Re: [PATCH V4] serial/uart/8250: Add tunable RX interrupt trigger I/F of FIFO buffers

[Stephen Warren <swarren@wwwdotorg.org>]

On 03/19/2014 04:15 AM, Yoshihiro YUNOMAE wrote:
> Add tunable RX interrupt trigger I/F of FIFO buffers.
> Serial devices are used as not only message communication devices but control
> or sending communication devices. For the latter uses, normally small data
> will be exchanged, so user applications want to receive data unit as soon as
> possible for real-time tendency. If we have a sensor which sends a 1 byte data
> each time and must control a device based on the sensor feedback, the RX
> interrupt should be triggered for each data.
> 
> According to HW specification of serial UART devices, RX interrupt trigger
> can be changed, but the trigger is hard-coded. For example, RX interrupt trigger
> in 16550A can be set to 1, 4, 8, or 14 bytes for HW, but current driver sets
> the trigger to only 8bytes.

> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 69932b7..fc17fb2 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -531,11 +531,10 @@ static void serial8250_clear_fifos(struct uart_8250_port *p)
>  
>  void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p)
>  {
> -	unsigned char fcr;
> -
>  	serial8250_clear_fifos(p);
> -	fcr = uart_config[p->port.type].fcr;
> -	serial_out(p, UART_FCR, fcr);
> +	if (!p->fcr)
> +		p->fcr = uart_config[p->port.type].fcr;

Rather than sprinkling this initialization all over the place, can't the
fcr value be set up one time, when the port is first created?

> +static int convert_fcr2val(struct uart_8250_port *up, unsigned char fcr)
> +{
> +	unsigned char trig_raw = fcr & UART_FCR_TRIGGER_MASK;
> +
> +	switch (up->port.type) {
> +	case PORT_16550A:
> +		if (trig_raw == UART_FCR_R_TRIG_00)
> +			return 1;
> +		else if (trig_raw == UART_FCR_R_TRIG_01)
> +			return 4;
> +		else if (trig_raw == UART_FCR_R_TRIG_10)
> +			return 8;
> +		else if (trig_raw == UART_FCR_R_TRIG_11)
> +			return 14;
> +		break;
> +	}
> +	return -EOPNOTSUPP;
> +}

Rather than implementing this translation inside this one function
(well, and convert_val2rxtrig() too), perhaps it would make sense to put
the values into uart_config[] and just look them up here. That would
better isolate the type-specific data into one place.

The comment right before UART_FCR_R_TRIG_00 in
include/uapi/linux/serial_reg.h might help when filling in any extra
fields in uart_config[].