======================================
Coresight - HW Assisted Tracing on ARM
======================================

   :Author:   Mathieu Poirier <mathieu.poirier@linaro.org>
   :Date:     September 11th, 2014

Introduction
------------

Coresight is an umbrella of technologies allowing for the debugging of ARM
based SoC.  It includes solutions for JTAG and HW assisted tracing.  This
document is concerned with the latter.

HW assisted tracing is becoming increasingly useful when dealing with systems
that have many SoCs and other components like GPU and DMA engines.  ARM has
developed a HW assisted tracing solution by means of different components, each
being added to a design at synthesis time to cater to specific tracing needs.
Components are generally categorised as source, link and sinks and are
(usually) discovered using the AMBA bus.

"Sources" generate a compressed stream representing the processor instruction
path based on tracing scenarios as configured by users.  From there the stream
flows through the coresight system (via ATB bus) using links that are connecting
the emanating source to a sink(s).  Sinks serve as endpoints to the coresight
implementation, either storing the compressed stream in a memory buffer or
creating an interface to the outside world where data can be transferred to a
host without fear of filling up the onboard coresight memory buffer.

At typical coresight system would look like this::

  *****************************************************************
 **************************** AMBA AXI  ****************************===||
  *****************************************************************    ||
        ^                    ^                            |            ||
        |                    |                            *            **
     0000000    :::::     0000000    :::::    :::::    @@@@@@@    ||||||||||||
     0 CPU 0<-->: C :     0 CPU 0<-->: C :    : C :    @ STM @    || System ||
  |->0000000    : T :  |->0000000    : T :    : T :<--->@@@@@     || Memory ||
  |  #######<-->: I :  |  #######<-->: I :    : I :      @@@<-|   ||||||||||||
  |  # ETM #    :::::  |  # PTM #    :::::    :::::       @   |
  |   #####      ^ ^   |   #####      ^ !      ^ !        .   |   |||||||||
  | |->###       | !   | |->###       | !      | !        .   |   || DAP ||
  | |   #        | !   | |   #        | !      | !        .   |   |||||||||
  | |   .        | !   | |   .        | !      | !        .   |      |  |
  | |   .        | !   | |   .        | !      | !        .   |      |  *
  | |   .        | !   | |   .        | !      | !        .   |      | SWD/
  | |   .        | !   | |   .        | !      | !        .   |      | JTAG
  *****************************************************************<-|
 *************************** AMBA Debug APB ************************
  *****************************************************************
   |    .          !         .          !        !        .    |
   |    .          *         .          *        *        .    |
  *****************************************************************
 ******************** Cross Trigger Matrix (CTM) *******************
  *****************************************************************
   |    .     ^              .                            .    |
   |    *     !              *                            *    |
  *****************************************************************
 ****************** AMBA Advanced Trace Bus (ATB) ******************
  *****************************************************************
   |          !                        ===============         |
   |          *                         ===== F =====<---------|
   |   :::::::::                         ==== U ====
   |-->:: CTI ::<!!                       === N ===
   |   :::::::::  !                        == N ==
   |    ^         *                        == E ==
   |    !  &&&&&&&&&       IIIIIII         == L ==
   |------>&& ETB &&<......II     I        =======
   |    !  &&&&&&&&&       II     I           .
   |    !                    I     I          .
   |    !                    I REP I<..........
   |    !                    I     I
   |    !!>&&&&&&&&&       II     I           *Source: ARM ltd.
   |------>& TPIU  &<......II    I            DAP = Debug Access Port
           &&&&&&&&&       IIIIIII            ETM = Embedded Trace Macrocell
               ;                              PTM = Program Trace Macrocell
               ;                              CTI = Cross Trigger Interface
               *                              ETB = Embedded Trace Buffer
          To trace port                       TPIU= Trace Port Interface Unit
                                              SWD = Serial Wire Debug

While on target configuration of the components is done via the APB bus,
all trace data are carried out-of-band on the ATB bus.  The CTM provides
a way to aggregate and distribute signals between CoreSight components.

The coresight framework provides a central point to represent, configure and
manage coresight devices on a platform.  This first implementation centers on
the basic tracing functionality, enabling components such ETM/PTM, funnel,
replicator, TMC, TPIU and ETB.  Future work will enable more
intricate IP blocks such as STM and CTI.


