.. SPDX-License-Identifier: GPL-2.0

=============================================
CoreSight Embedded Cross Trigger (CTI & CTM).
=============================================

    :Author:   Mike Leach <mike.leach@linaro.org>
    :Date:     November 2019

Hardware Description
--------------------

The CoreSight Cross Trigger Interface (CTI) is a hardware device that takes
individual input and output hardware signals known as triggers to and from
devices and interconnects them via the Cross Trigger Matrix (CTM) to other
devices via numbered channels, in order to propagate events between devices.

e.g.::

 0000000  in_trigs  :::::::
 0 C   0----------->:     :             +======>(other CTI channel IO)
 0  P  0<-----------:     :             v
 0   U 0  out_trigs :     : Channels  *****      :::::::
 0000000            : CTI :<=========>*CTM*<====>: CTI :---+
 #######  in_trigs  :     : (id 0-3)  *****      :::::::   v
 # ETM #----------->:     :                         ^   #######
 #     #<-----------:     :                         +---# ETR #
 ####### out_trigs  :::::::                             #######

The CTI driver enables the programming of the CTI to attach triggers to
channels. When an input trigger becomes active, the attached channel will
become active. Any output trigger attached to that channel will also
become active. The active channel is propagated to other CTIs via the CTM,
activating connected output triggers there, unless filtered by the CTI
channel gate.

It is also possible to activate a channel using system software directly
programming registers in the CTI.

The CTIs are registered by the system to be associated with CPUs and/or other
CoreSight devices on the trace data path. When these devices are enabled the
attached CTIs will also be enabled. By default/on power up the CTIs have
no programmed trigger/channel attachments, so will not affect the system
until explicitly programmed.

The hardware trigger connections between CTIs and devices is implementation
defined, unless the CPU/ETM combination is a v8 architecture, in which case
the connections have an architecturally defined standard layout.

The hardware trigger signals can also be connected to non-CoreSight devices
(e.g. UART), or be propagated off chip as hardware IO lines.

All the CTI devices are associated with a CTM. On many systems there will be a
single effective CTM (one CTM, or multiple CTMs all interconnected), but it is
possible that systems can have nets of CTIs+CTM that are not interconnected by
a CTM to each other. On these systems a CTM index is declared to associate
CTI devices that are interconnected via a given CTM.

Sysfs files and directories
---------------------------

The CTI devices appear on the existing CoreSight bus alongside the other
CoreSight devices::

    >$ ls /sys/bus/coresight/devices
     cti_cpu0  cti_cpu2  cti_sys0  etm0  etm2  funnel0  replicator0  tmc_etr0
     cti_cpu1  cti_cpu3  cti_sys1  etm1  etm3  funnel1  tmc_etf0     tpiu0

The ``cti_cpu<N>`` named CTIs are associated with a CPU, and any ETM used by
that core. The ``cti_sys<N>`` CTIs are general system infrastructure CTIs that
can be associated with other CoreSight devices, or other system hardware
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
  connections subsystem triggers0 triggers1  uevent

*Key file items are:-*
   * ``enable``: enables/disables the CTI. Read to determine current state.
     If this shows as enabled (1), but ``powered`` shows unpowered (0), then
     the enable indicates a request to enabled when the device is powered.
   * ``ctmid`` : associated CTM - only relevant if system has multiple CTI+CTM
     clusters that are not interconnected.
   * ``nr_trigger_cons`` : total connections - triggers<N> directories.
   * ``powered`` : Read to determine if the CTI is currently powered.

*Sub-directories:-*
   * ``triggers<N>``: contains list of triggers for an individual connection.
   * ``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
~~~~~~~~~~~~~~~~~~~~~~~

Individual trigger connection information. This describes trigger signals for
CoreSight and non-CoreSight connections.

Each triggers directory has a set of parameters describing the triggers for
the connection.

   * ``name`` : name of connection
   * ``in_signals`` : input trigger signal indexes used in this connection.
   * ``in_types`` : functional types for in signals.
   * ``out_signals`` : output trigger signals for this connection.
   * ``out_types`` : functional types for out signals.

e.g::

    >$ ls ./cti_cpu0/triggers0/
    in_signals  in_types  name  out_signals  out_types
    >$ cat ./cti_cpu0/triggers0/name
    cpu0
    >$ cat ./cti_cpu0/triggers0/out_signals
    0-2
    >$ cat ./cti_cpu0/triggers0/out_types
    pe_edbgreq pe_dbgrestart pe_ctiirq
    >$ cat ./cti_cpu0/triggers0/in_signals
    0-1
    >$ cat ./cti_cpu0/triggers0/in_types
    pe_dbgtrigger pe_pmuirq

If a connection has zero signals in either the 'in' or 'out' triggers then
those parameters will be omitted.

Channels API Directory
~~~~~~~~~~~~~~~~~~~~~~

This provides an easy way to attach triggers to channels, without needing
the multiple register operations that are required if manipulating the
'regs' sub-directory elements directly.

