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D36979.diff

diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c
--- a/sys/dev/uart/uart_dev_ns8250.c
+++ b/sys/dev/uart/uart_dev_ns8250.c
@@ -210,6 +210,8 @@
ns8250_flush(struct uart_bas *bas, int what)
{
uint8_t fcr;
+ uint8_t lsr;
+ int drain = 0;
fcr = FCR_ENABLE;
#ifdef CPU_XBURST
@@ -221,6 +223,23 @@
fcr |= FCR_RCV_RST;
uart_setreg(bas, REG_FCR, fcr);
uart_barrier(bas);
+
+ /*
+ * Detect and work around emulated UARTs which don't implement the
+ * FCR register; on these systems we need to drain the FIFO since
+ * the flush we request doesn't happen. One such system is the
+ * Firecracker VMM, aka. the rust-vmm/vm-superio emulation code:
+ * https://github.com/rust-vmm/vm-superio/issues/83
+ */
+ lsr = uart_getreg(bas, REG_LSR);
+ if ((lsr & LSR_TEMT) && (what & UART_FLUSH_TRANSMITTER))
+ drain |= UART_DRAIN_TRANSMITTER;
+ if ((lsr & LSR_RXRDY) && (what & UART_FLUSH_RECEIVER))
+ drain |= UART_DRAIN_RECEIVER;
+ if (drain != 0) {
+ printf("ns8250: UART FCR is broken\n");
+ ns8250_drain(bas, drain);
+ }
}
static int

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