Re: Extending serial port linux driver to toggle RS485 direction pin (GPIO)

Hi Greg, Hi Ivan,

as would like to extend the RS485 behavior on my raspberry PI I did some "deep-diving" into the amba-pl011 driver and soon recognized the missing hardware support to detect "FIFO empty". Googling around I found the following patch, describing exactly my problem:
https://lore.kernel.org/linux-arm-kernel/20200107072831.GB1014453@kroah.com/T/ (local)

As I'm not familiar with your processes, I kindly ask you where I can find the patched sources of this Soft-Rs485-Amba driver and if it is planned to be intergrated in a future kernel release?

Bests
Jochen

On 02/11/2021 19:58 Greg KH [off-list ref] wrote:

 
On Tue, Nov 02, 2021 at 06:55:56PM +0100, Jochen wrote:

quoted

Hello,

I have a RS485 hardware shield connected to the hardware uart of a raspberry PI3, where you have to toggle the RS485 driver DIRECTION-pin from within your software during write-commands. The DIR-Pin is connected to a GPIO pin of the PI.

As I do not want to do that in every application software, I thought it could be a good idea to enhance the serial-port driver (locally on my PI) with that functionality. Looking to the sources of amba-pl011.c I thought the "pl011_write"-function could be the right place to do so....but to be honest it seems not to work

Could you please give me a hint where to do that best in the serial port driver? (or is there already a RS485 driver with configurable Dir-GPIO-pin).
Is there a documentation of the serial-port architecture available which could help me to solve my problem?

Other drivers do this today, using gpio pins for this.  One example is
the drivers/tty/serial/ar933x_uart.c driver (look at the
ar933x_uart_tx_chars() function)

You could do much the same in the pl011_rs485_tx_start() function when
SER_RS485_RX_DURING_TX and SER_RS485_RTS_ON_SEND are checked, right?

As for making it "generic", I think there are other drivers that allow
the gpio pins to be selected as part of their device tree, look in the
drivers/tty/serial/ directory for the use of gpio values in lots of
different drivers.

hope this helps,

greg k-h