linux-kernel - Re: [PATCH] serial: amba-pl011: add RS485 support

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Date:   Wed, 16 Jun 2021 08:18:01 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>, gregkh@...uxfoundation.org
Cc:     linux@...linux.org.uk, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: amba-pl011: add RS485 support

On 10. 06. 21, 15:50, Lino Sanfilippo wrote:
> Add basic support for RS485: Provide a callback to configure rs485
> settings. Handle the RS485 specific part in the functions
> pl011_rs485_tx_start() and pl011_rs485_tx_stop() which extend the generic
> start/stop callbacks.
> Beside via IOCTL from userspace RS485 can be enabled by means of the
> device tree property "rs485-enabled-at-boot-time".
> 
> Signed-off-by: Lino Sanfilippo <LinoSanfilippo@....de>
> ---
> This patch has been tested with a Raspberry Pi CM3.
> 
>   drivers/tty/serial/amba-pl011.c | 143 +++++++++++++++++++++++++++++++-
>   1 file changed, 140 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> index 78682c12156a..36e8b938cdba 100644
> --- a/drivers/tty/serial/amba-pl011.c
> +++ b/drivers/tty/serial/amba-pl011.c
...
> @@ -1380,6 +1415,31 @@ static bool pl011_tx_char(struct uart_amba_port *uap, unsigned char c,
>   	return true;
>   }
>   
> +static void pl011_rs485_tx_start(struct uart_amba_port *uap)
> +{
> +	struct uart_port *port = &uap->port;
> +	u32 cr;
> +
> +	/* Enable transmitter */
> +	cr = pl011_read(uap, REG_CR);
> +	cr |= UART011_CR_TXE;
> +	/* Disable receiver if half-duplex */
> +	if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
> +		cr &= ~UART011_CR_RXE;
> +
> +	if (port->rs485.flags & SER_RS485_RTS_ON_SEND)
> +		cr &= ~UART011_CR_RTS;
> +	else
> +		cr |= UART011_CR_RTS;
> +
> +	pl011_write(cr, uap, REG_CR);
> +
> +	if (port->rs485.delay_rts_before_send)
> +		mdelay(port->rs485.delay_rts_before_send);

This is up to 1 second delay with interrupts disabled. Definitely not 
nice. 8250 clamps this to 100 ms at least, why did you choose 1000 ms?

> +
> +	uap->rs485_tx_started = true;
> +}
> +
>   /* Returns true if tx interrupts have to be (kept) enabled  */
>   static bool pl011_tx_chars(struct uart_amba_port *uap, bool from_irq)
>   {
...
> @@ -1941,6 +2021,7 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
>   	unsigned int lcr_h, old_cr;
>   	unsigned long flags;
>   	unsigned int baud, quot, clkdiv;
> +	unsigned int bits;
>   
>   	if (uap->vendor->oversampling)
>   		clkdiv = 8;
> @@ -1968,25 +2049,32 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
>   	switch (termios->c_cflag & CSIZE) {
>   	case CS5:
>   		lcr_h = UART01x_LCRH_WLEN_5;
> +		bits = 7;
>   		break;
>   	case CS6:
>   		lcr_h = UART01x_LCRH_WLEN_6;
> +		bits = 8;
>   		break;
>   	case CS7:
>   		lcr_h = UART01x_LCRH_WLEN_7;
> +		bits = 9;
>   		break;
>   	default: // CS8
>   		lcr_h = UART01x_LCRH_WLEN_8;
> +		bits = 10;
>   		break;
>   	}
> -	if (termios->c_cflag & CSTOPB)
> +	if (termios->c_cflag & CSTOPB) {
>   		lcr_h |= UART01x_LCRH_STP2;
> +		bits++;
> +	}
>   	if (termios->c_cflag & PARENB) {
>   		lcr_h |= UART01x_LCRH_PEN;
>   		if (!(termios->c_cflag & PARODD))
>   			lcr_h |= UART01x_LCRH_EPS;
>   		if (termios->c_cflag & CMSPAR)
>   			lcr_h |= UART011_LCRH_SPS;
> +		bits++;
>   	}

You can do simply:
   bits = tty_get_frame_size(termios->c_cflag);
now:
https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git/commit/?h=tty-testing&id=3ec2ff37230e1c961d4b0d0118dd23c46b5bcdbb


thanks,
-- 
js