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+ Coresight - HW Assisted Tracing on ARM
+ ======================================
+
+ Author: Mathieu Poirier <mathieu.poirier@linaro.org>
+ Date: August 27th 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 systhesis time to cater to specific tracing needs.
+Compoments 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 transfered to a
+host without fear of filling up the onboard coresight memory buffer.
+
+
+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:
+
+struct coresight_device *coresight_register(struct coresight_desc *desc);
+void coresight_unregister(struct coresight_device *csdev);
+
+The registering function is taking a "struct coresight_device *csdev" and
+register the device with the core framework. The unregister function takes
+a reference to a "strut coresight_device", obtained at registration time.
+
+If everything goes well during the registration process the new devices will
+show up under /sys/kernel/debug/coresight/, as showns here for a TC2 platform:
+
+root@linaro-developer:~# ls /sys/kernel/debug/coresight/
+00000000.replicator 20030000.tpiu 2201c000.ptm 2203c000.etm 2203e000.etm
+20010000.etb 20040000.funnel 2201d000.ptm 2203d000.etm
+root@linaro-developer:~#
+
+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 coresight_ops_entry **debugfs_ops;
+ struct module *owner;
+};
+
+
+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
+"**debugfs_ops" is a list of debugfs entries pertaining to operations
+specific to that component only. "Implementation defined" customisations are
+expected to be accessed and controlled using those entries.
+
+Last but not least, "struct module *owner" is expected to be set to reflect
+the information carried in "THIS_MODULE".
+
+How to use
+----------
+
+Before trace collection can start, a coresight sink needs to be identify.
+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 debugfs:
+
+root@linaro-developer:/sys/kernel/debug/coresight# ls
+00000000.replicator 20030000.tpiu 2201c000.ptm 2203c000.etm 2203e000.etm
+20010000.etb 20040000.funnel 2201d000.ptm 2203d000.etm
+root@linaro-developer:/sys/kernel/debug/coresight# ls 20010000.etb
+active status trigger_cntr
+root@linaro-developer:/sys/kernel/debug/coresight# echo 1 > 20010000.etb/active
+root@linaro-developer:/sys/kernel/debug/coresight# cat 20010000.etb/active
+1
+root@linaro-developer:/sys/kernel/debug/coresight#
+
+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@linaro-developer:/sys/kernel/debug/coresight# echo 1 > 2201c000.ptm/enable
+root@linaro-developer:/sys/kernel/debug/coresight# cat 2201c000.ptm/enable
+1
+root@linaro-developer:/sys/kernel/debug/coresight# 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@linaro-developer:/sys/kernel/debug/coresight#
+
+Trace collection is stopped the same way:
+
+root@linaro-developer:/sys/kernel/debug/coresight# echo 0 > 2201c000.ptm/enable
+root@linaro-developer:/sys/kernel/debug/coresight#
+
+The content of the ETB buffer can be harvested directly from /dev:
+
+root@linaro-developer:/sys/kernel/debug/coresight# 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@linaro-developer:/sys/kernel/debug/coresight#
+
+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