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>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240910045419.1252277-3-alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
---
include/hw/char/sifive_uart.h | 16 +++++-
hw/char/sifive_uart.c | 97 ++++++++++++++++++++++++++++++++---
2 files changed, 105 insertions(+), 8 deletions(-)
diff --git a/include/hw/char/sifive_uart.h b/include/hw/char/sifive_uart.h
index 7f6c79f8bd..0846cf6218 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 +68,20 @@ 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;
};
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..aeb45d3601 100644
--- a/hw/char/sifive_uart.c
+++ b/hw/char/sifive_uart.c
@@ -26,6 +26,8 @@
#include "hw/char/sifive_uart.h"
#include "hw/qdev-properties-system.h"
+#define TX_INTERRUPT_TRIGGER_DELAY_NS 100
+
/*
* Not yet implemented:
*
@@ -64,6 +66,72 @@ 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 *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 */
+ characters = fifo8_peek_bufptr(&s->tx_fifo,
+ fifo8_num_used(&s->tx_fifo), &numptr);
+ ret = qemu_chr_fe_write(&s->chr, characters, 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 +
+ TX_INTERRUPT_TRIGGER_DELAY_NS);
+}
+
static uint64_t
sifive_uart_read(void *opaque, hwaddr addr, unsigned int size)
{
@@ -82,7 +150,7 @@ 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 +174,10 @@ 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 +197,13 @@ 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 +270,9 @@ 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 +282,18 @@ 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)
@@ -222,8 +304,8 @@ static void sifive_uart_reset_hold(Object *obj, ResetType type)
static const VMStateDescription vmstate_sifive_uart = {
.name = TYPE_SIFIVE_UART,
- .version_id = 1,
- .minimum_version_id = 1,
+ .version_id = 2,
+ .minimum_version_id = 2,
.fields = (const VMStateField[]) {
VMSTATE_UINT8_ARRAY(rx_fifo, SiFiveUARTState,
SIFIVE_UART_RX_FIFO_SIZE),
@@ -233,6 +315,9 @@ static const VMStateDescription vmstate_sifive_uart = {
VMSTATE_UINT32(txctrl, SiFiveUARTState),
VMSTATE_UINT32(rxctrl, SiFiveUARTState),
VMSTATE_UINT32(div, SiFiveUARTState),
+ VMSTATE_UINT32(txfifo, SiFiveUARTState),
+ VMSTATE_FIFO8(tx_fifo, SiFiveUARTState),
+ VMSTATE_TIMER_PTR(fifo_trigger_handle, SiFiveUARTState),
VMSTATE_END_OF_LIST()
},
};
--
2.47.0