Behavior of serial usb driver when unplugged
A message that is not in the archive (sent off-list or not yet received): [email protected]
A message that is not in the archive (sent off-list or not yet received): [email protected]
I think that the fix to send tty_hangup when the USB-to-serial adapter is disconnected really should go in (there was about a 2-line patch posted a while ago to add this). The application I'm working on required this patch in order to detect that the USB serial port it's talking to has been disconnected from the machine - otherwise the behavior you get is rather silly, if you're blocked on a read it never returns after the device was unplugged, and writes return -ENODEV; basically if you don't write anything there is no way whatsoever to detect that the port is gone. It sounds like the poster's application is another where the current behavior of the USB serial driver makes things a real pain..
----- Original Message -----
From: "Greg KH" <[email protected]>
Newsgroups: fa.linux.kernel
To: "Byron Stanoszek" <[email protected]>
Cc: <[email protected]>; <[email protected]>
Sent: Wednesday, June 09, 2004 1:32 PM
Subject: Re: [linux-usb-devel] Behavior of serial usb driver when unplugged
Quoted 4 lines · Byron Stanoszek
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Hi all,
I'm currently using Linux (2.6.7-rc3) in an embedded system with a
8-port
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Sealevel SeaLink 2802 USB device. This is a 8-port RS-232/422 device
that
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allocates
/dev/ttyUSB0through/dev/ttyUSB7when plugged in.If I have a process talking to one of the ports, e.g. 'cat <
/dev/ttyUSB0',
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and
I unplug the USB hub, all ports except ttyUSB0 unregister properly.That's because userspace still has that port open.
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Without killing the 'cat' process, plugging the hub back in will make it allocate
/dev/ttyUSB1through/dev/ttyUSB8, thereby offsetting each USB port# by 1.Yup, glad it's all working properly for you.
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When killing the 'cat' process at this point, the kernel reports:
drivers/usb/serial/ftdi_sio.c: error from flowcontrol urbdrivers/usb/serial/ftdi_sio.c: Error from DTR LOW urbdrivers/usb/serial/ftdi_sio.c: Error from RTS LOW urband then unregisters
/dev/ttyUSB0.Nice.
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Is there a way to allow "hotplug" of a USB device to reuse
/dev/ttyUSB0regardless if an application still has that particular tty open?Nope. Why would you want to do such a thing?
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If not, is there a way I could make the serial subsystem can send an EIO errno or some other notification when the serial device is disconnected?
It's been discussed that the
tty_hangup()should be done for the port when this happens. I haven't messed with it to test it out, as no one has complained in the 5+ years of usb-serial drivers being in the kernel :)Also, you could use something like udev to make the name always show up the same for the device node no matter if it was called ttyUSB0 or ttyUSB1, which might solve some of your problems (but not others it sounds like.)
thanks,
greg k-h
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