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authorLukas Wunner <lukas@wunner.de>2022-09-27 14:52:34 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2022-11-03 05:44:04 +0300
commit038ee49fef18710bedd38b531d173ccd746b2d8d (patch)
treeed2407e64613e17b46f95059eb914d7b8ce231d3 /drivers/tty/serial/8250/8250_omap.c
parent30a0b95b1335e12efef89dd78518ed3e4a71a763 (diff)
downloadlinux-038ee49fef18710bedd38b531d173ccd746b2d8d.tar.xz
serial: 8250: 8250_omap: Avoid RS485 RTS glitch on ->set_termios()
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>
Diffstat (limited to 'drivers/tty/serial/8250/8250_omap.c')
-rw-r--r--drivers/tty/serial/8250/8250_omap.c7
1 files changed, 4 insertions, 3 deletions
diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 41b8c6b27136..68f5a167377f 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -292,6 +292,7 @@ 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) {
/*
@@ -308,7 +309,7 @@ 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);
@@ -324,7 +325,8 @@ 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);
@@ -669,7 +671,6 @@ 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);