ip = 0;
>       s->txctrl = 0;
>       s->rxctrl = 0;
>       s->div = 0;
> +
>       s->rx_fifo_len = 0;
> +
> +    memset(s->rx_fifo, 0, SIFIVE_UART_RX_FIFO_SIZE);
> +    fifo8_create(&s->tx_fifo, SIFIVE_UART_TX_FIFO_SIZE);
>   }
>   
>   static void sifive_uart_reset_hold(Object *obj, ResetType type)

Hi Alistair,

On 9/9/24 04:13, Alistair Francis wrote:
> The current approach of using qemu_chr_fe_write() and ignoring the
> return values results in dropped characters [1].
> 
> Let's update the SiFive UART to use a async sifive_uart_xmit() function
> to transmit the characters and apply back pressure to the guest with
> the SIFIVE_UART_TXFIFO_FULL status.
> 
> This should avoid dropped characters and more realisticly model the
> hardware.
> 
> 1: https://gitlab.com/qemu-project/qemu/-/issues/2114
> 
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> Tested-by: Thomas Huth <thuth@redhat.com>
> ---
>   include/hw/char/sifive_uart.h | 17 ++++++-
>   hw/char/sifive_uart.c         | 88 +++++++++++++++++++++++++++++++++--
>   2 files changed, 99 insertions(+), 6 deletions(-)
> 
> diff --git a/include/hw/char/sifive_uart.h b/include/hw/char/sifive_uart.h
> index 7f6c79f8bd..b43109bb8b 100644
> --- a/include/hw/char/sifive_uart.h
> +++ b/include/hw/char/sifive_uart.h
> @@ -24,6 +24,7 @@
>   #include "hw/qdev-properties.h"
>   #include "hw/sysbus.h"
>   #include "qom/object.h"
> +#include "qemu/fifo8.h"
>   
>   enum {
>       SIFIVE_UART_TXFIFO        = 0,
> @@ -48,9 +49,13 @@ enum {
>       SIFIVE_UART_IP_RXWM       = 2  /* Receive watermark interrupt pending */
>   };
>   
> +#define SIFIVE_UART_TXFIFO_FULL    0x80000000
> +
>   #define SIFIVE_UART_GET_TXCNT(txctrl)   ((txctrl >> 16) & 0x7)
>   #define SIFIVE_UART_GET_RXCNT(rxctrl)   ((rxctrl >> 16) & 0x7)
> +
>   #define SIFIVE_UART_RX_FIFO_SIZE 8
> +#define SIFIVE_UART_TX_FIFO_SIZE 8
>   
>   #define TYPE_SIFIVE_UART "riscv.sifive.uart"
>   OBJECT_DECLARE_SIMPLE_TYPE(SiFiveUARTState, SIFIVE_UART)
> @@ -63,13 +68,21 @@ struct SiFiveUARTState {
>       qemu_irq irq;
>       MemoryRegion mmio;
>       CharBackend chr;
> -    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> -    uint8_t rx_fifo_len;
> +
> +    uint32_t txfifo;
>       uint32_t ie;
>       uint32_t ip;
>       uint32_t txctrl;
>       uint32_t rxctrl;
>       uint32_t div;
> +
> +    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> +    uint8_t rx_fifo_len;
> +
> +    Fifo8 tx_fifo;
> +
> +    QEMUTimer *fifo_trigger_handle;
> +    uint64_t char_tx_time;

Unfortunately some fields now need to be migrated and
tracked in vmstate_sifive_uart[].

>   };
>   
>   SiFiveUARTState *sifive_uart_create(MemoryRegion *address_space, hwaddr base,
> diff --git a/hw/char/sifive_uart.c b/hw/char/sifive_uart.c
> index 7fc6787f69..ab899b60d6 100644
> --- a/hw/char/sifive_uart.c
> +++ b/hw/char/sifive_uart.c
> @@ -64,6 +64,72 @@ static void sifive_uart_update_irq(SiFiveUARTState *s)
>       }
>   }
>   
> +static gboolean sifive_uart_xmit(void *do_not_use, GIOCondition cond,
> +                                 void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +    int ret;
> +    const uint8_t *charecters;

"characters" ;)

