When 'etm->instructions_sample_period' is less than
'tidq->period_instructions', the function cs_etm__sample() cannot handle
this case properly with its logic.

Let's see below flow as an example:

- If we set itrace option '--itrace=i4', then function cs_etm__sample()
  has variables with initialized values:

  tidq->period_instructions = 0
  etm->instructions_sample_period = 4

- When the first packet is coming:

  packet->instr_count = 10; the number of instructions executed in this
  packet is 10, thus update period_instructions as below:

  tidq->period_instructions = 0 + 10 = 10
  instrs_over = 10 - 4 = 6
  offset = 10 - 6 - 1 = 3
  tidq->period_instructions = instrs_over = 6

- When the second packet is coming:

  packet->instr_count = 10; in the second pass, assume 10 instructions
  in the trace sample again:

  tidq->period_instructions = 6 + 10 = 16
  instrs_over = 16 - 4 = 12
  offset = 10 - 12 - 1 = -3  -> the negative value
  tidq->period_instructions = instrs_over = 12

So after handle these two packets, there have below issues:

The first issue is that cs_etm__instr_addr() returns the address within
the current trace sample of the instruction related to offset, so the
offset is supposed to be always unsigned value.  But in fact, function
cs_etm__sample() might calculate a negative offset value (in handling
the second packet, the offset is -3) and pass to cs_etm__instr_addr()
with u64 type with a big positive integer.

The second issue is it only synthesizes 2 samples for sample period = 4.
In theory, every packet has 10 instructions so the two packets have
total 20 instructions, 20 instructions should generate 5 samples
(4 x 5 = 20).  This is because cs_etm__sample() only calls once
cs_etm__synth_instruction_sample() to generate instruction sample per
range packet.

This patch fixes the logic in function cs_etm__sample(); the basic
idea for handling coming packet is:

- To synthesize the first instruction sample, it combines the left
  instructions from the previous packet and the head of the new
  packet; then generate continuous samples with sample period;
- At the tail of the new packet, if it has the rest instructions,
  these instructions will be left for the sequential sample.

Suggested-by: Mike Leach <redacted>
Signed-off-by: Leo Yan <redacted>
---
 tools/perf/util/cs-etm.c | 87 ++++++++++++++++++++++++++++++++--------
 1 file changed, 70 insertions(+), 17 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index b2f31390126a..4b7d6c36ce3c 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c

@@ -1356,9 +1356,12 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
        struct cs_etm_auxtrace *etm = etmq->etm;
        int ret;
        u8 trace_chan_id = tidq->trace_chan_id;
-       u64 instrs_executed = tidq->packet->instr_count;
+       u64 instrs_prev;

-       tidq->period_instructions += instrs_executed;
+       /* Get instructions remainder from previous packet */
+       instrs_prev = tidq->period_instructions;
+
+       tidq->period_instructions += tidq->packet->instr_count;

        /*
         * Record a branch when the last instruction in

@@ -1376,26 +1379,76 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
                 * TODO: allow period to be defined in cycles and clock time
                 */

-               /* Get number of instructions executed after the sample point */
-               u64 instrs_over = tidq->period_instructions -
-                       etm->instructions_sample_period;
+               /*
+                * Below diagram demonstrates the instruction samples
+                * generation flows:
+                *
+                *    Instrs     Instrs       Instrs       Instrs
+                *   Sample(n)  Sample(n+1)  Sample(n+2)  Sample(n+3)
+                *    |            |            |            |
+                *    V            V            V            V
+                *   --------------------------------------------------
+                *            ^                                  ^
+                *            |                                  |
+                *         Period                             Period
+                *    instructions(Pi)                   instructions(Pi')
+                *
+                *            |                                  |
+                *            \---------------- -----------------/
+                *                             V
+                *                 tidq->packet->instr_count
+                *
+                * Instrs Sample(n...) are the synthesised samples occurring
+                * every etm->instructions_sample_period instructions - as
+                * defined on the perf command line.  Sample(n) is being the
+                * last sample before the current etm packet, n+1 to n+3
+                * samples are generated from the current etm packet.
+                *
+                * tidq->packet->instr_count represents the number of
+                * instructions in the current etm packet.
+                *
+                * Period instructions (Pi) contains the the number of
+                * instructions executed after the sample point(n) from the
+                * previous etm packet.  This will always be less than
+                * etm->instructions_sample_period.
+                *
+                * When generate new samples, it combines with two parts
+                * instructions, one is the tail of the old packet and another
+                * is the head of the new coming packet, to generate
+                * sample(n+1); sample(n+2) and sample(n+3) consume the
+                * instructions with sample period.  After sample(n+3), the rest
+                * instructions will be used by later packet and it is assigned
+                * to tidq->period_instructions for next round calculation.
+                */

                /*
-                * Calculate the address of the sampled instruction (-1 as
-                * sample is reported as though instruction has just been
-                * executed, but PC has not advanced to next instruction)
+                * Get the initial offset into the current packet instructions;
+                * entry conditions ensure that instrs_prev is less than
+                * etm->instructions_sample_period.
                 */
-               u64 offset = (instrs_executed - instrs_over - 1);
-               u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
-                                             tidq->packet, offset);
+               u64 offset = etm->instructions_sample_period - instrs_prev;
+               u64 addr;

-               ret = cs_etm__synth_instruction_sample(
-                       etmq, tidq, addr, etm->instructions_sample_period);
-               if (ret)
-                       return ret;
+               while (tidq->period_instructions >=
+                               etm->instructions_sample_period) {
+                       /*
+                        * Calculate the address of the sampled instruction (-1
+                        * as sample is reported as though instruction has just
+                        * been executed, but PC has not advanced to next
+                        * instruction)
+                        */
+                       addr = cs_etm__instr_addr(etmq, trace_chan_id,
+                                                 tidq->packet, offset - 1);
+                       ret = cs_etm__synth_instruction_sample(
+                               etmq, tidq, addr,
+                               etm->instructions_sample_period);
+                       if (ret)
+                               return ret;

-               /* Carry remaining instructions into next sample period */
-               tidq->period_instructions = instrs_over;
+                       offset += etm->instructions_sample_period;
+                       tidq->period_instructions -=
+                               etm->instructions_sample_period;
+               }
        }

        if (etm->sample_branches) {
--

2.17.1