Signed-off-by: Paul Chavent <paul.chav...@onera.fr>
---
Documentation/pps/pps.txt | 15 +++++++++++++++
drivers/usb/serial/generic.c | 9 +++++++++
2 files changed, 24 insertions(+)
diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
index d35dcdd..67b9a94 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
+synchronisation.
+
+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 1f31e6b..877d6e0 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -568,6 +568,15 @@ void usb_serial_handle_dcd_change(struct usb_serial_port
*usb_port,
{
struct tty_port *port = &usb_port->port;
+ 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);
+ }
+ }
+
dev_dbg(&usb_port->dev, "%s - status %d\n", __func__, status);
if (status)
--
1.7.12.1
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