perf/amd/ibs: Remove pointless sample period check

Valid perf event sample period value for IBS PMUs (Fetch and Op both)
is limited to multiple of 0x10. perf_ibs_init() has this check:

  if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
          return -EINVAL;

But it's broken since hwc->sample_period will always be 0 when
event->attr.sample_freq is 0 (irrespective of event->attr.freq value.)

One option to fix this is to change the condition:

  - if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
  + if (!event->attr.freq && hwc->sample_period & 0x0f)

However, that will break all userspace tools which have been using IBS
event with sample_period not multiple of 0x10.

Another option is to remove the condition altogether and mask lower
nibble _silently_, same as what current code is inadvertently doing.
I'm preferring this approach as it keeps the existing behavior.

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/20250115054438.1021-3-ravi.bangoria@amd.com

arch/x86/events/amd/ibs.c

@@ -307,13 +307,8 @@ static int perf_ibs_init(struct perf_event *event)
 		if (config & perf_ibs->cnt_mask)
 			/* raw max_cnt may not be set */
 			return -EINVAL;
-		if (!event->attr.sample_freq && hwc->sample_period & 0x0f)
-			/*
-			 * lower 4 bits can not be set in ibs max cnt,
-			 * but allowing it in case we adjust the
-			 * sample period to set a frequency.
-			 */
-			return -EINVAL;
+
+		/* Silently mask off lower nibble. IBS hw mandates it. */
 		hwc->sample_period &= ~0x0FULL;
 		if (!hwc->sample_period)
 			hwc->sample_period = 0x10;