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* [PATCH AUTOSEL 4.9 1/5] serial: 8250: 8250_omap: Avoid RS485 RTS glitch on ->set_termios()
@ 2022-11-23 12:45 Sasha Levin
  2022-11-23 12:45 ` [PATCH AUTOSEL 4.9 2/5] xen/platform-pci: add missing free_irq() in error path Sasha Levin
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: Sasha Levin @ 2022-11-23 12:45 UTC (permalink / raw)
  To: linux-kernel, stable
  Cc: Lukas Wunner, Jan Kiszka, Su Bao Cheng, Matthias Schiffer,
	Greg Kroah-Hartman, Sasha Levin, jirislaby, ilpo.jarvinen, tony,
	andriy.shevchenko, linux-serial

From: Lukas Wunner <lukas@wunner.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@siemens.com>
Cc: Su Bao Cheng <baocheng.su@siemens.com>
Tested-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/6554b0241a2c7fd50f32576fdbafed96709e11e8.1664278942.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.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 c551407bee07..7795e6401a93 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


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end of thread, other threads:[~2022-11-23 12:58 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2022-11-23 12:45 [PATCH AUTOSEL 4.9 1/5] serial: 8250: 8250_omap: Avoid RS485 RTS glitch on ->set_termios() Sasha Levin
2022-11-23 12:45 ` [PATCH AUTOSEL 4.9 2/5] xen/platform-pci: add missing free_irq() in error path Sasha Levin
2022-11-23 12:45 ` [PATCH AUTOSEL 4.9 3/5] platform/x86: asus-wmi: add missing pci_dev_put() in asus_wmi_set_xusb2pr() Sasha Levin
2022-11-23 12:45 ` [PATCH AUTOSEL 4.9 4/5] tcp: configurable source port perturb table size Sasha Levin
2022-11-23 12:45 ` [PATCH AUTOSEL 4.9 5/5] net: usb: qmi_wwan: add Telit 0x103a composition Sasha Levin

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