This is an RFC patch to explore the solution to a problem we have in the CoreSight ETM/ETE PMU.
CoreSight ETM trace allows instruction level tracing of Arm CPUs. The ETM generates the CPU excecution trace and pumps it into CoreSight AMBA Trace Bus and is collected by a different CoreSight component (traditionally CoreSight TMC-ETR /ETB/ETF), called "sink". Important to note that there is no guarantee that every CPU has a dedicated sink. Thus multiple ETMs could pump the trace data into the same "sink" and thus they apply additional formatting of the trace data for the user to decode it properly and attribute the trace data to the corresponding ETM.
However, with the introduction of Arm Trace buffer Extensions (TRBE), we now have a dedicated per-CPU architected sink for collecting the trace. Since the TRBE is always per-CPU, it doesn't apply any formatting of the trace. The support for this driver is under review [1].
Now a system could have a per-cpu TRBE and one or more shared TMC-ETRs on the system. A user could choose a "specific" sink for a perf session (e.g, a TMC-ETR) or the driver could automatically select the nearest sink for a given ETM. It is possible that some ETMs could end up using TMC-ETR (e.g, if the TRBE is not usable on the CPU) while the others using TRBE in a single perf session. Thus we now have "formatted" trace collected from TMC-ETR and "unformatted" trace collected from TRBE. However, we don't get into a situation where a single event could end up using TMC-ETR & TRBE. i.e, any AUX buffer is guaranteed to be either RAW or FORMATTED, but not a mix of both.
As for perf decoding, we need to know the type of the data in the individual AUX buffers, so that it can set up the "OpenCSD" (library for decoding CoreSight trace) decoder instance appropriately. Thus the perf.data file must conatin the hints for the tool to decode the data correctly.
Since this is a runtime variable, and perf tool doesn't have a control on what sink gets used (in case of automatic sink selection), we need this information made available from the PMU driver for each AUX record.
This patch is an attempt to solve the problem by, adding an AUX flag for each AUX record to indicate the type of the trace in them. It can be defined as a PMU specific flag, which each PMU could interpret in its on way (e.g, PERF_AUX_FLAG_PMU_FLAG_1 or could be a dedicated flag for the CoreSight in a "generic" form PERF_AUX_FLAG_ALT_FMT (Thanks Mike Leach for the name).
We are looking for suggestions on how best to solve this problem and happy to explore other options if there is a preferred way of solving this.
[1] https://lkml.kernel.org/r/1610511498-4058-1-git-send-email-anshuman.khandual...
Suzuki K Poulose (1): perf: Handle multiple formatted AUX records
drivers/hwtracing/coresight/coresight-etm-perf.c | 2 ++ include/linux/coresight.h | 1 + include/uapi/linux/perf_event.h | 1 + 3 files changed, 4 insertions(+)