tty/serial: atmel: RS485 & ISO7816: wait for TXRDY before sending data

tty/serial: atmel: RS485 & ISO7816: wait for TXRDY before sending data

[ Upstream commit 2319b7a11873310c984443a9744ca111ca3c1dde ]

At this moment, TXEMPTY is checked before sending data on RS485 and ISO7816
modes. However, TXEMPTY is risen when FIFO (if used) or the Transmit Shift
Register are empty, even though TXRDY might be up and controller is able to
receive data. Since the controller sends data only when TXEMPTY is ready,
on RS485, when DMA is not used, the RTS pin is driven low after each byte.
With this patch, the characters will be transmitted when TXRDY is up and
so, RTS pin will remain high between bytes.
The performance improvement on RS485 is about 8% with a baudrate of 300.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Link: https://lore.kernel.org/r/20200107111656.26308-1-codrin.ciubotariu@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: b7166cf930e6 ("tty: serial: atmel: Preserve previous USART mode if RS485 disabled")
Signed-off-by: Sasha Levin <sashal@kernel.org>