Re: Re: [PATCH V5.1] serial/uart/8250: Add tunable RX interrupt trigger I/F of FIFO buffers

[Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>]

Hi Stephen,

Thank you for your review.

(2014/04/16 2:39), Stephen Warren wrote:
> On 04/15/2014 02:06 AM, Yoshihiro YUNOMAE wrote:
>> /* I found a bug in V5, so I resend this as V5.1. Please do not review V5. */
>>
>> 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.h b/drivers/tty/serial/8250/8250.h
>
>> +#define FCR_RX_TRIG_OFFSET	6
>
> Isn't "SHIFT" the usual term rather than "OFFSET". Why not add this

OK, I'll rename this.

> define into include/uapi/linux/serial_reg.h, along with all the other
> UART_FCR_TRIGGER_* defines?

I thought these definition will not be used in userland.
I'll move these in serial_reg.h.

>> +#define FCR_RX_TRIG_BITS(x)	(((x) >> FCR_RX_TRIG_OFFSET) & 0x3)
>
> Perhaps use UART_FCR_TRIGGER_MASK rather than a hard-coded "3" (and then
> you'd have to mask before shifting).

OK, I'll define FCR_RX_TRIG_BITS as follows:

#define UART_FCR_R_TRIG_SHIFT    6
#define UART_FCR_R_TRIG_BITS(x) \
     (((x) & UART_FCR_TRIGGER_MASK) >> UART_FCR_R_TRIG_SHIFT)

>> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
>
>> @@ -2275,10 +2276,9 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
>
>>   	if (up->capabilities & UART_CAP_FIFO && port->fifosize > 1) {
>> -		fcr = uart_config[port->type].fcr;
>> -		if ((baud < 2400 && !up->dma) || fifo_bug) {
>> -			fcr &= ~UART_FCR_TRIGGER_MASK;
>> -			fcr |= UART_FCR_TRIGGER_1;
>> +		/* NOTE: If fifo_bug is not set, a uaser can set RX_trigger. */
>> +		if ((baud < 2400 && !up->dma &&
>> +		    (up->fcr == uart_config[port->type].fcr)) || up->fifo_bug) {
>> +			up->fcr &= ~UART_FCR_TRIGGER_MASK;
>> +			up->fcr |= UART_FCR_TRIGGER_1;
>>   		}
>>   	}
>
> Does the "(up->fcr == uart_config[port->type].fcr)" term prevent the
> user from changing the trigger level multiple times? Perhaps this is
> intended?

No, this means that if a user changed FCR value before setting termios,
use the changed value because the user think changed value is always
set. But, I thought this is not straightforward and it cannot help
when the user want to use default FCR value.
Could I add FCR changed flag(user_changed_fcr) in uart_8250_port
structure and check the flag here?
Or shouldn't the driver check the user changing?

>> +static int convert_fcr2val(struct uart_8250_port *up)
>
> "val" is rather generic here, and doesn't describe what the function is
> doing. What about fcr_get_rxtrig_bytes()?

Sure.

>> +{
>> +	const struct serial8250_config *conf_type = &uart_config[up->port.type];
>> +	unsigned char rx_trig_raw = up->fcr & UART_FCR_TRIGGER_MASK;
>> +	unsigned char val;
>> +
>> +	val = conf_type->rx_trig_byte[FCR_RX_TRIG_BITS(rx_trig_raw)];
>
> Given that FCR_RX_TRIG_BITS() does all the required shifting/masking,
> why not just:
>
> 	val = conf_type->rx_trig_byte[FCR_RX_TRIG_BITS(up->fcr)];

That's true. I'll fix it.

>> +static int convert_val2rxtrig(struct uart_8250_port *up, unsigned char val)
>
> Perhaps name this bytes_to_fcr_txtrig()?

OK, I'll rename it to bytes_to_fcr_rxtrig().

>> +static void register_dev_spec_attr_grp(struct uart_8250_port *up)
>> +{
>> +	switch (up->port.type) {
>> +	case PORT_16550A:
>> +	case PORT_16650V2:
>> +	case PORT_16654:
>> +	case PORT_16750:
>> +	case PORT_TEGRA:
>
> Why not replace that with:
>
> const struct serial8250_config *conf_type = &uart_config[up->port.type];
> if (conf_type->rx_trig_byte[0]) {
>
> (or something like that anyway)
>
> That means this switch statement doesn't need to be updated when new
> UART types are modified to set up the rx_trig_byte field.

Nice idea! I'll use your idea.

Thanks,
Yoshihiro YUNOMAE

-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@hitachi.com