Re: [PATCH/RFC] serial: Add ioctl to enable auto rs485 mode with some Exar UARTs

[Laurent Pinchart <laurentp@cse-semaphore.com>]

Hi Matthias,

On Friday 19 December 2008 00:48:34 Matthias Fuchs wrote:
> Hi,
>
> please see my patch for enabling the RS485 half-duplex control below.
> Please note that I am using this on a PowerPC platform. So I needed to
> add the ioctl to the PowerPC header. As Wolfgang stated, it is
> already in the x86 header. I am not sure if I have to post the modification
> on the powerpc header to the PowerPC list or if it will be accepted here
> as well.  .... But first I will have to see what you think of this patch
> :-)

You will have to submit ioctls.h modifications as a separate patch to the 
powerpc-dev mailing list.

The patch looks good, a few comments below.

> Matthias
>
>
> Some Exar UARTs support a auto rs485 mode. In this mode
> the UART's RTS# pin is activated during transmitting and
> can be used to enable a rs485 line driver. This has nothing
> to do with attempts to do this by manually by asserting/
> deasserting handshake lines.
>
> Signed-off-by: Matthias Fuchs <mfuchs@ma-fu.de>
> ---
>  arch/powerpc/include/asm/ioctls.h |    2 +
>  drivers/serial/8250.c             |   59
> +++++++++++++++++++++++++++++++++++++ 2 files changed, 61 insertions(+), 0
> deletions(-)
>
> diff --git a/arch/powerpc/include/asm/ioctls.h
> b/arch/powerpc/include/asm/ioctls.h index 279a622..1842186 100644
> --- a/arch/powerpc/include/asm/ioctls.h
> +++ b/arch/powerpc/include/asm/ioctls.h
> @@ -89,6 +89,8 @@
>  #define TIOCSBRK	0x5427  /* BSD compatibility */
>  #define TIOCCBRK	0x5428  /* BSD compatibility */
>  #define TIOCGSID	0x5429  /* Return the session ID of FD */
> +#define TIOCGRS485	0x542e
> +#define TIOCSRS485	0x542f
>  #define TIOCGPTN	_IOR('T',0x30, unsigned int) /* Get Pty Number (of
> pty-mux device) */ #define TIOCSPTLCK	_IOW('T',0x31, int)  /* Lock/unlock
> Pty */
>
> diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
> index 303272a..e6df50f 100644
> --- a/drivers/serial/8250.c
> +++ b/drivers/serial/8250.c
> @@ -2511,6 +2511,64 @@ serial8250_type(struct uart_port *port)
>  	return uart_config[type].name;
>  }
>
> +static int
> +serial8250_ioctl_port(struct uart_port *port, unsigned int cmd, unsigned
> long arg) +{
> +	struct uart_8250_port *up = (struct uart_8250_port *)port;
> +
> +	switch (cmd) {
> +	case TIOCSRS485:
> +	{
> +		struct serial_rs485 rs485ctrl;
> +		unsigned char fctr;
> +
> +		if (port->type != PORT_16850)
> +			return -ENODEV;

According to the ioctl manpage, -ENOTTY might be better:

"ENOTTY The specified request does not apply to the kind of object that the 
descriptor d references."

> +		if (copy_from_user(&rs485ctrl,
> +				   (struct serial_rs485 *)arg,
> +				   sizeof(rs485ctrl)))
> +			return -EFAULT;
> +
> +		serial_outp(up, UART_LCR, 0xbf);
> +		fctr = serial_inp(up, UART_FCTR);
> +		if (rs485ctrl.flags & SER_RS485_ENABLED)
> +			fctr |= 0x08;
> +		else
> +			fctr &= ~0x08;
> +		serial_outp(up, UART_FCTR, fctr);
> +		serial_outp(up, UART_LCR, 0);

Doesn't this require locking ? serial8250_break_ctl and serial8250_set_termios 
both take the port.lock spinlock to prevent races from messing up with 
UART_LCR.

> +		return 0;
> +	}
> +
> +	case TIOCGRS485:
> +	{
> +		struct serial_rs485 rs485ctrl;
> +
> +		if (port->type != PORT_16850)
> +			return -ENODEV;

What about setting rs485ctrl.flags to 0 and returning with success when RS485 
auto direction control is not supported by the hardware ? I'm not sure if the 
preferred semantic for get requests on unsupported features is to return an 
error or return a 'feature disabled' value.

> +		serial_outp(up, UART_LCR, 0xbf);
> +		if (serial_inp(up, UART_FCTR) & 0x08)
> +			rs485ctrl.flags = SER_RS485_ENABLED;
> +		else
> +			rs485ctrl.flags = 0;
> +		serial_outp(up, UART_LCR, 0);

Ditto, doesn't this require locking ?

> +
> +		if (copy_to_user((struct serial_rs485 *)arg,
> +				 &rs485ctrl,
> +				 sizeof(rs485ctrl)))
> +			return -EFAULT;
> +		return 0;
> +	}
> +
> +	default:
> +		return -ENOIOCTLCMD;
> +	}
> +
> +	return 0;
> +}
> +
>  static struct uart_ops serial8250_pops = {
>  	.tx_empty	= serial8250_tx_empty,
>  	.set_mctrl	= serial8250_set_mctrl,
> @@ -2529,6 +2587,7 @@ static struct uart_ops serial8250_pops = {
>  	.request_port	= serial8250_request_port,
>  	.config_port	= serial8250_config_port,
>  	.verify_port	= serial8250_verify_port,
> +	.ioctl		= serial8250_ioctl_port,
>  #ifdef CONFIG_CONSOLE_POLL
>  	.poll_get_char = serial8250_get_poll_char,
>  	.poll_put_char = serial8250_put_poll_char,

-- 
Laurent Pinchart
CSE Semaphore Belgium

Chaussee de Bruxelles, 732A
B-1410 Waterloo
Belgium

T +32 (2) 387 42 59
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