diff options
| -rw-r--r-- | Documentation/pps/pps.txt | 15 | ||||
| -rw-r--r-- | drivers/usb/serial/generic.c | 10 |
2 files changed, 25 insertions, 0 deletions
diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt index d35dcdd82ff6..c03b1be5eb15 100644 --- a/Documentation/pps/pps.txt +++ b/Documentation/pps/pps.txt @@ -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 -------------- diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c index 1f31e6b4c251..3a5dac879094 100644 --- a/drivers/usb/serial/generic.c +++ b/drivers/usb/serial/generic.c @@ -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)) |