Acronyms and Classification
---------------------------

Acronyms:

PTM:
    Program Trace Macrocell
ETM:
    Embedded Trace Macrocell
STM:
    System trace Macrocell
ETB:
    Embedded Trace Buffer
ITM:
    Instrumentation Trace Macrocell
TPIU:
     Trace Port Interface Unit
TMC-ETR:
        Trace Memory Controller, configured as Embedded Trace Router
TMC-ETF:
        Trace Memory Controller, configured as Embedded Trace FIFO
CTI:
    Cross Trigger Interface

Classification:

Source:
   ETMv3.x ETMv4, PTMv1.0, PTMv1.1, STM, STM500, ITM
Link:
   Funnel, replicator (intelligent or not), TMC-ETR
Sinks:
   ETBv1.0, ETB1.1, TPIU, TMC-ETF
Misc:
   CTI


Device Tree Bindings
--------------------

See Documentation/devicetree/bindings/arm/coresight.txt for details.

As of this writing drivers for ITM, STMs and CTIs are not provided but are
expected to be added as the solution matures.


Framework and implementation
----------------------------

The coresight framework provides a central point to represent, configure and
manage coresight devices on a platform.  Any coresight compliant device can
register with the framework for as long as they use the right APIs:

.. c:function:: struct coresight_device *coresight_register(struct coresight_desc *desc);
.. c:function:: void coresight_unregister(struct coresight_device *csdev);

The registering function is taking a ``struct coresight_desc *desc`` and
register the device with the core framework. The unregister function takes
a reference to a ``struct coresight_device *csdev`` obtained at registration time.

If everything goes well during the registration process the new devices will
show up under /sys/bus/coresight/devices, as showns here for a TC2 platform::

    root:~# ls /sys/bus/coresight/devices/
    replicator  20030000.tpiu    2201c000.ptm  2203c000.etm  2203e000.etm
    20010000.etb         20040000.funnel  2201d000.ptm  2203d000.etm
    root:~#

The functions take a ``struct coresight_device``, which looks like this::

    struct coresight_desc {
            enum coresight_dev_type type;
            struct coresight_dev_subtype subtype;
            const struct coresight_ops *ops;
            struct coresight_platform_data *pdata;
            struct device *dev;
            const struct attribute_group **groups;
    };


The "coresight_dev_type" identifies what the device is, i.e, source link or
sink while the "coresight_dev_subtype" will characterise that type further.

The ``struct coresight_ops`` is mandatory and will tell the framework how to
perform base operations related to the components, each component having
a different set of requirement. For that ``struct coresight_ops_sink``,
``struct coresight_ops_link`` and ``struct coresight_ops_source`` have been
provided.

The next field ``struct coresight_platform_data *pdata`` is acquired by calling
``of_get_coresight_platform_data()``, as part of the driver's _probe routine and
``struct device *dev`` gets the device reference embedded in the ``amba_device``::

    static int etm_probe(struct amba_device *adev, const struct amba_id *id)
    {
     ...
     ...
     drvdata->dev = &adev->dev;
     ...
    }

Specific class of device (source, link, or sink) have generic operations
that can be performed on them (see ``struct coresight_ops``). The ``**groups``
is a list of sysfs entries pertaining to operations
specific to that component only.  "Implementation defined" customisations are
expected to be accessed and controlled using those entries.

Device Naming scheme
--------------------

The devices that appear on the "coresight" bus were named the same as their
parent devices, i.e, the real devices that appears on AMBA bus or the platform bus.
Thus the names were based on the Linux Open Firmware layer naming convention,
which follows the base physical address of the device followed by the device
type. e.g::

    root:~# ls /sys/bus/coresight/devices/
     20010000.etf  20040000.funnel      20100000.stm     22040000.etm
     22140000.etm  230c0000.funnel      23240000.etm     20030000.tpiu
     20070000.etr  20120000.replicator  220c0000.funnel
     23040000.etm  23140000.etm         23340000.etm

However, with the introduction of ACPI support, the names of the real
devices are a bit cryptic and non-obvious. Thus, a new naming scheme was
introduced to use more generic names based on the type of the device. The
following rules apply::

  1) Devices that are bound to CPUs, are named based on the CPU logical
     number.

     e.g, ETM bound to CPU0 is named "etm0"

  2) All other devices follow a pattern, "<device_type_prefix>N", where :

	<device_type_prefix> 	- A prefix specific to the type of the device
	N			- a sequential number assigned based on the order
				  of probing.

	e.g, tmc_etf0, tmc_etr0, funnel0, funnel1

Thus, with the new scheme the devices could appear as ::

    root:~# ls /sys/bus/coresight/devices/
     etm0     etm1     etm2         etm3  etm4      etm5      funnel0
     funnel1  funnel2  replicator0  stm0  tmc_etf0  tmc_etr0  tpiu0

Some of the examples below might refer to old naming scheme and some
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.

