Restart the "Things not to do" list.

1 files changed, 38 insertions, 0 deletions

diff --git a/HACKING b/HACKING
index 4f91d979..804e7be6 100644
--- a/HACKING
+++ b/HACKING
@@ -374,6 +374,44 @@ structure should be UTC (with leap-second corrections) not just Unix
 seconds since the epoch.  The report-generator function for D
 does *not* apply a timezone offset.
 
+** Blind alleys
+
+Things we've considered doing and rejected.
+
+*** Allowing clients to ship arbitrary control strings to a GPS
+
+Tempting -- it would allow us to do sirfmon-like things with the
+daemon running -- but a bad idea.  It would make denial-of-service 
+attacks on applications using the GPS far too easy.  For examples,
+suppose the control string were a baud-rate change?
+
+*** Using libusb to do USB device discovery
+
+There has been some consideration of going to the cross-platform libusb
+library to do USB device discovery. This would create an external
+dependency that gpsd doesn't now have, and bring more complexity on
+board than is probably desirable.
+
+We've chosen instead to rely on the local hotplug system.  That way
+gosd can concentrate solely on knowing about GPSes.
+
+*** Setting FIFO threshold to 1 to reduce jitter in serial-message times
+
+When using gpsd as a time reference, one of the things we'd like to do
+is make the amount of lag in the message path from GPS to GPS small
+and with as little jitter as possible, so we can correct for it with
+a constant offset.
+
+A possibility we considered is to set the FIFO threshold on the serial
+device UART to 1 using TIOCGSERIAL/TIOCSSERIAL.  This would, in
+effect, disable transmission buffering, increasing lag but decreasing
+jitter.
+
+But it's almost certainly not worth the work.  Rob Janssen, our timekeeping
+expert, reckons that at 4800bps the UART buffering can cause at most
+about 15msec of jitter.  This is, observably, swamped by other less
+controllable sources of variation.
+
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