From deff81bf13b8b71c59b0eb9b071934076eda943a Mon Sep 17 00:00:00 2001
From: zbb <zbb@FreeBSD.org>
Date: Sat, 26 Oct 2013 17:24:59 +0000
Subject: [PATCH] Wait for DesignWare UART transfers completion before
 accessing line control

When using DW UART with BUSY detection it is necessary to wait
until all serial transfers are finished before manipulating the
line control. LCR will not be affected when UART is busy.
In addition, if Divisor Latch Access Bit is being set in order to
modify UART divisors:
1. We will get BUSY interrupt if interrupts are enabled.
2. Because LCR will not be affected the THR and (even worse) IER
   contents will be corrupted. This will lead to console hang.

Approved by:	cognet (mentor)
---
 sys/dev/ic/ns16550.h           |  1 +
 sys/dev/uart/uart_dev_ns8250.c | 26 +++++++++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/sys/dev/ic/ns16550.h b/sys/dev/ic/ns16550.h
index 659f59184970..33a7dd14b82b 100644
--- a/sys/dev/ic/ns16550.h
+++ b/sys/dev/ic/ns16550.h
@@ -185,6 +185,7 @@
 #define DW_REG_USR	31	/* DesignWare derived Uart Status Reg */
 #define com_usr		39	/* Octeon 16750/16550 Uart Status Reg */
 #define REG_USR		com_usr
+#define USR_BUSY	1	/* Uart Busy. Serial transfer in progress */
 #define USR_TXFIFO_NOTFULL 2    /* Uart TX FIFO Not full */
 
 /* 16950 register #1.  Access enabled by ACR[7].  Also requires !LCR[7]. */
diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c
index 211d11321860..249be4ce0e33 100644
--- a/sys/dev/uart/uart_dev_ns8250.c
+++ b/sys/dev/uart/uart_dev_ns8250.c
@@ -647,11 +647,35 @@ int
 ns8250_bus_param(struct uart_softc *sc, int baudrate, int databits,
     int stopbits, int parity)
 {
+	struct ns8250_softc *ns8250;
 	struct uart_bas *bas;
-	int error;
+	int error, limit;
 
+	ns8250 = (struct ns8250_softc*)sc;
 	bas = &sc->sc_bas;
 	uart_lock(sc->sc_hwmtx);
+	/*
+	 * When using DW UART with BUSY detection it is necessary to wait
+	 * until all serial transfers are finished before manipulating the
+	 * line control. LCR will not be affected when UART is busy.
+	 */
+	if (ns8250->busy_detect != 0) {
+		/*
+		 * Pick an arbitrary high limit to avoid getting stuck in
+		 * an infinite loop in case when the hardware is broken.
+		 */
+		limit = 10 * 1024;
+		while (((uart_getreg(bas, DW_REG_USR) & USR_BUSY) != 0) &&
+		    --limit)
+			DELAY(4);
+
+		if (limit <= 0) {
+			/* UART appears to be stuck */
+			uart_unlock(sc->sc_hwmtx);
+			return (EIO);
+		}
+	}
+
 	error = ns8250_param(bas, baudrate, databits, stopbits, parity);
 	uart_unlock(sc->sc_hwmtx);
 	return (error);