Re: Modem control lines for RTSCTS hardware flow control via rts-gpio and cts-gpio with IMX

[Richard Genoud <richard.genoud@gmail.com>]

Hi,

Le 14/01/2022 à 07:19, Tomasz Moń a écrit :
> On 14.01.2022 04:08, Tim Harvey wrote:
>> So I believe in order to support using gpios for rts/cts in the imx
>> uart driver I must find the right place to call imx_uart_rts_active
>> and imx_uart_rts_inactive when the FIFO is not full and full
>> respectively. I'm not that familiar with the Linux uart driver
>> framework - am I on the right track and if so any ideas where this is
>> best done?
> 
> It is not really the driver (and thus FIFO level), but rather the amount
> of free space in tty buffer (checked by Line Discipline workqueue) that
> determines when to throttle (set RTS inactive). This mostly works fine,
> but fails [1] when the RX interrupt frequency is too high [2].
> 
> The throttle/unthrottle request, when termios CRTSCTS is set, is seen by
> the driver as the call to .set_mctrl (imx_uart_set_mctrl) with TIOCM_RTS
> bit cleared/set in mctrl parameter. Currently imx_uart_set_mctrl() only
> controls the UCR2_CTS and UCR2_CTSC bits based on mctrl.
> 
> To support your case you would most likely have to add the gpio handling
> in imx_uart_set_mctrl(). However, I am unaware what other issues you
> might encounter (i.e. if it is not done there yet simply because nobody
> had that use case or if there is some deeper problem).
> 
> [1] https://lore.kernel.org/linux-serial/10e723c0-a28b-de0d-0632-0bd250478313@camlingroup.com/
> [2] https://lore.kernel.org/linux-serial/20220104103203.2033673-1-tomasz.mon@camlingroup.com/
> 
> Best Regards,
> Tomasz Mon
> 

I'd suggest to start testing with a serial port connected to nothing, and check the pins values
with a scope or a voltmeter.
Setting pins values from userspace can done quite easily with :
#include <unistd.h>
#include <termios.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

void usage(char *prog)
{
	printf("usage: %s serial_port id 0/1 sleep_time_sec\n", prog);
	printf("%s\n", "1:DTR	DTR (data terminal ready)");
	printf("%s\n", "2:RTS	RTS (request to send)");
	printf("%s\n", "3:Both");
}

int main(int argc, char **argv)
{
	int fd;
	unsigned status = 0;
	int enable;
	int err;

	if (argc < 5) {
		usage(argv[0]);
		return -1;
	}

	fd = open(argv[1], O_RDWR | O_NOCTTY);

	enable = atoi(argv[3]);

	if (fd < 0)
		return -1;


	switch(atoi(argv[2])) {
	case 0:
		if (enable)
			status |= TIOCM_LE;
		break;
	case 1:
		if (enable)
			status |= TIOCM_DTR;
		break;
	case 2:
		if (enable)
			status |= TIOCM_RTS;
		break;
	case 3:
		if (enable)
			status |= TIOCM_DTR | TIOCM_RTS;
		break;
	default:
		printf("unknown signal\n");
	}

	err = ioctl(fd, TIOCMSET, &status);
	sleep(atoi(argv[4]));
out:
	if (fd > -1)
		close(fd);

	return err;
}

regards,
Richard