Re: [RFC 1/4] drivers/nvdimm: Add perf interface to expose nvdimm performance stats

On 5/12/21 10:57 PM, Peter Zijlstra wrote:

On Wed, May 12, 2021 at 10:08:21PM +0530, Kajol Jain wrote:

quoted

+static void nvdimm_pmu_read(struct perf_event *event)
+{
+	struct nvdimm_pmu *nd_pmu = to_nvdimm_pmu(event->pmu);
+
+	/* jump to arch/platform specific callbacks if any */
+	if (nd_pmu && nd_pmu->read)
+		nd_pmu->read(event, nd_pmu->dev);
+}
+
+static void nvdimm_pmu_del(struct perf_event *event, int flags)
+{
+	struct nvdimm_pmu *nd_pmu = to_nvdimm_pmu(event->pmu);
+
+	/* jump to arch/platform specific callbacks if any */
+	if (nd_pmu && nd_pmu->del)
+		nd_pmu->del(event, flags, nd_pmu->dev);
+}
+
+static int nvdimm_pmu_add(struct perf_event *event, int flags)
+{
+	struct nvdimm_pmu *nd_pmu = to_nvdimm_pmu(event->pmu);
+
+	if (flags & PERF_EF_START)
+		/* jump to arch/platform specific callbacks if any */
+		if (nd_pmu && nd_pmu->add)
+			return nd_pmu->add(event, flags, nd_pmu->dev);
+	return 0;
+}

What's the value add here? Why can't you directly set driver pointers? I
also don't really believe ->{add,del,read} can be optional and still
have a sane driver.

Hi Peter,

  The intend for adding these callbacks  is to give flexibility to the
arch/platform specific driver code to use its own routine for getting 
counter data or specific checks/operations. Arch/platform driver code
would have different method to get the counter data like IBM pseries
nmem* device which uses a hypervisor call(hcall).

But yes the current read/add/del functions are not adding value. We
could  add an arch/platform specific function which could handle the
capturing of the counter data and do the rest of the operation here,
is this approach better?

Thanks,
Kajol Jain