RTS toggling in serial port from kernel space

bizoo · 01-29-2010, 09:26 AM

HI

Driven by my needs , i need to toggle only the flow control (RTS, CTS) bits and have nothing to do with the rest of the functionality (e.g. normaltx , rx operation).

I am presuming that there exists a line discipline from user space for toggling RTS as
ioctl (*file, cmd, *unsigned long )
can be used easily to toggle RTS.

However, the use of IOCTL in kernel space for the same purpose brings about no change on RTS (oscilloscope shows no change).

google led me to this
http://www.archivum.info/linux-kerne...dling-RTS.html

and according to that i should make changes directly in the driver.

Can some one give me some more pointers on that? i actually have not seen the serial driver from inside and am hoping if there do exists some other work around so any pointers will be appreciated.
The snippet of my module which tries to toggle the pin in kernel is attached for pointing to possible mistakes

thx

Code:

int x=0;

int actual_module()
{   int k;
    mm_segment_t oldfs;
		f=filp_open("/dev/ttyUSB0",O_RDWR | O_NDELAY,0);
		if ((IS_ERR(f)))
		{printk("device open fail \n");
		 return 1;
		}
		f->f_pos=0; 
		oldfs=get_fs();
		set_fs(KERNEL_DS);	
		tty=(struct tty_struct*)f->private_data;
		x |= TIOCM_RTS;
		//f->f_op->ioctl(f->f_dentry->d_inode, f, TIOCMSET, (unsigned long)&x);
    k=tty->driver->ops->ioctl(tty, f,TIOCMSET, (unsigned long)&x);
		printk("return value is %d \n", k);
		mdelay(1000);
	  x &= ~TIOCM_RTS;
  	set_fs(oldfs);
		return 0;
}

theNbomr · 01-31-2010, 06:30 PM

If you know the nature of the serial hardware, you should be able to use one of the userspace facilities such as io_perm + outb/inb to tickle the right registers in the UART. I am not aware of any ioctl or other calls that permit manipulation of flow control and modem control signals.
Is your application limited to a specific hardware architecture? If so, which?

--- rod.

bizoo · 01-31-2010, 11:55 PM

Well
i can easily do it from user space but the time taken between issuing the command and execution is too large (200 us). Thus i am trying to toggle the flow control pins where this delay , though unpredictable, will be on average less.

And i have no special hardware in mind,
the normal x86 processors willl do
thx

theNbomr · 02-02-2010, 10:15 AM

I'm only speculating here, but my hunch is that the longish time lag is a result of the overhead of making a system call. In order to eliminate this, you will have to write substantial portions of the application as a kernel driver. Are you sure Linux is the right OS for this?

--- rod.