coresight: docs: Add information about the topology representations

Update the CoreSight documents to describe the new connections directory
and the links between CoreSight devices in this directory.
Signed-off-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200518180242.7916-7-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/trace/coresight/coresight-ect.rst

@@ -72,7 +72,7 @@ capable of generating or using trigger signals.::
 
   >$ ls /sys/bus/coresight/devices/etm0/cti_cpu0
   channels  ctmid  enable  nr_trigger_cons mgmt  power powered  regs
-  subsystem triggers0 triggers1  uevent
+  connections subsystem triggers0 triggers1  uevent
 
 *Key file items are:-*
    * ``enable``: enables/disables the CTI. Read to determine current state.
@@ -88,6 +88,9 @@ capable of generating or using trigger signals.::
    * ``channels``: Contains the channel API - CTI main programming interface.
    * ``regs``: Gives access to the raw programmable CTI regs.
    * ``mgmt``: the standard CoreSight management registers.
+   * ``connections``: Links to connected *CoreSight* devices. The number of
+     links can be 0 to ``nr_trigger_cons``. Actual number given by ``nr_links``
+     in this directory.
 
 
 triggers<N> directories

Documentation/trace/coresight/coresight.rst

@@ -241,6 +241,91 @@ to the newer scheme, to give a confirmation that what you see on your
 system is not unexpected. One must use the "names" as they appear on
 the system under specified locations.
 
+Topology Representation
+-----------------------
+
+Each CoreSight component has a ``connections`` directory which will contain
+links to other CoreSight components. This allows the user to explore the trace
+topology and for larger systems, determine the most appropriate sink for a
+given source. The connection information can also be used to establish
+which CTI devices are connected to a given component. This directory contains a
+``nr_links`` attribute detailing the number of links in the directory.
+
+For an ETM source, in this case ``etm0`` on a Juno platform, a typical
+arrangement will be::
+
+  linaro-developer:~# ls - l /sys/bus/coresight/devices/etm0/connections
+  <file details>  cti_cpu0 -> ../../../23020000.cti/cti_cpu0
+  <file details>  nr_links
+  <file details>  out:0 -> ../../../230c0000.funnel/funnel2
+
+Following the out port to ``funnel2``::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/funnel2/connections
+  <file details> in:0 -> ../../../23040000.etm/etm0
+  <file details> in:1 -> ../../../23140000.etm/etm3
+  <file details> in:2 -> ../../../23240000.etm/etm4
+  <file details> in:3 -> ../../../23340000.etm/etm5
+  <file details> nr_links
+  <file details> out:0 -> ../../../20040000.funnel/funnel0
+
+And again to ``funnel0``::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/funnel0/connections
+  <file details> in:0 -> ../../../220c0000.funnel/funnel1
+  <file details> in:1 -> ../../../230c0000.funnel/funnel2
+  <file details> nr_links
+  <file details> out:0 -> ../../../20010000.etf/tmc_etf0
+
+Finding the first sink ``tmc_etf0``. This can be used to collect data
+as a sink, or as a link to propagate further along the chain::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/tmc_etf0/connections
+  <file details> cti_sys0 -> ../../../20020000.cti/cti_sys0
+  <file details> in:0 -> ../../../20040000.funnel/funnel0
+  <file details> nr_links
+  <file details> out:0 -> ../../../20150000.funnel/funnel4
+
+via ``funnel4``::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/funnel4/connections
+  <file details> in:0 -> ../../../20010000.etf/tmc_etf0
+  <file details> in:1 -> ../../../20140000.etf/tmc_etf1
+  <file details> nr_links
+  <file details> out:0 -> ../../../20120000.replicator/replicator0
+
+and a ``replicator0``::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/replicator0/connections
+  <file details> in:0 -> ../../../20150000.funnel/funnel4
+  <file details> nr_links
+  <file details> out:0 -> ../../../20030000.tpiu/tpiu0
+  <file details> out:1 -> ../../../20070000.etr/tmc_etr0
+
+Arriving at the final sink in the chain, ``tmc_etr0``::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/tmc_etr0/connections
+  <file details> cti_sys0 -> ../../../20020000.cti/cti_sys0
+  <file details> in:0 -> ../../../20120000.replicator/replicator0
+  <file details> nr_links
+
+As described below, when using sysfs it is sufficient to enable a sink and
+a source for successful trace. The framework will correctly enable all
+intermediate links as required.
+
+Note: ``cti_sys0`` appears in two of the connections lists above.
+CTIs can connect to multiple devices and are arranged in a star topology
+via the CTM. See (:doc:`coresight-ect`) [#fourth]_ for further details.
+Looking at this device we see 4 connections::
+
+  linaro-developer:~# ls -l /sys/bus/coresight/devices/cti_sys0/connections
+  <file details> nr_links
+  <file details> stm0 -> ../../../20100000.stm/stm0
+  <file details> tmc_etf0 -> ../../../20010000.etf/tmc_etf0
+  <file details> tmc_etr0 -> ../../../20070000.etr/tmc_etr0
+  <file details> tpiu0 -> ../../../20030000.tpiu/tpiu0
+
+
 How to use the tracer modules
 -----------------------------