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

From: Jochen Mades <jochen@mades.net>
To: Greg KH <gregkh@linuxfoundation.org>,
	"sistik@3ksolutions.sk" <sistik@3ksolutions.sk>
Cc: linux-serial@vger.kernel.org
Subject: Re: Extending serial port linux driver to toggle RS485 direction pin (GPIO)
Date: Tue, 16 Nov 2021 07:43:52 +0100 (CET)	[thread overview]
Message-ID: <236176398.157022.1637045032216@webmail.strato.com> (raw)
In-Reply-To: <YYGKbfvFki8VN4HN@kroah.com>

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/

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 <gregkh@linuxfoundation.org> wrote:
> 
>  
> On Tue, Nov 02, 2021 at 06:55:56PM +0100, Jochen wrote:
> > 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