linux-kernel - [PATCH] serial: 8250_omap: remove wait loop from Errata i202 workaround

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Date:   Thu, 13 Oct 2022 13:23:39 +0200
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Tony Lindgren <tony@...mide.com>,
        Peter Hurley <peter@...leysoftware.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>
Subject: [PATCH] serial: 8250_omap: remove wait loop from Errata i202 workaround

We were occasionally seeing the "Errata i202: timedout" on an AM335x
board when repeatedly opening and closing a UART connected to an active
sender. As new input may arrive at any time, it is possible to miss the
"RX FIFO empty" condition, forcing the loop to wait until it times out.

Nothing in the i202 Advisory states that such a wait is even necessary;
other FIFO clear functions like serial8250_clear_fifos() do not wait
either. For this reason, it seems safe to remove the wait, fixing the
mentioned issue.

Fixes: 61929cf0169d ("tty: serial: Add 8250-core based omap driver")
Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>
---
 drivers/tty/serial/8250/8250_omap.c | 17 -----------------
 1 file changed, 17 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 41b8c6b27136..484f791617af 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -193,27 +193,10 @@ static void omap8250_set_mctrl(struct uart_port *port, unsigned int mctrl)
 static void omap_8250_mdr1_errataset(struct uart_8250_port *up,
 				     struct omap8250_priv *priv)
 {
-	u8 timeout = 255;
-
 	serial_out(up, UART_OMAP_MDR1, priv->mdr1);
 	udelay(2);
 	serial_out(up, UART_FCR, up->fcr | UART_FCR_CLEAR_XMIT |
 			UART_FCR_CLEAR_RCVR);
-	/*
-	 * Wait for FIFO to empty: when empty, RX_FIFO_E bit is 0 and
-	 * TX_FIFO_E bit is 1.
-	 */
-	while (UART_LSR_THRE != (serial_in(up, UART_LSR) &
-				(UART_LSR_THRE | UART_LSR_DR))) {
-		timeout--;
-		if (!timeout) {
-			/* Should *never* happen. we warn and carry on */
-			dev_crit(up->port.dev, "Errata i202: timedout %x\n",
-				 serial_in(up, UART_LSR));
-			break;
-		}
-		udelay(1);
-	}
 }
 
 static void omap_8250_get_divisor(struct uart_port *port, unsigned int baud,
-- 
2.25.1