linux-kernel - Re: [PATCH 2 1/9] serial: core: move RS485 configuration tasks from drivers into core

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Date:   Thu, 17 Feb 2022 12:33:54 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     gregkh@...uxfoundation.org, jirislaby@...nel.org,
        u.kleine-koenig@...gutronix.de, linux@...linux.org.uk,
        richard.genoud@...il.com, nicolas.ferre@...rochip.com,
        alexandre.belloni@...tlin.com, ludovic.desroches@...rochip.com,
        shawnguo@...nel.org, s.hauer@...gutronix.de, kernel@...gutronix.de,
        festevam@...il.com, linux-imx@....com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...s.st.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 2 1/9] serial: core: move RS485 configuration tasks from
 drivers into core

On Wed, Feb 16, 2022 at 01:17:55AM +0100, Lino Sanfilippo wrote:
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1282,8 +1282,26 @@ static int uart_set_rs485_config(struct uart_port *port,
>  	if (copy_from_user(&rs485, rs485_user, sizeof(*rs485_user)))
>  		return -EFAULT;
>  
> +	/* pick sane settings if the user hasn't */
> +	if (!(rs485.flags & SER_RS485_RTS_ON_SEND) ==
> +	    !(rs485.flags & SER_RS485_RTS_AFTER_SEND)) {
> +		rs485.flags |= SER_RS485_RTS_ON_SEND;
> +		rs485.flags &= ~SER_RS485_RTS_AFTER_SEND;
> +	}

The policy you're enforcing here is:  If settings are nonsensical,
always default to active-high polarity.

However some drivers currently enforce a completely different policy:
E.g. with 8250_lpc18xx.c, if SER_RS485_RTS_ON_SEND is set, use active-high
(and fix up SER_RS485_RTS_AFTER_SEND), else use active-low polarity.
This yields a different result compared to your policy in case both bits
are cleared.

Similarly, sc16is7xx.c defaults to active-low if SER_RS485_RTS_AFTER_SEND
is set, else active-high polarity.  This yields a different result compared
to your policy in case both bits are set.

You risk breaking existing user space applications with this change
if those applications specify nonsensical polarity settings.

I happen to have created a similar commit to this one a month ago
and I came to the conclusion that all one can do is offer a library
function uart_sanitize_rs485_mode() which drivers may elect to call
if that helper's policy is identical to what they're doing now:

https://github.com/l1k/linux/commit/637984111e42

> +
> +	rs485.delay_rts_before_send = min_t(unsigned int,
> +					    rs485.delay_rts_before_send,
> +					    SER_RS485_MAX_RTS_DELAY);
> +	rs485.delay_rts_after_send = min_t(unsigned int,
> +					   rs485.delay_rts_after_send,
> +					   SER_RS485_MAX_RTS_DELAY);

Nonsensical delays may not only be passed in from user space via ioctl(),
but through the device tree.  That's another reason to use a library
function:  It can be called both from the ioctl() as well as after (or in)
uart_get_rs485_mode().

> +	/* Return clean padding area to userspace */
> +	memset(rs485.padding, 0, sizeof(rs485.padding));

Unlike the polarity and delay handling, this one makes sense.

Thanks,

Lukas