> +    uint32_t numptr = 0;
> +
> +    /* instant drain the fifo when there's no back-end */
> +    if (!qemu_chr_fe_backend_connected(&s->chr)) {
> +        fifo8_reset(&s->tx_fifo);
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    if (fifo8_is_empty(&s->tx_fifo)) {
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    /* Don't pop the FIFO in case the write fails */
> +    charecters = fifo8_peek_bufptr(&s->tx_fifo,
> +                                   fifo8_num_used(&s->tx_fifo), &numptr);
> +    ret = qemu_chr_fe_write(&s->chr, charecters, numptr);
> +
> +    if (ret >= 0) {
> +        /* We wrote the data, actually pop the fifo */
> +        fifo8_pop_bufptr(&s->tx_fifo, ret, NULL);
> +    }
> +
> +    if (!fifo8_is_empty(&s->tx_fifo)) {
> +        guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,
> +                                        sifive_uart_xmit, s);
> +        if (!r) {
> +            fifo8_reset(&s->tx_fifo);
> +            return G_SOURCE_REMOVE;
> +        }
> +    }
> +
> +    /* Clear the TX Full bit */
> +    if (!fifo8_is_full(&s->tx_fifo)) {
> +        s->txfifo &= ~SIFIVE_UART_TXFIFO_FULL;
> +    }
> +
> +    sifive_uart_update_irq(s);
> +    return G_SOURCE_REMOVE;

Alex suggested to see if we can have a generic (abstract?) FIFO char
implementation. I might have a look later when I get the PL011 series
in.

> +}
> +
> +static void sifive_uart_write_tx_fifo(SiFiveUARTState *s, const uint8_t *buf,
> +                                      int size)
> +{
> +    uint64_t current_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +
> +    if (size > fifo8_num_free(&s->tx_fifo)) {
> +        size = fifo8_num_free(&s->tx_fifo);
> +        qemu_log_mask(LOG_GUEST_ERROR, "sifive_uart: TX FIFO overflow");
> +    }
> +
> +    fifo8_push_all(&s->tx_fifo, buf, size);
> +
> +    if (fifo8_is_full(&s->tx_fifo)) {
> +        s->txfifo |= SIFIVE_UART_TXFIFO_FULL;
> +    }
> +
> +    timer_mod(s->fifo_trigger_handle, current_time +
> +              (s->char_tx_time * 4));
> +}
> +
>   static uint64_t
>   sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
>   {
> @@ -82,7 +148,7 @@ sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
>           return 0x80000000;
>   
>       case SIFIVE_UART_TXFIFO:
> -        return 0; /* Should check tx fifo */
> +        return s->txfifo;
>       case SIFIVE_UART_IE:
>           return s->ie;
>       case SIFIVE_UART_IP:
> @@ -106,12 +172,10 @@ sifive_uart_write(void *opaque, hwaddr addr,
>   {
>       SiFiveUARTState *s = opaque;
>       uint32_t value = val64;
> -    unsigned char ch = value;
>   
>       switch (addr) {
>       case SIFIVE_UART_TXFIFO:
> -        qemu_chr_fe_write(&s->chr, &ch, 1);
> -        sifive_uart_update_irq(s);
> +        sifive_uart_write_tx_fifo(s, (uint8_t *) &value, 1);
>           return;
>       case SIFIVE_UART_IE:
>           s->ie = val64;
> @@ -131,6 +195,13 @@ sifive_uart_write(void *opaque, hwaddr addr,
>                     __func__, (int)addr, (int)value);
>   }
>   
> +static void fifo_trigger_update(void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +
> +    sifive_uart_xmit(NULL, G_IO_OUT, s);
> +}
> +
>   static const MemoryRegionOps sifive_uart_ops = {
>       .read = sifive_uart_read,
>       .write = sifive_uart_write,
> @@ -197,6 +268,9 @@ static void sifive_uart_realize(DeviceState *dev, Error **errp)
>   {
>       SiFiveUARTState *s = SIFIVE_UART(dev);
>   
> +    s->fifo_trigger_handle = timer_new_ns(QEMU_CLOCK_VIRTUAL,
> +                                          fifo_trigger_update, s);
> +
>       qemu_chr_fe_set_handlers(&s->chr, sifive_uart_can_rx, sifive_uart_rx,
>                                sifive_uart_event, sifive_uart_be_change, s,
>                                NULL, true);
> @@ -206,12 +280,18 @@ static void sifive_uart_realize(DeviceState *dev, Error **errp)
>   static void sifive_uart_reset_enter(Object *obj, ResetType type)
>   {
>       SiFiveUARTState *s = SIFIVE_UART(obj);
> +
> +    s->txfifo = 0;
>       s->ie = 0;
>       s->ip = 0;
>       s->txctrl = 0;
>       s->rxctrl = 0;
>       s->div = 0;
> +
>       s->rx_fifo_len = 0;
> +
> +    memset(s->rx_fifo, 0, SIFIVE_UART_RX_FIFO_SIZE);
> +    fifo8_create(&s->tx_fifo, SIFIVE_UART_TX_FIFO_SIZE);
>   }
>   
>   static void sifive_uart_reset_hold(Object *obj, ResetType type)

