linux-kernel - Re: [PATCH V1 RESEND 1/3] perf/imx_ddr: Add system PMU identifier for userspace

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Message-ID: <20200520073304.GA23534@willie-the-truck>
Date:   Wed, 20 May 2020 08:33:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Joakim Zhang <qiangqing.zhang@....com>, john.garry@...wei.com,
        mark.rutland@....com, shawnguo@...nel.org, linux-imx@....com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V1 RESEND 1/3] perf/imx_ddr: Add system PMU identifier
 for userspace

On Tue, May 19, 2020 at 12:51:25PM -0600, Rob Herring wrote:
> On Tue, May 12, 2020 at 03:31:13PM +0800, Joakim Zhang wrote:
> > +static ssize_t ddr_perf_identifier_show(struct device *dev,
> > +					struct device_attribute *attr,
> > +					char *page)
> > +{
> > +	struct ddr_pmu *pmu = dev_get_drvdata(dev);
> > +
> > +	return sprintf(page, "%s\n", pmu->devtype_data->identifier);
> 
> Why do we need yet another way to identify the SoC from userspace?

I also really dislike this. What's the preferred way to identify the SoC
from userspace? It's needed so that the perf userspace tool can describe
perf events that are supported for the PMU, as this isn't probe-able
directly from the hardware. We have the same issue with the SMMUv3 PMCG [1],
and so we need to solve the problem for both DT and ACPI.

Will

[1] https://lore.kernel.org/r/1587120634-19666-1-git-send-email-john.garry@huawei.com