AMBA PL011 do not have hardware support for RS485. This implementation is
for drive enable signal (DE), which switch direction of RS485 driver chip.
This signal si drived by RTS pin. Correct multiplexor settings have to be
provided to Device Tree. Usually it is 'ctsrts', which is used for enabling
of HW flow control, too.

DE signal is switched by starting transmition from serial core and data
transfer is initiated by first hrtimer if there is delay before send
enabled.

There is missing FIFO empty interrupt in PL011. It is replaced by second
hrtimer which is started if there are no more data in port transmit buffer.
Notice that port transmit buffer is not the same as HW TX FIFO. Time of
this timmer is set to char send time and it is running until fifo is empty.
This kind of implementation cause that there can be unwanted delay of one
timer tick before DE signal is switched. This is used to prevent data loss
during transmit. Second timer can start first if there is delay after send
enabled.

Signed-off-by: Ivan Sistik <redacted>
---
 arch/arm/configs/bcm2835_defconfig |   1 +
 drivers/tty/serial/Kconfig         |  12 +
 drivers/tty/serial/amba-pl011.c    | 470 ++++++++++++++++++++++++++++-
 3 files changed, 480 insertions(+), 3 deletions(-)

diff --git a/arch/arm/configs/bcm2835_defconfig b/arch/arm/configs/bcm2835_defconfig
index e9bc88937b1e..514888681913 100644
--- a/arch/arm/configs/bcm2835_defconfig
+++ b/arch/arm/configs/bcm2835_defconfig

@@ -78,6 +78,7 @@ CONFIG_SERIAL_8250_SHARE_IRQ=y
 CONFIG_SERIAL_8250_BCM2835AUX=y
 CONFIG_SERIAL_AMBA_PL011=y
 CONFIG_SERIAL_AMBA_PL011_CONSOLE=y
+CONFIG_SERIAL_AMBA_PL011_SOFT_RS485=y
 CONFIG_SERIAL_DEV_BUS=y
 CONFIG_TTY_PRINTK=y
 CONFIG_I2C_CHARDEV=y

diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index df8bd0c7b97d..3cecd8a75691 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig

@@ -73,6 +73,18 @@ config SERIAL_AMBA_PL011_CONSOLE
 	  your boot loader (lilo or loadlin) about how to pass options to the
 	  kernel at boot time.)
 
+config SERIAL_AMBA_PL011_SOFT_RS485
+	bool "RS485 software direction switching for ARM AMBA PL011 serial"
+	depends on SERIAL_AMBA_PL011=y