linux-kernel - Re: [PATCH v6 2/2] drivers/perf: hisi: Add driver for HiSilicon PCIe PMU

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Date:   Wed, 16 Jun 2021 10:23:43 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "liuqi (BA)" <liuqi115@...wei.com>
Cc:     Krzysztof Wilczyński <kw@...ux.com>,
        Linuxarm <linuxarm@...wei.com>, will@...nel.org,
        mark.rutland@....com, bhelgaas@...gle.com,
        linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, zhangshaokun@...ilicon.com
Subject: Re: [PATCH v6 2/2] drivers/perf: hisi: Add driver for HiSilicon PCIe
 PMU

On Wed, Jun 16, 2021 at 09:09:40AM +0800, liuqi (BA) wrote:
> On 2021/6/12 7:33, Krzysztof Wilczyński wrote:

> > > +static ssize_t hisi_pcie_event_sysfs_show(struct device *dev,
> > > +				   struct device_attribute *attr, char *buf)
> > > +{
> > > +	struct dev_ext_attribute *eattr;
> > > +
> > > +	eattr = container_of(attr, struct dev_ext_attribute, attr);
> > > +
> > > +	return sysfs_emit(buf, "config=0x%lx\n", (unsigned long)eattr->var);
> > > +}
> > 
> > I am not that familiar with the perf drivers, thus I might be completely
> > wrong here, but usually for sysfs objects a single value is preferred,
> > so that this "config=" technically would not be needed, unless this is
> > somewhat essential to the consumers of this attribute to know what the
> > value is?  What do you think?
>
> "config=" is a supported for userspace tool, it is a kind of alias, so
> cannot be remover here, thanks.

I don't understand this part.  This is brand-new functionality for the
kernel, so there's no requirement to maintain compatibility for
existing userspace tools.

If there's a similar sysfs show function for other perf drivers, and
you need to be compatible with *that*, fine.  But if this is merely
about being compatible with userspace that uses out-of-tree kernel
functionality, that's not a real factor.

Bjorn