On 21/03/2012 3:08 PM, Greg KH wrote:
On Wed, Mar 21, 2012 at 02:47:56PM -0400, Gerald Villemure wrote:
Currently if we want to read the status of the PINs on a serial
port, I need to open the file (e.g. /dev/ttyUSB0) unfortunately in
doing so we *change* RTS/DTR
Hm, just opening the port shouldn't change the line settings. Is this
unique to the pl2303 driver, or does it also happen on a "normal" serial
port as well?
I tryed it on: USB:pl2303,cp210x,ftdi_sio and also on the regular
motherboard /dev/ttyS0
In every case DTR and RTS go high as soon as OPEN is called on the
device file.
I hacked the pl2303.c files to add the following param:
parm: openmode:Intitial settings on open, default=3
(0=!DTR_!RTS, 1=DTS_!RTS, 2=!DTR_RTS, 3=DTR_RTS) (int)
That way you can load the module using something like this:
insmod /.../pl2303.ko debug openmode=0
But its an ugly hack... If you want it let me know.
How difficult would it be to extent the usb_serial driver to expose
the pin status like this:
/sys/class/tty/ttyUSB0/device/RTS (rw, Request To Send)
/sys/class/tty/ttyUSB0/device/CTS (r, Clear To Send)
/sys/class/tty/ttyUSB0/device/DSR (r, Data Set Ready)
/sys/class/tty/ttyUSB0/device/DCD (r, Data Carrier Detect)
/sys/class/tty/ttyUSB0/device/DTR (rw, Data Terminal Ready)
/sys/class/tty/ttyUSB0/device/RI (r, Ring Indicator)
We could then simply read the file /sys/class/tty/ttyUSB0/device/DCD
and get a 1 if it is high.
That would imply that at all times, even if the port is not open, the
driver needs to be watching the line settings of the device? For usb to
serial devices, this can take a lot of USB traffic to do so, and might
make people who care about power usage unhappy.
Or we could just do the querying of the device if the sysfs file is
open...
That is correct, you poll the device ONLY when the sysfs file is opened.
This would open up the use of the serial port PIN's to scripting.
We could also control RTS/DTS by using something like:
echo 1> /sys/class/tty/ttyUSB0/device/DTR
I realize that if we LIMIT our usage of the serial port to ONLY
serial communication then there is not value is this extension. But
the moment you start using it to drive DIY electronics it would be a
VERY welcomed addition.
There is the POSIX interface for serial ports, which we can't ignore, so
we need to keep that. But, to extend this to all serial ports, not just
USB ones, might be nice for some users.
I would agree.
So, care to work on a patch to the tty core that implements this to see
if it's even viable? :)
I am far from being a proper kernel hacker. I would not know where to
start.
Gérald
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