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

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Message-ID: <YJwP9ByvAcDPixVN@hirez.programming.kicks-ass.net>
Date:   Wed, 12 May 2021 19:27:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kajol Jain <kjain@...ux.ibm.com>
Cc:     mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org,
        maddy@...ux.vnet.ibm.com, santosh@...six.org,
        aneesh.kumar@...ux.ibm.com, vaibhav@...ux.ibm.com,
        dan.j.williams@...el.com, ira.weiny@...el.com,
        atrajeev@...ux.vnet.ibm.com, tglx@...utronix.de
Subject: Re: [RFC 1/4] drivers/nvdimm: Add perf interface to expose nvdimm
 performance stats

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.