Adding a new section giving information on how coresight has been integrated with the perf subsystem along with a general idea of how to control tracing from the perf tool cmd line. Signed-off-by: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx> --- Documentation/trace/coresight.txt | 116 +++++++++++++++++++++++++++++++++++++- 1 file changed, 113 insertions(+), 3 deletions(-) diff --git a/Documentation/trace/coresight.txt b/Documentation/trace/coresight.txt index 0a5c3290e732..330020082b02 100644 --- a/Documentation/trace/coresight.txt +++ b/Documentation/trace/coresight.txt @@ -193,10 +193,120 @@ 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 +There is two ways to use the coresight framework: 1) using the perf cmd line +tool and 2) interacting directly with the coresight devices using the sysFS +interface. The latter will slowly be faded out as more functionality become +available from the perf cmd line tool but for the time being both are still +supported. The following sections provide details on using both methods. + +1) Using perf framework: + +Coresight tracers like ETM and PTM are represented using the Perf framework's +Performance Monitoring Unit (PMU). As such the perf framework takes charge of +controlling when tracing happens based on when the process(es) of interest are +scheduled. When configure 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 amnout ETM/PTM IP block in a system (usually equal to the +amount of processor core), the "cs_etm" PMU will be listed only once. + +Before a trace can be configured and started a Coresight sink needs to be +selected using the sysFS method (see below). This is only temporary until +sink selection can be made from the command line tool. + +linaro@linaro-nano:~$ ls /sys/bus/coresight/devices +20010000.etb 20030000.tpiu 20040000.funnel 2201c000.ptm +2201d000.ptm 2203c000.etm 2203d000.etm 2203e000.etm replicator + +linaro@linaro-nano:~$ echo 1 > /sys/bus/coresight/devices/20010000.etb/enable_sink + +Once a sink has been selected configuring a Coresight PMU works the same way as +any other PMU. As such tracing can happen for a single CPU, a group of CPU, per +thread or a combination of those: + +linaro@linaro-nano:~$ perf record -e cs_etm// --per-thread <command> + +linaro@linaro-nano:~$ perf record -C 0,2-3 -e cs_etm// <command> + +Tracing limited to user and kernel space can also be used to narrow the amount +of collected tracers: + +linaro@linaro-nano:~$ perf record -e cs_etm//u --per-thread <command> + +linaro@linaro-nano:~$ perf record -C 0,2-3 -e cs_etm//k <command> + +As of this writing two ETM/PTM specific options have are available: cycle +accurate and timestamp (please refer to the Embedded Trace Macrocell reference +manual for details on these options). By default both are disabled but using +the "cycacc" and "timestamp" mnemonic within the double '/' will see those +options configure for the upcoming trace run: + +linaro@linaro-nano:~$ perf record -e cs_etm/cycacc/ --per-thread <command> + +linaro@linaro-nano:~$ perf record -C 0,2-3 -e cs_etm/cycacc,timestamp/ <command> + +The Coresight PMUs can be configured to work in "full trace" or "snapshot" mode. +In full trace mode trade acquisition is enable from beginning to end with trace +data being recorded continuously: + +linaro@linaro-nano:~$ perf record -e cs_etm// dd if=/dev/random of=./test.txt bs=1k count=1000 + +Since this can lead to a significant amount of data and that some device are +limited in disk space snapshot mode can be used instead. In snapshot mode +traces are still collected in the ring buffer but not communicated to user +space. The ring buffer is allowed to wrap around, providing the latest +information before an event of interest happens. Significant events are +communicated by sending a USR2 signal to the user space command line tool. +From there the tool will stop trace collection and harvest data from the ring +buffer before re-enabling traces. Snapshot mode can be invoked using '-S' when +launching a trace collection: + +linaro@linaro-nano:~$ perf record -S -e cs_etm// dd if=/dev/random of=./test.txt bs=1k count=1000 + +Trace data collected during trace runs end up in the centennial "perf.data" +file. Trace configuration information necessary for trace decoding are also +embedded in the "perf.data" file. Two new headers, 'PERF_RECORD_AUXTRACE_INFO' +and 'PERF_RECORD_AUXTRACE' have been added to list of event types in order to +find out where the different sections start. + +It is worth nothing that a set of metadata information exists for each tracer +that participated in a trace run. As such if 5 processors have been engaged, +5 set of metadata will be found in the perf.data file. This is to ensure that +tracer decompression tools have all the information they need in order to +process the trace data. + +Metadata information is collected directly from the ETM/PTM management registers +using the sysFS interface. Since there is not way for the perf command line +tool to associate a CPU with a tracer, a symbolic link has been created between +the cs_etm sysFS event directory and each Coresight tracer: + +linaro@linaro-nano:~$ ls /sys/bus/event_source/devices/cs_etm +cpu0 cpu1 cpu2 cpu3 cpu4 format perf_event_mux_interval_ms +power subsystem type uevent + +linaro@linaro-nano:~$ ls /sys/bus/event_source/devices/cs_etm/cpu0/mgmt/ +etmccer etmccr etmcr etmidr etmscr etmtecr1 etmtecr2 +etmteevr etmtraceidr etmtssvr + +2) Using the sysFS interface: + +Most, if not all, configuration registers are made available to users via the +sysFS interface. Until all coresight ETM drivers have been converted to perf, +it will also be possible to start and stop traces from sysFS. + +As with the perf method described above, a coresight sink needs to be identify +before trace collection can commence. Using the sysFS method _only_, 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: +class will have an "enable_sink" entry in sysfs: root:/sys/bus/coresight/devices# ls replicator 20030000.tpiu 2201c000.ptm 2203c000.etm 2203e000.etm -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