Hi Alistair,

On 9/9/24 04:13, Alistair Francis wrote:
> The current approach of using qemu_chr_fe_write() and ignoring the
> return values results in dropped characters [1].
> 
> Let's update the SiFive UART to use a async sifive_uart_xmit() function
> to transmit the characters and apply back pressure to the guest with
> the SIFIVE_UART_TXFIFO_FULL status.
> 
> This should avoid dropped characters and more realisticly model the
> hardware.
> 
> 1: https://gitlab.com/qemu-project/qemu/-/issues/2114
> 
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> Tested-by: Thomas Huth <thuth@redhat.com>
> ---
>   include/hw/char/sifive_uart.h | 17 ++++++-
>   hw/char/sifive_uart.c         | 88 +++++++++++++++++++++++++++++++++--
>   2 files changed, 99 insertions(+), 6 deletions(-)
> 
> diff --git a/include/hw/char/sifive_uart.h b/include/hw/char/sifive_uart.h
> index 7f6c79f8bd..b43109bb8b 100644
> --- a/include/hw/char/sifive_uart.h
> +++ b/include/hw/char/sifive_uart.h
> @@ -24,6 +24,7 @@
>   #include "hw/qdev-properties.h"
>   #include "hw/sysbus.h"
>   #include "qom/object.h"
> +#include "qemu/fifo8.h"
>   
>   enum {
>       SIFIVE_UART_TXFIFO        = 0,
> @@ -48,9 +49,13 @@ enum {
>       SIFIVE_UART_IP_RXWM       = 2  /* Receive watermark interrupt pending */
>   };
>   
> +#define SIFIVE_UART_TXFIFO_FULL    0x80000000
> +
>   #define SIFIVE_UART_GET_TXCNT(txctrl)   ((txctrl >> 16) & 0x7)
>   #define SIFIVE_UART_GET_RXCNT(rxctrl)   ((rxctrl >> 16) & 0x7)
> +
>   #define SIFIVE_UART_RX_FIFO_SIZE 8
> +#define SIFIVE_UART_TX_FIFO_SIZE 8
>   
>   #define TYPE_SIFIVE_UART "riscv.sifive.uart"
>   OBJECT_DECLARE_SIMPLE_TYPE(SiFiveUARTState, SIFIVE_UART)
> @@ -63,13 +68,21 @@ struct SiFiveUARTState {
>       qemu_irq irq;
>       MemoryRegion mmio;
>       CharBackend chr;
> -    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> -    uint8_t rx_fifo_len;
> +
> +    uint32_t txfifo;
>       uint32_t ie;
>       uint32_t ip;
>       uint32_t txctrl;
>       uint32_t rxctrl;
>       uint32_t div;
> +
> +    uint8_t rx_fifo[SIFIVE_UART_RX_FIFO_SIZE];
> +    uint8_t rx_fifo_len;
> +
> +    Fifo8 tx_fifo;
> +
> +    QEMUTimer *fifo_trigger_handle;
> +    uint64_t char_tx_time;

Unfortunately some fields now need to be migrated and
tracked in vmstate_sifive_uart[].

>   };
>   
>   SiFiveUARTState *sifive_uart_create(MemoryRegion *address_space, hwaddr base,
> diff --git a/hw/char/sifive_uart.c b/hw/char/sifive_uart.c
> index 7fc6787f69..ab899b60d6 100644
> --- a/hw/char/sifive_uart.c
> +++ b/hw/char/sifive_uart.c
> @@ -64,6 +64,72 @@ static void sifive_uart_update_irq(SiFiveUARTState *s)
>       }
>   }
>   
> +static gboolean sifive_uart_xmit(void *do_not_use, GIOCondition cond,
> +                                 void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +    int ret;
> +    const uint8_t *charecters;

"characters" ;)

> +    uint32_t numptr = 0;
> +
> +    /* instant drain the fifo when there's no back-end */
> +    if (!qemu_chr_fe_backend_connected(&s->chr)) {
> +        fifo8_reset(&s->tx_fifo);
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    if (fifo8_is_empty(&s->tx_fifo)) {
> +        return G_SOURCE_REMOVE;
> +    }
> +
> +    /* Don't pop the FIFO in case the write fails */
> +    charecters = fifo8_peek_bufptr(&s->tx_fifo,
> +                                   fifo8_num_used(&s->tx_fifo), &numptr);
> +    ret = qemu_chr_fe_write(&s->chr, charecters, numptr);
> +
> +    if (ret >= 0) {
> +        /* We wrote the data, actually pop the fifo */
> +        fifo8_pop_bufptr(&s->tx_fifo, ret, NULL);
> +    }
> +
> +    if (!fifo8_is_empty(&s->tx_fifo)) {
> +        guint r = qemu_chr_fe_add_watch(&s->chr, G_IO_OUT | G_IO_HUP,
> +                                        sifive_uart_xmit, s);
> +        if (!r) {
> +            fifo8_reset(&s->tx_fifo);
> +            return G_SOURCE_REMOVE;
> +        }
> +    }
> +
> +    /* Clear the TX Full bit */
> +    if (!fifo8_is_full(&s->tx_fifo)) {
> +        s->txfifo &= ~SIFIVE_UART_TXFIFO_FULL;
> +    }
> +
> +    sifive_uart_update_irq(s);
> +    return G_SOURCE_REMOVE;

