Re: [PATCH 00/10] coresight: Add new API to allocate trace source ID values

[Mathieu Poirier]

Good morning,

On Tue, Mar 08, 2022 at 08:49:50PM +0000, Mike Leach wrote:
> The current method for allocating trace source ID values to sources is
> to use a fixed algorithm for CPU based sources of (cpu_num * 2 + 0x10).
> The STM is allocated ID 0x1.
> 
> This fixed algorithm is used in both the CoreSight driver code, and by
> perf when writing the trace metadata in the AUXTRACE_INFO record.
> 
> The method needs replacing as currently:-
> 1. It is inefficient in using available IDs.
> 2. Does not scale to larger systems with many cores and the algorithm
> has no limits so will generate invalid trace IDs for cpu number > 44.
> 
> Additionally requirements to allocate additional system IDs on some
> systems have been seen.
> 
> This patch set  introduces an API that allows the allocation of trace IDs
> in a dynamic manner.
> 
> Architecturally reserved IDs are never allocated, and the system is
> limited to allocating only valid IDs.
> 
> Each of the current trace sources ETM3.x, ETM4.x and STM is updated to use
> the new API.
> 
> perf handling is changed so that the ID associated with the CPU is read
> from sysfs. The ID allocator is notified when perf events start and stop
> so CPU based IDs are kept constant throughout any perf session.
> 
> For the ETMx.x devices IDs are allocated on certain events
> a) When using sysfs, an ID will be allocated on hardware enable, and freed
> when the sysfs reset is written.
> b) When using perf, ID is allocated on hardware enable, and freed on
> hardware disable.
> 
> For both cases the ID is allocated when sysfs is read to get the current
> trace ID. This ensures that consistent decode metadata can be extracted
> from the system where this read occurs before device enable.
> 
> Note: This patchset breaks backward compatibility for perf record.
> Because the method for generating the AUXTRACE_INFO meta data has
> changed, using an older perf record will result in metadata that
> does not match the trace IDs used in the recorded trace data.
> This mismatch will cause subsequent decode to fail. Older versions of
> perf will still be able to decode data generated by the updated system.
> 

I have started looking at this set, comments to follow shortly.

Thanks,
Mathieu

> 
> Applies to coresight/next [b54f53bc11a5]
> Tested on DB410c
> 
> Mike Leach (10):
>   coresight: trace-id: Add API to dynamically assign trace ID values
>   coresight: trace-id: Set up source trace ID map for system
>   coresight: stm: Update STM driver to use Trace ID api
>   coresight: etm4x: Use trace ID API to dynamically allocate trace ID
>   coresight: etm3x: Use trace ID API to allocate IDs
>   coresight: perf: traceid: Add perf notifiers for trace ID
>   perf: cs-etm: Update event to read trace ID from sysfs
>   coresight: Remove legacy Trace ID allocation mechanism
>   coresight: etmX.X: stm: Remove unused legacy source trace ID ops
>   coresight: trace-id: Add debug & test macros to trace id allocation
> 
>  drivers/hwtracing/coresight/Makefile          |   2 +-
>  drivers/hwtracing/coresight/coresight-core.c  |  64 ++---
>  .../hwtracing/coresight/coresight-etm-perf.c  |  16 +-
>  drivers/hwtracing/coresight/coresight-etm.h   |   3 +-
>  .../coresight/coresight-etm3x-core.c          |  93 ++++---
>  .../coresight/coresight-etm3x-sysfs.c         |  28 +-
>  .../coresight/coresight-etm4x-core.c          |  63 ++++-
>  .../coresight/coresight-etm4x-sysfs.c         |  32 ++-
>  drivers/hwtracing/coresight/coresight-etm4x.h |   3 +
>  drivers/hwtracing/coresight/coresight-priv.h  |   1 +
>  drivers/hwtracing/coresight/coresight-stm.c   |  49 +---
>  .../hwtracing/coresight/coresight-trace-id.c  | 255 ++++++++++++++++++
>  .../hwtracing/coresight/coresight-trace-id.h  |  69 +++++
>  include/linux/coresight-pmu.h                 |  12 -
>  include/linux/coresight.h                     |   3 -
>  tools/perf/arch/arm/util/cs-etm.c             |  12 +-
>  16 files changed, 530 insertions(+), 175 deletions(-)
>  create mode 100644 drivers/hwtracing/coresight/coresight-trace-id.c
>  create mode 100644 drivers/hwtracing/coresight/coresight-trace-id.h
> 
> -- 
> 2.17.1
>