[PATCH v2 0/3] perf cs-etm: Handle valid-but-zero timestamps

@@ -2473,6 +2473,10 @@ static bool cs_etm__is_timeless_decoding(struct cs_etm_auxtrace *etm)
 	struct evlist *evlist = etm->session->evlist;
 	bool timeless_decoding = true;
 
+	/* Override timeless mode with user input from --itrace=Z */
+	if (etm->synth_opts.timeless_decoding)
+		return true;
+
 	/*
 	 * Circle through the list of event and complain if we find one
 	 * with the time bit set.

@@ -2819,6 +2819,14 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
 	if (err)
 		goto err_free_etm;
 
+	if (session->itrace_synth_opts->set) {
+		etm->synth_opts = *session->itrace_synth_opts;
+	} else {
+		itrace_synth_opts__set_default(&etm->synth_opts,
+				session->itrace_synth_opts->default_no_sample);
+		etm->synth_opts.callchain = false;
+	}
+
 	etm->session = session;
 	etm->machine = &session->machines.host;
 

@@ -2863,14 +2871,6 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
 		return 0;
 	}
 
-	if (session->itrace_synth_opts->set) {
-		etm->synth_opts = *session->itrace_synth_opts;
-	} else {
-		itrace_synth_opts__set_default(&etm->synth_opts,
-				session->itrace_synth_opts->default_no_sample);
-		etm->synth_opts.callchain = false;
-	}
-
 	err = cs_etm__synth_events(etm, session);
 	if (err)
 		goto err_delete_thread;

@@ -6,6 +6,7 @@
  * Author: Mathieu Poirier <mathieu.poirier@linaro.org>
  */
 
+#include <asm/bug.h>
 #include <linux/coresight-pmu.h>
 #include <linux/err.h>
 #include <linux/list.h>

@@ -17,6 +18,7 @@
 
 #include "cs-etm.h"
 #include "cs-etm-decoder.h"
+#include "debug.h"
 #include "intlist.h"
 
 /* use raw logging */

@@ -294,7 +296,8 @@ cs_etm_decoder__do_soft_timestamp(struct cs_etm_queue *etmq,
 static ocsd_datapath_resp_t
 cs_etm_decoder__do_hard_timestamp(struct cs_etm_queue *etmq,
 				  const ocsd_generic_trace_elem *elem,
-				  const uint8_t trace_chan_id)
+				  const uint8_t trace_chan_id,
+				  const ocsd_trc_index_t indx)
 {
 	struct cs_etm_packet_queue *packet_queue;
 

@@ -313,14 +316,33 @@ cs_etm_decoder__do_hard_timestamp(struct cs_etm_queue *etmq,
 		return OCSD_RESP_CONT;
 	}
 
-	/*
-	 * This is the first timestamp we've seen since the beginning of traces
-	 * or a discontinuity.  Since timestamps packets are generated *after*
-	 * range packets have been generated, we need to estimate the time at
-	 * which instructions started by subtracting the number of instructions
-	 * executed to the timestamp.
-	 */
-	packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
+
+	if (!elem->timestamp) {
+		/*
+		 * Zero timestamps can be seen due to misconfiguration or hardware bugs.
+		 * Warn once, and don't try to subtract instr_count as it would result in an
+		 * underflow.
+		 */
+		packet_queue->cs_timestamp = 0;
+		WARN_ONCE(true, "Zero Coresight timestamp found at Idx:%" OCSD_TRC_IDX_STR
+				". Decoding may be improved with --itrace=Z...\n", indx);
+	} else if (packet_queue->instr_count > elem->timestamp) {
+		/*
+		 * Sanity check that the elem->timestamp - packet_queue->instr_count would not
+		 * result in an underflow. Warn and clamp at 0 if it would.
+		 */
+		packet_queue->cs_timestamp = 0;
+		pr_err("Timestamp calculation underflow at Idx:%" OCSD_TRC_IDX_STR "\n", indx);
+	} else {
+		/*
+		 * This is the first timestamp we've seen since the beginning of traces
+		 * or a discontinuity.  Since timestamps packets are generated *after*
+		 * range packets have been generated, we need to estimate the time at
+		 * which instructions started by subtracting the number of instructions
+		 * executed to the timestamp.
+		 */
+		packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
+	}
 	packet_queue->next_cs_timestamp = elem->timestamp;
 	packet_queue->instr_count = 0;
 

@@ -542,7 +564,7 @@ cs_etm_decoder__set_tid(struct cs_etm_queue *etmq,
 
 static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
 				const void *context,
-				const ocsd_trc_index_t indx __maybe_unused,
+				const ocsd_trc_index_t indx,
 				const u8 trace_chan_id __maybe_unused,
 				const ocsd_generic_trace_elem *elem)
 {

@@ -579,7 +601,8 @@ static ocsd_datapath_resp_t cs_etm_decoder__gen_trace_elem_printer(
 		break;
 	case OCSD_GEN_TRC_ELEM_TIMESTAMP:
 		resp = cs_etm_decoder__do_hard_timestamp(etmq, elem,
-							 trace_chan_id);
+							 trace_chan_id,
+							 indx);
 		break;
 	case OCSD_GEN_TRC_ELEM_PE_CONTEXT:
 		resp = cs_etm_decoder__set_tid(etmq, packet_queue,