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;
};
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)
--
2.46.0Hi 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)
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)
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