linux-kernel - [PATCH AUTOSEL 4.14 02/10] serial: 8250: 8250_omap: Avoid RS485 RTS glitch on ->set

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Date:   Wed, 23 Nov 2022 07:45:10 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Lukas Wunner <lukas@...ner.de>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        Su Bao Cheng <baocheng.su@...mens.com>,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sasha Levin <sashal@...nel.org>, jirislaby@...nel.org,
        ilpo.jarvinen@...ux.intel.com, tony@...mide.com,
        andriy.shevchenko@...ux.intel.com, linux-serial@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 02/10] serial: 8250: 8250_omap: Avoid RS485 RTS glitch on ->set_termios()

From: Lukas Wunner <lukas@...ner.de>

[ Upstream commit 038ee49fef18710bedd38b531d173ccd746b2d8d ]

RS485-enabled UART ports on TI Sitara SoCs with active-low polarity
exhibit a Transmit Enable glitch on ->set_termios():

omap8250_restore_regs(), which is called from omap_8250_set_termios(),
sets the TCRTLR bit in the MCR register and clears all other bits,
including RTS.  If RTS uses active-low polarity, it is now asserted
for no reason.

The TCRTLR bit is subsequently cleared by writing up->mcr to the MCR
register.  That variable is always zero, so the RTS bit is still cleared
(incorrectly so if RTS is active-high).

(up->mcr is not, as one might think, a cache of the MCR register's
current value.  Rather, it only caches a single bit of that register,
the AFE bit.  And it only does so if the UART supports the AFE bit,
which OMAP does not.  For details see serial8250_do_set_termios() and
serial8250_do_set_mctrl().)

Finally at the end of omap8250_restore_regs(), the MCR register is
restored (and RTS deasserted) by a call to up->port.ops->set_mctrl()
(which equals serial8250_set_mctrl()) and serial8250_em485_stop_tx().

So there's an RTS glitch between setting TCRTLR and calling
serial8250_em485_stop_tx().  Avoid by using a read-modify-write
when setting TCRTLR.

While at it, drop a redundant initialization of up->mcr.  As explained
above, the variable isn't used by the driver and it is already
initialized to zero because it is part of the static struct
serial8250_ports[] declared in 8250_core.c.  (Static structs are
initialized to zero per section 6.7.8 nr. 10 of the C99 standard.)

Cc: Jan Kiszka <jan.kiszka@...mens.com>
Cc: Su Bao Cheng <baocheng.su@...mens.com>
Tested-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Signed-off-by: Lukas Wunner <lukas@...ner.de>
Link: https://lore.kernel.org/r/6554b0241a2c7fd50f32576fdbafed96709e11e8.1664278942.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/tty/serial/8250/8250_omap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index e32afaa94d36..c5af1495cd2e 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -259,6 +259,7 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
 {
 	struct omap8250_priv *priv = up->port.private_data;
 	struct uart_8250_dma	*dma = up->dma;
+	u8 mcr = serial8250_in_MCR(up);

 	if (dma && dma->tx_running) {
 		/*
@@ -275,7 +276,7 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
 	serial_out(up, UART_EFR, UART_EFR_ECB);

 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_A);
-	serial8250_out_MCR(up, UART_MCR_TCRTLR);
+	serial8250_out_MCR(up, mcr | UART_MCR_TCRTLR);
 	serial_out(up, UART_FCR, up->fcr);

 	omap8250_update_scr(up, priv);
@@ -291,7 +292,8 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
 	serial_out(up, UART_LCR, 0);

 	/* drop TCR + TLR access, we setup XON/XOFF later */
-	serial8250_out_MCR(up, up->mcr);
+	serial8250_out_MCR(up, mcr);
+
 	serial_out(up, UART_IER, up->ier);

 	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
@@ -600,7 +602,6 @@ static int omap_8250_startup(struct uart_port *port)

 	pm_runtime_get_sync(port->dev);

-	up->mcr = 0;
 	serial_out(up, UART_FCR, UART_FCR_CLEAR_RCVR | UART_FCR_CLEAR_XMIT);

 	serial_out(up, UART_LCR, UART_LCR_WLEN8);
-- 
2.35.1