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/6554b0241a2c7fd50f32576fdbafed96709e11e8.166427894... 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 3f7379f16a36..483fff3a95c9 100644 --- a/drivers/tty/serial/8250/8250_omap.c +++ b/drivers/tty/serial/8250/8250_omap.c @@ -293,6 +293,7 @@ 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) { /* @@ -309,7 +310,7 @@ 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); @@ -325,7 +326,8 @@ 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); @@ -684,7 +686,6 @@ 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);