A number of files provide this API::

   >$ ls ./cti_sys0/channels/
   chan_clear         chan_inuse      chan_xtrigs_out     trigin_attach
   chan_free          chan_pulse      chan_xtrigs_reset   trigin_detach
   chan_gate_disable  chan_set        chan_xtrigs_sel     trigout_attach
   chan_gate_enable   chan_xtrigs_in  trig_filter_enable  trigout_detach
   trigout_filtered

Most access to these elements take the form::

  echo <chan> [<trigger>] > /<device_path>/<operation>

where the optional <trigger> is only needed for trigXX_attach | detach
operations.

e.g.::

   >$ echo 0 1 > ./cti_sys0/channels/trigout_attach
   >$ echo 0 > ./cti_sys0/channels/chan_set

Attaches trigout(1) to channel(0), then activates channel(0) generating a
set state on cti_sys0.trigout(1)


*API operations*

   * ``trigin_attach, trigout_attach``: Attach a channel to a trigger signal.
   * ``trigin_detach, trigout_detach``: Detach a channel from a trigger signal.
   * ``chan_set``: Set the channel - the set state will be propagated around
     the CTM to other connected devices.
   * ``chan_clear``: Clear the channel.
   * ``chan_pulse``: Set the channel for a single CoreSight clock cycle.
   * ``chan_gate_enable``: Write operation sets the CTI gate to propagate
     (enable) the channel to other devices. This operation takes a channel
     number. CTI gate is enabled for all channels by default at power up. Read
     to list the currently enabled channels on the gate.
   * ``chan_gate_disable``: Write channel number to disable gate for that
     channel.
   * ``chan_inuse``: Show the current channels attached to any signal
   * ``chan_free``: Show channels with no attached signals.
   * ``chan_xtrigs_sel``: write a channel number to select a channel to view,
     read to show the selected channel number.
   * ``chan_xtrigs_in``: Read to show the input triggers attached to
     the selected view channel.
   * ``chan_xtrigs_out``:Read to show the output triggers attached to
     the selected view channel.
   * ``trig_filter_enable``: Defaults to enabled, disable to allow potentially
     dangerous output signals to be set.
   * ``trigout_filtered``: Trigger out signals that are prevented from being
     set if filtering ``trig_filter_enable`` is enabled. One use is to prevent
     accidental ``EDBGREQ`` signals stopping a core.
   * ``chan_xtrigs_reset``: Write 1 to clear all channel / trigger programming.
     Resets device hardware to default state.


The example below attaches input trigger index 1 to channel 2, and output
trigger index 6 to the same channel. It then examines the state of the
channel / trigger connections using the appropriate sysfs attributes.

The settings mean that if either input trigger 1, or channel 2 go active then
trigger out 6 will go active. We then enable the CTI, and use the software
channel control to activate channel 2. We see the active channel on the
``choutstatus`` register and the active signal on the ``trigoutstatus``
register. Finally clearing the channel removes this.

e.g.::

   .../cti_sys0/channels# echo 2 1 > trigin_attach
   .../cti_sys0/channels# echo 2 6 > trigout_attach
   .../cti_sys0/channels# cat chan_free
   0-1,3
   .../cti_sys0/channels# cat chan_inuse
   2
   .../cti_sys0/channels# echo 2 > chan_xtrigs_sel
   .../cti_sys0/channels# cat chan_xtrigs_trigin
   1
   .../cti_sys0/channels# cat chan_xtrigs_trigout
   6
   .../cti_sys0/# echo 1 > enable
   .../cti_sys0/channels# echo 2 > chan_set
   .../cti_sys0/channels# cat ../regs/choutstatus
   0x4
   .../cti_sys0/channels# cat ../regs/trigoutstatus
   0x40
   .../cti_sys0/channels# echo 2 > chan_clear
   .../cti_sys0/channels# cat ../regs/trigoutstatus
   0x0
   .../cti_sys0/channels# cat ../regs/choutstatus
   0x0