How to use the tracer modules
-----------------------------

There are two ways to use the Coresight framework:

1. using the perf cmd line tools.
2. interacting directly with the Coresight devices using the sysFS interface.

Preference is given to the former as using the sysFS interface
requires a deep understanding of the Coresight HW.  The following sections
provide details on using both methods.

1) Using the sysFS interface:

Before trace collection can start, a coresight sink needs to be identified.
There is no limit on the amount of sinks (nor sources) that can be enabled at
any given moment.  As a generic operation, all device pertaining to the sink
class will have an "active" entry in sysfs::

    root:/sys/bus/coresight/devices# ls
    replicator  20030000.tpiu    2201c000.ptm  2203c000.etm  2203e000.etm
    20010000.etb         20040000.funnel  2201d000.ptm  2203d000.etm
    root:/sys/bus/coresight/devices# ls 20010000.etb
    enable_sink  status  trigger_cntr
    root:/sys/bus/coresight/devices# echo 1 > 20010000.etb/enable_sink
    root:/sys/bus/coresight/devices# cat 20010000.etb/enable_sink
    1
    root:/sys/bus/coresight/devices#

At boot time the current etm3x driver will configure the first address
comparator with "_stext" and "_etext", essentially tracing any instruction
that falls within that range.  As such "enabling" a source will immediately
trigger a trace capture::

    root:/sys/bus/coresight/devices# echo 1 > 2201c000.ptm/enable_source
    root:/sys/bus/coresight/devices# cat 2201c000.ptm/enable_source
    1
    root:/sys/bus/coresight/devices# cat 20010000.etb/status
    Depth:          0x2000
    Status:         0x1
    RAM read ptr:   0x0
    RAM wrt ptr:    0x19d3   <----- The write pointer is moving
    Trigger cnt:    0x0
    Control:        0x1
    Flush status:   0x0
    Flush ctrl:     0x2001
    root:/sys/bus/coresight/devices#

Trace collection is stopped the same way::

    root:/sys/bus/coresight/devices# echo 0 > 2201c000.ptm/enable_source
    root:/sys/bus/coresight/devices#

The content of the ETB buffer can be harvested directly from /dev::

    root:/sys/bus/coresight/devices# dd if=/dev/20010000.etb \
    of=~/cstrace.bin
    64+0 records in
    64+0 records out
    32768 bytes (33 kB) copied, 0.00125258 s, 26.2 MB/s
    root:/sys/bus/coresight/devices#

The file cstrace.bin can be decompressed using "ptm2human", DS-5 or Trace32.

Following is a DS-5 output of an experimental loop that increments a variable up
to a certain value.  The example is simple and yet provides a glimpse of the
wealth of possibilities that coresight provides.
::

