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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> |
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| .. | ||
| hvc | ||
| ipwireless | ||
| serdev | ||
| serial | ||
| vt | ||
| amiserial.c | ||
| ehv_bytechan.c | ||
| goldfish.c | ||
| Kconfig | ||
| Makefile | ||
| mips_ejtag_fdc.c | ||
| moxa.c | ||
| mxser.c | ||
| n_gsm.c | ||
| n_hdlc.c | ||
| n_null.c | ||
| n_tty.c | ||
| nozomi.c | ||
| pty.c | ||
| rpmsg_tty.c | ||
| synclink_gt.c | ||
| sysrq.c | ||
| tty_audit.c | ||
| tty_baudrate.c | ||
| tty_buffer.c | ||
| tty_io.c | ||
| tty_ioctl.c | ||
| tty_jobctrl.c | ||
| tty_ldisc.c | ||
| tty_ldsem.c | ||
| tty_mutex.c | ||
| tty_port.c | ||
| tty.h | ||
| ttynull.c | ||
| vcc.c | ||