[PATCH] powerpc/perf: Add missing break in power7_marked_instr_event()

* [PATCH] powerpc/perf: Add missing break in power7_marked_instr_event()
@ 2018-09-20  9:41 Michael Ellerman
  2018-09-20  9:59 ` Madhavan Srinivasan
  2018-10-04  6:14 ` Michael Ellerman
  0 siblings, 2 replies; 6+ messages in thread
From: Michael Ellerman @ 2018-09-20  9:41 UTC (permalink / raw)
  To: linuxppc-dev; +Cc: maddy, paulus

In power7_marked_instr_event() there is a switch case that is missing
a break or an explicit fallthrough, it's not immediately clear which
it should be.

The function determines based on the PMU event code, whether the event
is a "marked" event (which then requires us to configure the PMU in a
certain way). On Power7 there is no specific bit(s) in the event to
tell us that, we just have to know.

Rather than having a full list of every event and whether they are
marked, we pull apart the event code and for events with certain
values of certain fields we can say that those are all marked events.

We take the psel (bits 0-7) of the event, and look at bits 4-7. For a
value of 6 we say that if the entire psel == 0x64 then if the pmc == 3
the event is marked, else not, and otherwise we continue.

It is then that we fallthrough to the 8 case, where we return true if
the unit == 0xd.

The question is should the 6 case also fallthrough and check for
unit == 0xd, or should it return.

Looking at the full list of events we see that there are zero events
where (psel >> 4) == 0x6 and unit == 0xd.

So the answer is it doesn't really matter, there are no valid event
codes that will return a different result whether we fallthrough or
break.

But equally, testing the 6 case events against unit == 0xd is slightly
bogus, as there are no such events. So to make the code clearer, and
avoid any future confusion, have the 6 case break rather than falling
through.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/powerpc/perf/power7-pmu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/perf/power7-pmu.c b/arch/powerpc/perf/power7-pmu.c
index 7963658dbc22..6dbae9884ec4 100644
--- a/arch/powerpc/perf/power7-pmu.c
+++ b/arch/powerpc/perf/power7-pmu.c
@@ -238,6 +238,7 @@ static int power7_marked_instr_event(u64 event)
 	case 6:
 		if (psel == 0x64)
 			return pmc >= 3;
+		break;
 	case 8:
 		return unit == 0xd;
 	}
-- 
2.17.1

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