[PATCH] 8250: Improve workaround for UARTs that don't

[Will Newton]

Recent changes to tighten the check for UARTs that don't correctly
re-assert THRE caused problems when such a UART was opened for the second
time - the bug could only successfully be detected at first initialization.
This patch stores the information about the bug in the bugs field of the
port structure when the port is first started up so subsequent opens can
check this bit even if the test for the bug fails.

Signed-off-by: Will Newton <will.newton@xxxxxxxxx>
Acked-by: Alex Williamson <alex.williamson@xxxxxx>

---
 drivers/serial/8250.c |   16 ++++++++++++----
 drivers/serial/8250.h |    1 +
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 342e12f..9ccc563 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1908,15 +1908,23 @@ static int serial8250_startup(struct uart_port *port)
 		 * kick the UART on a regular basis.
 		 */
 		if (!(iir1 & UART_IIR_NO_INT) && (iir & UART_IIR_NO_INT)) {
+			up->bugs |= UART_BUG_THRE;
 			pr_debug("ttyS%d - using backup timer\n", port->line);
-			up->timer.function = serial8250_backup_timeout;
-			up->timer.data = (unsigned long)up;
-			mod_timer(&up->timer, jiffies +
-				poll_timeout(up->port.timeout) + HZ / 5);
 		}
 	}

 	/*
+	 * The above check will only give an accurate result the first time
+	 * the port is opened so this value needs to be preserved.
+	 */
+	if (up->bugs & UART_BUG_THRE) {
+		up->timer.function = serial8250_backup_timeout;
+		up->timer.data = (unsigned long)up;
+		mod_timer(&up->timer, jiffies +
+			  poll_timeout(up->port.timeout) + HZ / 5);
+	}
+
+	/*
 	 * If the "interrupt" for this port doesn't correspond with any
 	 * hardware interrupt, we use a timer-based system.  The original
 	 * driver used to do this with IRQ0.
diff --git a/drivers/serial/8250.h b/drivers/serial/8250.h
index 78c0016..5202603 100644
--- a/drivers/serial/8250.h
+++ b/drivers/serial/8250.h
@@ -47,6 +47,7 @@ struct serial8250_config {
 #define UART_BUG_QUOT	(1 << 0)	/* UART has buggy quot LSB */
 #define UART_BUG_TXEN	(1 << 1)	/* UART has buggy TX IIR status */
 #define UART_BUG_NOMSR	(1 << 2)	/* UART has buggy MSR status bits (Au1x00) */
+#define UART_BUG_THRE	(1 << 3)	/* UART has buggy THRE reassertion */

 #define PROBE_RSA	(1 << 0)
 #define PROBE_ANY	(~0)
-- 
1.5.5.2

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