rts-gpios support in fsl_lpuart driver?

[Sherry Sun]

> Subject: Re: rts-gpios support in fsl_lpuart driver?
> 
> On Tue, Feb 10, 2026 at 12:48:05PM +0100, Alexander Dahl wrote:
> > Hello everyone,
> 
> Add sherry. look like we have not implemented it at lpuart.

Yes, currently the lpuart driver only supports auto RS-485 RTS mode.
We haven't received any requests for sw-controlled RTS mode (rts-gpio).
We can consider adding this feature in the future if such requests arise.

Best Regards
Sherry
> 
> Frank
> >
> > we are currently experimenting with the SolidRun 8XLite SoM featuring
> > an i.MX 8XLite SoC on a custom base board.  Due to how our peripheral
> > hardware is connected we need to use rs485 with uart3, which has no
> > native soc controlled rts line.  On different SoCs you just use
> > 'rts-gpios' in devicetree then, letting the kernel switch this line
> > through gpio.  Alas it does not work on this SoC, the RTS line is not
> > switched at all. :-/
> >
> > According to my analysis the i.MX8 family has different variants using
> > different uarts with different drivers.  For the variants using the
> > imx.c driver, you find dts files using rts-gpios, but for variants
> > using the fsl_lpuart.c driver you find none.
> >
> > The rts-gpios property is evaluated by the serial core in
> > `mctrl_gpio_init()` right?  And the actual switching is also done
> > through that mctrl_gpio interface, right?  As far as I understand the
> > source code, the serial core calls the .set_mctrl function pointer,
> > and then it is up to the driver what to do?  Some (like atmel_serial.c
> > and imx.c) call mctrl_gpio_set() in the function mapped to that
> > pointer, some (like fsl_lpuart.c) don't.  Correct so far?
> >
> > Up to this point I'm somewhat confused this is no generic feature in
> > the core which works for any uart, but there's probably a reason for
> > that?  Maybe interaction with the actual uart registers in some way?
> >
> > If it is like described, I'd say the fsl_lpuart driver currently just
> > does not support this usecase?  Is it possible to implement the
> > necessary things?  Is anybody already working on that?  I looked in my
> > mailing list archive and in the imx downstream kernel, but could not
> > find any hints.
> >
> > Any advice welcome. :-)
> >
> > Greets
> > Alex
> >