linux-kernel - Re: [PATCH] Bugfix RTS line config in RS485 mode is overwritten in pl011_set

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Date:   Mon, 3 Jan 2022 14:44:01 +0100
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     jmades <jochen@...es.net>, gregkh@...uxfoundation.org,
        Russell King <linux@...linux.org.uk>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Philipp Rosenberger <p.rosenberger@...bus.com>
Subject: Re: [PATCH] Bugfix RTS line config in RS485 mode is overwritten in
 pl011_set_mctrl() function.

On 02.01.22 at 19:28, Lukas Wunner wrote:
> On Sun, Jan 02, 2022 at 04:06:53PM +0100, Lino Sanfilippo wrote:
>> On 02.01.22 at 11:07, Lukas Wunner wrote:
>>> On Fri, Dec 31, 2021 at 05:15:14PM +0000, jmades wrote:
>>>> --- a/drivers/tty/serial/amba-pl011.c
>>>> +++ b/drivers/tty/serial/amba-pl011.c
>>>> @@ -1646,8 +1646,12 @@ static void pl011_set_mctrl(struct uart_port *port, unsigned int mctrl)
>>>>  	    container_of(port, struct uart_amba_port, port);
>>>>  	unsigned int cr;
>>>>
>>>> -	if (port->rs485.flags & SER_RS485_ENABLED)
>>>> -		mctrl &= ~TIOCM_RTS;
>>>> +	if (port->rs485.flags & SER_RS485_ENABLED) {
>>>> +		if (port->rs485.flags & SER_RS485_RTS_AFTER_SEND)
>>>> +			mctrl &= ~TIOCM_RTS;
>>>> +		else
>>>> +			mctrl |= TIOCM_RTS;
>>>> +	}
>>>>
>>>>  	cr = pl011_read(uap, REG_CR);
>>
>> Does this logic really have to be implemented in the driver?
>
> No, it doesn't have to be and indeed I'm working towards consolidating
> it in the serial core with this collection of patches:
>
> https://git.kernel.org/gregkh/tty/c/d3b3404df318
> https://lore.kernel.org/all/f49f945375f5ccb979893c49f1129f51651ac738.1641129062.git.lukas@wunner.de
> https://lore.kernel.org/all/e22089ab49e6e78822c50c8c4db46bf3ee885623.1641129328.git.lukas@wunner.de
> https://lore.kernel.org/all/bceeaba030b028ed810272d55d5fc6f3656ddddb.1641129752.git.lukas@wunner.de
> https://github.com/l1k/linux/commit/532ef2ad757f
>
> The last of these removes the rs485 logic from pl011_set_mctrl().
> I'll post it once the others (and Jochen Mades' patch) have landed.
>
> Even though the logic is eventually removed from pl011_set_mctrl(),
> Jochen's patch makes sense as a backportable fix for v5.15.
>
>
>> It looks as if the serial core already takes RS485 into account before
>> calling set_mctrls(). At least I get the impression when looking
>> at uart_tiocmset() and uart_port_dtr_rts(). Also other drivers like imx
>> simply seem to ignore RTS in case of RS485.
>
> The logic in uart_port_dtr_rts() is broken.  That's fixed by d3b3404df318,
> which is queued up in tty-next for v5.17.
>
> The pl011 driver papered over it with its own rs485-specific logic in
> pl011_set_mctrl().  But as Jochen Mades correctly pointed out, that
> only worked correctly if RTS is driven high on idle.
>
>
> The logic in uart_tiocmset() is correct, but not sufficient because
> uart_throttle(), uart_unthrottle and uart_set_termios() need to become
> rs485-aware as well.  That's also addressed by the above-linked
> GitHub commit.
>


Thanks for this information. I have one or two patches prepared a while ago that
aim into a similar direction (move RS485 related code out of the drivers into
the serial core). I will send them in the next days.

Best regards,
Lino