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Subject: Re: [RFC 1/4] drivers/nvdimm: Add perf interface to expose nvdimm
 performance stats
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org,
        linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org,
        maddy@linux.vnet.ibm.com, santosh@fossix.org,
        aneesh.kumar@linux.ibm.com, vaibhav@linux.ibm.com,
        dan.j.williams@intel.com, ira.weiny@intel.com,
        atrajeev@linux.vnet.ibm.com, tglx@linutronix.de
References: <20210512163824.255370-1-kjain@linux.ibm.com>
 <20210512163824.255370-2-kjain@linux.ibm.com>
 <YJwP9ByvAcDPixVN@hirez.programming.kicks-ass.net>
From:   kajoljain <kjain@linux.ibm.com>
Message-ID: <37015d53-050a-acef-2958-b1ff5d02800b@linux.ibm.com>
Date:   Thu, 13 May 2021 17:56:14 +0530
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On 5/12/21 10:57 PM, Peter Zijlstra wrote:
> On Wed, May 12, 2021 at 10:08:21PM +0530, Kajol Jain wrote:
>> +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