    Info                                    Tracing enabled
    Instruction     106378866       0x8026B53C      E52DE004        false   PUSH     {lr}
    Instruction     0       0x8026B540      E24DD00C        false   SUB      sp,sp,#0xc
    Instruction     0       0x8026B544      E3A03000        false   MOV      r3,#0
    Instruction     0       0x8026B548      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B54C      E59D3004        false   LDR      r3,[sp,#4]
    Instruction     0       0x8026B550      E3530004        false   CMP      r3,#4
    Instruction     0       0x8026B554      E2833001        false   ADD      r3,r3,#1
    Instruction     0       0x8026B558      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B55C      DAFFFFFA        true    BLE      {pc}-0x10 ; 0x8026b54c
    Timestamp                                       Timestamp: 17106715833
    Instruction     319     0x8026B54C      E59D3004        false   LDR      r3,[sp,#4]
    Instruction     0       0x8026B550      E3530004        false   CMP      r3,#4
    Instruction     0       0x8026B554      E2833001        false   ADD      r3,r3,#1
    Instruction     0       0x8026B558      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B55C      DAFFFFFA        true    BLE      {pc}-0x10 ; 0x8026b54c
    Instruction     9       0x8026B54C      E59D3004        false   LDR      r3,[sp,#4]
    Instruction     0       0x8026B550      E3530004        false   CMP      r3,#4
    Instruction     0       0x8026B554      E2833001        false   ADD      r3,r3,#1
    Instruction     0       0x8026B558      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B55C      DAFFFFFA        true    BLE      {pc}-0x10 ; 0x8026b54c
    Instruction     7       0x8026B54C      E59D3004        false   LDR      r3,[sp,#4]
    Instruction     0       0x8026B550      E3530004        false   CMP      r3,#4
    Instruction     0       0x8026B554      E2833001        false   ADD      r3,r3,#1
    Instruction     0       0x8026B558      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B55C      DAFFFFFA        true    BLE      {pc}-0x10 ; 0x8026b54c
    Instruction     7       0x8026B54C      E59D3004        false   LDR      r3,[sp,#4]
    Instruction     0       0x8026B550      E3530004        false   CMP      r3,#4
    Instruction     0       0x8026B554      E2833001        false   ADD      r3,r3,#1
    Instruction     0       0x8026B558      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B55C      DAFFFFFA        true    BLE      {pc}-0x10 ; 0x8026b54c
    Instruction     10      0x8026B54C      E59D3004        false   LDR      r3,[sp,#4]
    Instruction     0       0x8026B550      E3530004        false   CMP      r3,#4
    Instruction     0       0x8026B554      E2833001        false   ADD      r3,r3,#1
    Instruction     0       0x8026B558      E58D3004        false   STR      r3,[sp,#4]
    Instruction     0       0x8026B55C      DAFFFFFA        true    BLE      {pc}-0x10 ; 0x8026b54c
    Instruction     6       0x8026B560      EE1D3F30        false   MRC      p15,#0x0,r3,c13,c0,#1
    Instruction     0       0x8026B564      E1A0100D        false   MOV      r1,sp
    Instruction     0       0x8026B568      E3C12D7F        false   BIC      r2,r1,#0x1fc0
    Instruction     0       0x8026B56C      E3C2203F        false   BIC      r2,r2,#0x3f
    Instruction     0       0x8026B570      E59D1004        false   LDR      r1,[sp,#4]
    Instruction     0       0x8026B574      E59F0010        false   LDR      r0,[pc,#16] ; [0x8026B58C] = 0x80550368
    Instruction     0       0x8026B578      E592200C        false   LDR      r2,[r2,#0xc]
    Instruction     0       0x8026B57C      E59221D0        false   LDR      r2,[r2,#0x1d0]
    Instruction     0       0x8026B580      EB07A4CF        true    BL       {pc}+0x1e9344 ; 0x804548c4
    Info                                    Tracing enabled
    Instruction     13570831        0x8026B584      E28DD00C        false   ADD      sp,sp,#0xc
    Instruction     0       0x8026B588      E8BD8000        true    LDM      sp!,{pc}
    Timestamp                                       Timestamp: 17107041535

2) Using perf framework:

Coresight tracers are represented using the Perf framework's Performance
Monitoring Unit (PMU) abstraction.  As such the perf framework takes charge of
controlling when tracing gets enabled based on when the process of interest is
scheduled.  When configured in a system, Coresight PMUs will be listed when
queried by the perf command line tool:

	linaro@linaro-nano:~$ ./perf list pmu

		List of pre-defined events (to be used in -e):

		cs_etm//                                    [Kernel PMU event]

	linaro@linaro-nano:~$

Regardless of the number of tracers available in a system (usually equal to the
amount of processor cores), the "cs_etm" PMU will be listed only once.

