Add a loader tunable "hw.broken_txfifo" which enables a workaround for a

bug in old versions of QEMU (and Xen, and other places using QEMU code).
On those buggy emulated UARTs, the "TX idle" interrupt gets lost; with
this workaround, we spinwait for the TX to happen and then send ourselves
the interrupt.  It's ugly but it works, while minimizing the impact on
the code for the !broken_txfifo case.

MFC after:	2 weeks

1 files changed, 13 insertions, 1 deletions

diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c
index 5d56a68..eb36da1 100644
--- a/sys/dev/uart/uart_dev_ns8250.c
+++ b/sys/dev/uart/uart_dev_ns8250.c
@@ -31,6 +31,8 @@ __FBSDID("$FreeBSD$");
 #include <sys/systm.h>
 #include <sys/bus.h>
 #include <sys/conf.h>
+#include <sys/kernel.h>
+#include <sys/sysctl.h>
 #include <machine/bus.h>
 
 #include <dev/uart/uart.h>
@@ -845,6 +847,11 @@ ns8250_bus_setsig(struct uart_softc *sc, int sig)
 	return (0);
 }
 
+static int broken_txfifo = 0;
+SYSCTL_INT(_hw, OID_AUTO, broken_txfifo, CTLFLAG_RW | CTLFLAG_TUN,
+	&broken_txfifo, 0, "UART FIFO has QEMU emulation bug");
+TUNABLE_INT("hw.broken_txfifo", &broken_txfifo);
+
 static int
 ns8250_bus_transmit(struct uart_softc *sc)
 {
@@ -862,7 +869,12 @@ ns8250_bus_transmit(struct uart_softc *sc)
 		uart_setreg(bas, REG_DATA, sc->sc_txbuf[i]);
 		uart_barrier(bas);
 	}
-	sc->sc_txbusy = 1;
+	if (broken_txfifo)
+		ns8250_drain(bas, UART_DRAIN_TRANSMITTER);
+	else
+		sc->sc_txbusy = 1;
 	uart_unlock(sc->sc_hwmtx);
+	if (broken_txfifo)
+		uart_sched_softih(sc, SER_INT_TXIDLE);
 	return (0);
 }