From: Frank Chang <frank.chang@sifive.com>

Currently, the SiFive UART raises an IRQ whenever:

  1. ie.txwm is enabled.
  2. ie.rxwm is enabled and the Rx FIFO is not empty.

It does not check the watermark thresholds set by software. However,
since commit [1] changed the SiFive UART character printing from
synchronous to asynchronous, Tx overflows may occur, causing characters
to be dropped when running Linux because:

  1. The Linux SiFive UART driver sets the transmit watermark level to 1
     [2], meaning a transmit watermark interrupt is raised whenever a
     character is enqueued into the Tx FIFO.
  2. Upon receiving a transmit watermark interrupt, the Linux driver
     transfers up to a full Tx FIFO's worth of characters from the Linux
     serial transmit buffer [3], without checking the txdata.full flag
     before transferring multiple characters [4].

To fix this issue, we must honor the Tx/Rx watermark thresholds and
raise interrupts only when the Tx threshold is exceeded or the Rx
threshold is undercut.

[1] 53c1557b230986ab6320a58e1b2c26216ecd86d5
[2] https://github.com/torvalds/linux/blob/master/drivers/tty/serial/sifive.c#L1039
[3] https://github.com/torvalds/linux/blob/master/drivers/tty/serial/sifive.c#L538
[4] https://github.com/torvalds/linux/blob/master/drivers/tty/serial/sifive.c#L291

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Signed-off-by: Emmanuel Blot <emmanuel.blot@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250911160647.5710-2-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
---
 hw/char/sifive_uart.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/hw/char/sifive_uart.c b/hw/char/sifive_uart.c
index 9bc697a67b..138c31fcab 100644
--- a/hw/char/sifive_uart.c
+++ b/hw/char/sifive_uart.c
@@ -35,16 +35,17 @@
  */
 
 /* Returns the state of the IP (interrupt pending) register */
-static uint64_t sifive_uart_ip(SiFiveUARTState *s)
+static uint32_t sifive_uart_ip(SiFiveUARTState *s)
 {
-    uint64_t ret = 0;
+    uint32_t ret = 0;
 
-    uint64_t txcnt = SIFIVE_UART_GET_TXCNT(s->txctrl);
-    uint64_t rxcnt = SIFIVE_UART_GET_RXCNT(s->rxctrl);
+    uint32_t txcnt = SIFIVE_UART_GET_TXCNT(s->txctrl);
+    uint32_t rxcnt = SIFIVE_UART_GET_RXCNT(s->rxctrl);
 
-    if (txcnt != 0) {
+    if (fifo8_num_used(&s->tx_fifo) < txcnt) {
         ret |= SIFIVE_UART_IP_TXWM;
     }
+
     if (s->rx_fifo_len > rxcnt) {
         ret |= SIFIVE_UART_IP_RXWM;
     }
@@ -55,15 +56,14 @@ static uint64_t sifive_uart_ip(SiFiveUARTState *s)
 static void sifive_uart_update_irq(SiFiveUARTState *s)
 {
     int cond = 0;
-    if ((s->ie & SIFIVE_UART_IE_TXWM) ||
-        ((s->ie & SIFIVE_UART_IE_RXWM) && s->rx_fifo_len)) {
+    uint32_t ip = sifive_uart_ip(s);
+
+    if (((ip & SIFIVE_UART_IP_TXWM) && (s->ie & SIFIVE_UART_IE_TXWM)) ||
+        ((ip & SIFIVE_UART_IP_RXWM) && (s->ie & SIFIVE_UART_IE_RXWM))) {
         cond = 1;
     }
-    if (cond) {
-        qemu_irq_raise(s->irq);
-    } else {
-        qemu_irq_lower(s->irq);
-    }
+
+    qemu_set_irq(s->irq, cond);
 }
 
 static gboolean sifive_uart_xmit(void *do_not_use, GIOCondition cond,
-- 
2.51.0