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White Rabbit Switch - Software
Commit 7b2f47f7 authored by Alessandro Rubini's avatar Alessandro Rubini
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doc: update with new configuration and timing 

Signed-off-by: Alessandro Rubini's avatarAlessandro Rubini <rubini@gnudd.com>
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doc/wrs-user-manual.in
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......@@ -476,8 +476,161 @@ value is changed by the web interface, proper action is taken.
unknown facility names will generate a runtime error on the switch.
All three strings default to ``@t{daemon.info}''.
@item CONFIG_PORT00_PARAMS
@itemx CONFIG_PORT01_PARAMS
@itemx ...
@itemx CONFIG_PORT17_PARAMS
These configuration items are used to set up port parameters;
this includes the delays, the PTP role (not currently used,
though) and the fiber type as an integer index. Most likely
the default values work for you. See @ref{Timing Configuration}
for details.
@item CONFIG_SFP00_PARAMS
@itemx ...
@itemx CONFIG_SFP09_PARAMS
Configuration for @sc{sfp} models. You should fill values for
all @sc{sfp} models you are using in your @sc{wrs} and all
wavelengths you are using. See @ref{Timing Configuration}
for details.
@item CONFIG_FIBER00_PARAMS
@itemx ...
@itemx CONFIG_FIBER03_PARAMS
Configuration for fiber types. You are expected to have no more
than 4 fiber types installed in your deployment (if more, you need
to add items to the @i{dot-config} file). See @ref{Timing
Configuration} for details.
@item CONFIG_TIME_GM
@itemx CONFIG_TIME_FM
@itemx CONFIG_TIME_BC
The type of PTP clock this switch is. Only one of the three
items should be set (running ``@t{make menuconfig}'' offers
them as an exclusive choice). The options select a grand-master
with external reference, from GPS or Cesium or both; a free-running
master, used for isolated acquisition networks, withouth an
external reference; or a normal ``boundary-clock'' device that
is slave on some ports and master on other ports.
@item CONFIG_PTP_*
We support a set of PPSi configuration files, for different PTP
roles. The default option being selected by @i{Kconfig} is
@t{PTP_WR_DEFAULT} for @t{TIME_BC} devices and @t{PTP_WR_MASTER}
for @t{GM} and @t{FM} devices. The former configuration selects
a mandated slave PTP role on ports @t{wr0} and @t{wr1}, and
mandated master on all other ports; the latter conifiguration
is master on all ports. Please see the help provided within
@i{Kconfig} for more details about the various options we support.
@item CONFIG_PTP_CUSTOM_FILENAME
If you chose @t{CONFIG_PTP_CUSTOM} in the choice above, you
can provide your own filename for the PPSi configuration file;
the chosen name is expected to be intalled in the @sc{wrs}
filesystem.
@end table
@c ==========================================================================
@node Timing Configuration
@section Timing Configuration
This section describes how timing configuration works in the switch.
Please note that up to version 4.1 (included) of @t{wr-switch-sw} things
were different and not really documented.
Timing configuration now depends on three sets of @i{dot-config}
variables: per-port information, per-sfp information and fiber
description.
This is, for explanation's sake, an example of such items:
@smallexample
CONFIG_PORT09_PARAMS="name=wr9,tx=226214,rx=226758,role=slave,fiber=2"
CONFIG_SFP00_PARAMS="name=AXGE-1254-0531,tx=0,rx=0,wl_txrx=1310+1490"
CONFIG_FIBER02_PARAMS="alpha_1310_1490=2.6787e-04"
@end smallexample
When making timing calculation for port @t{wr9}, assuming the
auto-detected @sc{sfp} model is``AXGE-1254-0531'', the software
uses configuration in the following way:
@itemize @bullet
@item The port has known tx and rx delays (around 226ns); the
values depend on trace length and other hardware-specific details
and are determined by a calibration procedure. These values are
used as constant delays in the @i{tx} and @i{rx} directions.
@item The port is also configured as slave (information not used
at this point) and is deployed using fiber type 2 -- this number
is just a local enumeration of fiber types; most likely you'll be
using type ``0'' in every port.
@item The transceiver installed uses 1310nm light for tx
and 1490nm light for rx (this is part of the specification of the
transceiver, but cannot be auto-detected). Moreover it has 0 constant
delay in both tx and rx, determined by calibration.
@item The fiber type being used (type 2 here), when driven
with 1310nm and 1490nm wavelengths, features an @i{alpha} parameter of
0.00026787 (i.e. a ratio of 1.00026787) between the speed of light in
the two directions. This value depends on both the fiber type and
wavelength, and is determined, again, by calibration.
@end itemize
Please note that only one alpha value is provided, because the
opposite one (@t{alpha_1490_1310}) is calculated by software.
@subheading Other Deployments
The example above matches the choices we make at CERN, where our
White Rabbit networks are all run with a single monomodal fiber
and 1310/1490 light.
If you are using dual-fiber transceivers, which is acceptable for
short links, you use the same wavelength in both direction, over two
fibers of the same length. In this case you may choose to avoid
writing the @t{wl_txrx} parameter in @sc{sfp} configuration and the
@t{alpha_XX_XX} parameter in fiber configuration. The missing
parameters will cause warning messages on the console, but are not
fatal, and a default alpha of 0 is used.
If you are using a pair of transceivers with different wavelengths,
and long fibers, you should provide an appropriate value of alpha,
according to laboratory measures on your fiber type. The
@t{CONFIG_FIBERxx_PARAMS} items are parsed as a list of
comma-separated assignments, so you can specify and number of
wavelength pairs. The accuracy of your value depends on the length
of the fiber link. For a 10km fiber (100us round-trip) you need to know
alpha up to 1e-7 if you want the related uncertainty to be
less than 10ps.
@subheading Calibration
Calibration of per-port and per-@sc{sfp} delays is described in the
White Rabbit wiki:
@url{http://www.ohwr.org/projects/white-rabbit/wiki/Calibration}. The
procedure can measure the total constant delays, but splitting between
the what depends on the port and what depends on the transceiver is
arbitrary (also, the split between the tx and rx paths is arbitrary).
The delays used in this example come from values listed in the above
web page, and you should not be surprised by the fact that the
transceiver specifies the delays as zero. By performing the
calibration procedure using this very transceiver type, the hardware
engineers decided to assign the whole of the delay to the port (any
split of the measured value is equally right). Other transceiver
types can be calibrated to either positive or negative values, to cope
with the @i{difference} between them and the @t{AXGE} devices.
@c ############################################################################
@node Booting with Barebox
@chapter Booting with Barebox
......
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