USB: serial: invoke dcd_change ldisc's handler.

[sfrench/cifs-2.6.git] / Documentation / pps / pps.txt

diff --git a/Documentation/pps/pps.txt b/Documentation/pps/pps.txt
index d35dcdd82ff6a6c93b2dae00be717080b9b21cb1..c03b1be5eb1517b7b16cbddd6d2999101020ca53 100644 (file)
--- 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
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