Re: [PATCH] perf test: Fix perf test 11 hwmon endianess issue

[David Laight <david.laight.linux@gmail.com>]

On Fri, 31 Jan 2025 12:24:00 +0100
Thomas Richter <tmricht@linux.ibm.com> wrote:

> perf test 11 hwmon fails on s390 with this error
> 
>  # ./perf test -Fv 11
>  --- start ---
>  ---- end ----
>  11.1: Basic parsing test             : Ok
>  --- start ---
>  Testing 'temp_test_hwmon_event1'
>  Using CPUID IBM,3931,704,A01,3.7,002f
>  temp_test_hwmon_event1 -> hwmon_a_test_hwmon_pmu/temp_test_hwmon_event1/
>  FAILED tests/hwmon_pmu.c:189 Unexpected config for
>     'temp_test_hwmon_event1', 292470092988416 != 655361
>  ---- end ----
>  11.2: Parsing without PMU name       : FAILED!
>  --- start ---
>  Testing 'hwmon_a_test_hwmon_pmu/temp_test_hwmon_event1/'
>  FAILED tests/hwmon_pmu.c:189 Unexpected config for
>     'hwmon_a_test_hwmon_pmu/temp_test_hwmon_event1/',
>     292470092988416 != 655361
>  ---- end ----
>  11.3: Parsing with PMU name          : FAILED!
>  #
> 
> The root cause is in member test_event::config which is initialized
> to 0xA0001 or 655361. During event parsing a long list event parsing
> functions are called and end up with this gdb call stack:
...
> However member key::type_and_num is defined as union and bit field:
> 
>    union hwmon_pmu_event_key {
>         long type_and_num;
>         struct {
>                 int num :16;
>                 enum hwmon_type type :8;
>         };
>    };

That is entirely horrid.

I'm surprised this even compiles:
static size_t hwmon_pmu__event_hashmap_hash(long key, void *ctx __maybe_unused)
{
	return ((union hwmon_pmu_event_key)key).type_and_num;
}
It has to be just 'return key', but I'm not sure what the hashmap code is doing.

AFAICT the code is just trying to generate a value for the hashmap to hash on?
Why not just use (type << 16 | num) instead of 'trying to be clever' with a union?

	David