[PATCH 3/3] selftests/powerpc: ensure PMC reads are set and ordered on count_pmc

["Desnes A. Nunes do Rosario" <desnesn@linux.ibm.com>]

Function count_pmc() needs a memory barrier to ensure that PMC reads are
fully consistent. The lack of it can occasionally fail pmc56_overflow test,
since depending on the workload on the system, PMC5 & 6 can have past val-
ues from the time the counters are frozen and turned back on. These past
values will be accounted as overflows and make the test fail.

=========
test: pmc56_overflow
...
ebb_state:
...
>>pmc[5] count = 0xfd4cbc8c
>>pmc[6] count = 0xddd8b3b6
HW state:
MMCR0 0x0000000084000000 FC PMAE
MMCR2 0x0000000000000000
EBBHR 0x0000000010003f68
BESCR 0x8000000000000000 GE
...
PMC5  0x0000000000000000
PMC6  0x0000000000000000
SIAR  0x0000000010003398
...
  [3]: register SPRN_PMC2  = 0x0000000080000003
  [4]: register SPRN_PMC5  = 0x0000000000000000
  [5]: register SPRN_PMC6  = 0x0000000000000000
  [6]: register SPRN_PMC2  = 0x0000000080000003
>>[7]: register SPRN_PMC5  = 0x000000008f21266d
>>[8]: register SPRN_PMC6  = 0x000000000da80f8d
  [9]: register SPRN_PMC2  = 0x0000000080000003
>>[10]: register SPRN_PMC5  = 0x000000006e2b961f
>>[11]: register SPRN_PMC6  = 0x00000000d030a429
  [12]: register SPRN_PMC2  = 0x0000000080000003
  [13]: register SPRN_PMC5  = 0x0000000000000000
  [14]: register SPRN_PMC6  = 0x0000000000000000
...
PMC5/6 overflow 2
[FAIL] Test FAILED on line 87
failure: pmc56_overflow
=========

Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.ibm.com>
---
 tools/testing/selftests/powerpc/pmu/ebb/ebb.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/testing/selftests/powerpc/pmu/ebb/ebb.c b/tools/testing/selftests/powerpc/pmu/ebb/ebb.c
index bf6f25dfcf7b..6199f3cea0f9 100644
--- a/tools/testing/selftests/powerpc/pmu/ebb/ebb.c
+++ b/tools/testing/selftests/powerpc/pmu/ebb/ebb.c
@@ -258,6 +258,10 @@ int count_pmc(int pmc, uint32_t sample_period)
 	start_value = pmc_sample_period(sample_period);
 
 	val = read_pmc(pmc);
+
+	/* Ensure pmc value is consistent between freezes */
+	mb();
+
 	if (val < start_value)
 		ebb_state.stats.negative++;
 	else
-- 
2.21.1