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

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Message-ID: <YMmaOn8tEl7PcOE8@kroah.com>
Date:   Wed, 16 Jun 2021 08:29:14 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, linux@...linux.org.uk,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] serial: amba-pl011: add RS485 support

On Wed, Jun 16, 2021 at 08:18:01AM +0200, Jiri Slaby wrote:
> 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

I was wondering about that.

I'll go drop this patch from my tty-next branch and wait for a new
version that fixes the above delay issue and it can use this new call as
well.

thanks,

greg k-h