On Thu, 18 May 2017 18:24:32 +0100
Will Deacon [off-list ref] wrote:

+/* Perf callbacks */
+static int arm_spe_pmu_event_init(struct perf_event *event)
+{
+	u64 reg;
+	struct perf_event_attr *attr = &event->attr;
+	struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
+
+	/* This is, of course, deeply driver-specific */
+	if (attr->type != event->pmu->type)
+		return -ENOENT;
+
+	if (event->cpu >= 0 &&
+	    !cpumask_test_cpu(event->cpu, &spe_pmu->supported_cpus))
+		return -ENOENT;
+
+	if (arm_spe_event_to_pmsevfr(event) & PMSEVFR_EL1_RES0)
+		return -EOPNOTSUPP;
+
+	if (event->hw.sample_period < spe_pmu->min_period ||
+	    event->hw.sample_period & PMSIRR_EL1_IVAL_MASK)
+		return -EOPNOTSUPP;
+
+	if (attr->exclude_idle)
+		return -EOPNOTSUPP;
+
+	/*
+	 * Feedback-directed frequency throttling doesn't work when we
+	 * have a buffer of samples. We'd need to manually count the
+	 * samples in the buffer when it fills up and adjust the event
+	 * count to reflect that. Instead, force the user to specify a
+	 * sample period instead.
+	 */
+	if (attr->freq)
+		return -EINVAL;
+
+	reg = arm_spe_event_to_pmsfcr(event);
+	if ((reg & BIT(PMSFCR_EL1_FE_SHIFT)) &&
+	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_EVT))
+		return -EOPNOTSUPP;
+
+	if ((reg & BIT(PMSFCR_EL1_FT_SHIFT)) &&
+	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_TYP))
+		return -EOPNOTSUPP;
+
+	if ((reg & BIT(PMSFCR_EL1_FL_SHIFT)) &&
+	    !(spe_pmu->features & SPE_PMU_FEAT_FILT_LAT))
+		return -EOPNOTSUPP;

I've consistently brought up lack of proper user error messaging in all
previous submissions of this driver:

Jan. 10 2017 (first RFC):
https://www.spinics.net/lists/arm-kernel/msg553322.html

Jan. 13 (second RFC):
https://patchwork.kernel.org/patch/9515967/

[I just went ahead and re-read https://lkml.org/lkml/2015/8/26/661 and
still think it's not a reason to not do this]

30 Jan 2017 (first PATCH series):
https://www.spinics.net/lists/arm-kernel/msg559232.html

6 apr 2017 (PATCH v2 series):
https://lkml.org/lkml/2017/4/6/790

AFAICT, my comments hold, yet the driver still gets resubmitted without
them being addressed.  How do we get out of this loop?

Kim