Alex suggested to see if we can have a generic (abstract?) FIFO char
implementation. I might have a look later when I get the PL011 series
in.

> +}
> +
> +static void sifive_uart_write_tx_fifo(SiFiveUARTState *s, const uint8_t *buf,
> +                                      int size)
> +{
> +    uint64_t current_time = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
> +
> +    if (size > fifo8_num_free(&s->tx_fifo)) {
> +        size = fifo8_num_free(&s->tx_fifo);
> +        qemu_log_mask(LOG_GUEST_ERROR, "sifive_uart: TX FIFO overflow");
> +    }
> +
> +    fifo8_push_all(&s->tx_fifo, buf, size);
> +
> +    if (fifo8_is_full(&s->tx_fifo)) {
> +        s->txfifo |= SIFIVE_UART_TXFIFO_FULL;
> +    }
> +
> +    timer_mod(s->fifo_trigger_handle, current_time +
> +              (s->char_tx_time * 4));
> +}
> +
>   static uint64_t
>   sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
>   {
> @@ -82,7 +148,7 @@ sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
>           return 0x80000000;
>   
>       case SIFIVE_UART_TXFIFO:
> -        return 0; /* Should check tx fifo */
> +        return s->txfifo;
>       case SIFIVE_UART_IE:
>           return s->ie;
>       case SIFIVE_UART_IP:
> @@ -106,12 +172,10 @@ sifive_uart_write(void *opaque, hwaddr addr,
>   {
>       SiFiveUARTState *s = opaque;
>       uint32_t value = val64;
> -    unsigned char ch = value;
>   
>       switch (addr) {
>       case SIFIVE_UART_TXFIFO:
> -        qemu_chr_fe_write(&s->chr, &ch, 1);
> -        sifive_uart_update_irq(s);
> +        sifive_uart_write_tx_fifo(s, (uint8_t *) &value, 1);
>           return;
>       case SIFIVE_UART_IE:
>           s->ie = val64;
> @@ -131,6 +195,13 @@ sifive_uart_write(void *opaque, hwaddr addr,
>                     __func__, (int)addr, (int)value);
>   }
>   
> +static void fifo_trigger_update(void *opaque)
> +{
> +    SiFiveUARTState *s = opaque;
> +
> +    sifive_uart_xmit(NULL, G_IO_OUT, s);
> +}
> +
>   static const MemoryRegionOps sifive_uart_ops = {
>       .read = sifive_uart_read,
>       .write = sifive_uart_write,
> @@ -197,6 +268,9 @@ static void sifive_uart_realize(DeviceState *dev, Error **errp)
>   {
>       SiFiveUARTState *s = SIFIVE_UART(dev);
>   
> +    s->fifo_trigger_handle = timer_new_ns(QEMU_CLOCK_VIRTUAL,
> +                                          fifo_trigger_update, s);
> +
>       qemu_chr_fe_set_handlers(&s->chr, sifive_uart_can_rx, sifive_uart_rx,
>                                sifive_uart_event, sifive_uart_be_change, s,
>                                NULL, true);
> @@ -206,12 +280,18 @@ static void sifive_uart_realize(DeviceState *dev, Error **errp)
>   static void sifive_uart_reset_enter(Object *obj, ResetType type)
>   {
>       SiFiveUARTState *s = SIFIVE_UART(obj);
> +
> +    s->txfifo = 0;
>       s->ie = 0;
>       s->ip = 0;
>       s->txctrl = 0;
>       s->rxctrl = 0;
>       s->div = 0;
> +
>       s->rx_fifo_len = 0;
> +
> +    memset(s->rx_fifo, 0, SIFIVE_UART_RX_FIFO_SIZE);
> +    fifo8_create(&s->tx_fifo, SIFIVE_UART_TX_FIFO_SIZE);
>   }
>   
>   static void sifive_uart_reset_hold(Object *obj, ResetType type)