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USB: serial: invoke dcd_change ldisc's handler.

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The DCD pin of the serial port can receive a PPS signal. By calling
the port line discipline dcd handle, this patch allow to monitor PPS
through USB serial devices.

However the performance aren't as good as the uart drivers, so
document this point too.

Signed-off-by: Paul Chavent <paul.chavent@onera.fr>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Paul Chavent 2013-09-16 08:41:00 +02:00 committed by Greg Kroah-Hartman
parent d14654dff7
commit 833efc0ed1
2 changed files with 25 additions and 0 deletions
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15
Documentation/pps/pps.txt
View File

@ -66,6 +66,21 @@ In LinuxPPS the PPS sources are simply char devices usually mapped
into files /dev/pps0, /dev/pps1, etc..
PPS with USB to serial devices
------------------------------
It is possible to grab the PPS from an USB to serial device. However,
you should take into account the latencies and jitter introduced by
the USB stack. Users has reported clock instability around +-1ms when
synchronized with PPS through USB. This isn't suited for time server
synchronization.
If your device doesn't report PPS, you can check that the feature is
supported by its driver. Most of the time, you only need to add a call
to usb_serial_handle_dcd_change after checking the DCD status (see
ch341 and pl2303 examples).
Coding example
--------------

10
drivers/usb/serial/generic.c
View File

@ -570,6 +570,16 @@ void usb_serial_handle_dcd_change(struct usb_serial_port *usb_port,
dev_dbg(&usb_port->dev, "%s - status %d\n", __func__, status);
if (tty) {
struct tty_ldisc *ld = tty_ldisc_ref(tty);
if (ld) {
if (ld->ops->dcd_change)
ld->ops->dcd_change(tty, status);
tty_ldisc_deref(ld);
}
}
if (status)
wake_up_interruptible(&port->open_wait);
else if (tty && !C_CLOCAL(tty))