On Wed, 21 Mar 2012, 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? > > > 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... > > > 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. > > So, care to work on a patch to the tty core that implements this to see > if it's even viable? :) Alternatively, why not write a program that opens the serial port and provides an easy interface for reading and writing the values of the line settings (or are these the modem settings? -- I never can remember which are which). Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
