[PATCH 17/25] perf cs-etm: Allow history collection for samples with zero IP

[Leo Yan <leo.yan@arm.com>]

Perf clears the IP of a user-only PMU sample when its interrupt skids
into the kernel. The timestamp and TID remain valid, but the zero-IP
check prevents --itrace=L from attaching decoded branch history.

Remove the IP check and keep matching samples to trace by time and
thread.

Assisted-by: Codex:gpt-6
Signed-off-by: Leo Yan <leo.yan@arm.com>
---
 tools/perf/util/cs-etm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 00407a80933e1c0b76260aed5894dece50b9aecd..baa962085dba5226d9729a0ee8039374d5fcb1a0 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -3097,6 +3097,8 @@ static int cs_etm__br_stack_init(struct cs_etm_auxtrace *etm,
 /*
  * Add decoded branch history to an existing sample. The sample keeps its own
  * ip, callchain and event identity; only an absent branch stack is filled in.
+ * A user-only PMU sample can have a zero IP when the interrupt skids into
+ * the kernel; its thread and trace window still identify valid history.
  */
 static int cs_etm__process_sample(struct cs_etm_auxtrace *etm,
 				  struct perf_session *session,
@@ -3107,7 +3109,7 @@ static int cs_etm__process_sample(struct cs_etm_auxtrace *etm,
 	int err;
 
 	if (!etm->synth_opts.add_last_branch || sample->branch_stack ||
-	    !sample->ip || !sample->time || sample->time == (u64)-1)
+	    !sample->time || sample->time == (u64)-1)
 		return 0;
 
 	/* Adding branch history to existing samples supports the host only */

-- 
2.34.1