A Coresight PMU works the same way as any other PMU, i.e the name of the PMU is
listed along with configuration options within forward slashes '/'.  Since a
Coresight system will typically have more than one sink, the name of the sink to
work with needs to be specified as an event option.
On newer kernels the available sinks are listed in sysFS under
($SYSFS)/bus/event_source/devices/cs_etm/sinks/::

	root@localhost:/sys/bus/event_source/devices/cs_etm/sinks# ls
	tmc_etf0  tmc_etr0  tpiu0

On older kernels, this may need to be found from the list of coresight devices,
available under ($SYSFS)/bus/coresight/devices/::

	root:~# ls /sys/bus/coresight/devices/
	 etm0     etm1     etm2         etm3  etm4      etm5      funnel0
	 funnel1  funnel2  replicator0  stm0  tmc_etf0  tmc_etr0  tpiu0
	root@linaro-nano:~# perf record -e cs_etm/@tmc_etr0/u --per-thread program

As mentioned above in section "Device Naming scheme", the names of the devices could
look different from what is used in the example above. One must use the device names
as it appears under the sysFS.

The syntax within the forward slashes '/' is important.  The '@' character
tells the parser that a sink is about to be specified and that this is the sink
to use for the trace session.

More information on the above and other example on how to use Coresight with
the perf tools can be found in the "HOWTO.md" file of the openCSD gitHub
repository [#third]_.

2.1) AutoFDO analysis using the perf tools:

perf can be used to record and analyze trace of programs.

Execution can be recorded using 'perf record' with the cs_etm event,
specifying the name of the sink to record to, e.g::

    perf record -e cs_etm/@tmc_etr0/u --per-thread

The 'perf report' and 'perf script' commands can be used to analyze execution,
synthesizing instruction and branch events from the instruction trace.
'perf inject' can be used to replace the trace data with the synthesized events.
The --itrace option controls the type and frequency of synthesized events
(see perf documentation).

Note that only 64-bit programs are currently supported - further work is
required to support instruction decode of 32-bit Arm programs.


Generating coverage files for Feedback Directed Optimization: AutoFDO
---------------------------------------------------------------------

'perf inject' accepts the --itrace option in which case tracing data is
removed and replaced with the synthesized events. e.g.
::

	perf inject --itrace --strip -i perf.data -o perf.data.new

Below is an example of using ARM ETM for autoFDO.  It requires autofdo
(https://github.com/google/autofdo) and gcc version 5.  The bubble
sort example is from the AutoFDO tutorial (https://gcc.gnu.org/wiki/AutoFDO/Tutorial).
::

	$ gcc-5 -O3 sort.c -o sort
	$ taskset -c 2 ./sort
	Bubble sorting array of 30000 elements
	5910 ms

	$ perf record -e cs_etm/@tmc_etr0/u --per-thread taskset -c 2 ./sort
	Bubble sorting array of 30000 elements
	12543 ms
	[ perf record: Woken up 35 times to write data ]
	[ perf record: Captured and wrote 69.640 MB perf.data ]

	$ perf inject -i perf.data -o inj.data --itrace=il64 --strip
	$ create_gcov --binary=./sort --profile=inj.data --gcov=sort.gcov -gcov_version=1
	$ gcc-5 -O3 -fauto-profile=sort.gcov sort.c -o sort_autofdo
	$ taskset -c 2 ./sort_autofdo
	Bubble sorting array of 30000 elements
	5806 ms


How to use the STM module
-------------------------

Using the System Trace Macrocell module is the same as the tracers - the only
difference is that clients are driving the trace capture rather
than the program flow through the code.

As with any other CoreSight component, specifics about the STM tracer can be
found in sysfs with more information on each entry being found in [#first]_::

    root@genericarmv8:~# ls /sys/bus/coresight/devices/stm0
    enable_source   hwevent_select  port_enable     subsystem       uevent
    hwevent_enable  mgmt            port_select     traceid
    root@genericarmv8:~#

Like any other source a sink needs to be identified and the STM enabled before
being used::

    root@genericarmv8:~# echo 1 > /sys/bus/coresight/devices/tmc_etf0/enable_sink
    root@genericarmv8:~# echo 1 > /sys/bus/coresight/devices/stm0/enable_source

From there user space applications can request and use channels using the devfs
interface provided for that purpose by the generic STM API::

    root@genericarmv8:~# ls -l /dev/stm0
    crw-------    1 root     root       10,  61 Jan  3 18:11 /dev/stm0
    root@genericarmv8:~#

Details on how to use the generic STM API can be found here [#second]_.

.. [#first] Documentation/ABI/testing/sysfs-bus-coresight-devices-stm

.. [#second] Documentation/trace/stm.rst

.. [#third] https://github.com/Linaro/perf-opencsd