linux-kernel - Re: [PATCH v2 2/5] perf: Allow a PMU to have a parent.

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Date:   Wed, 29 Mar 2023 11:03:06 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        linux-cxl@...r.kernel.org, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, mark.rutland@....com, will@...nel.org,
        dan.j.williams@...el.com, linuxarm@...wei.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCH v2 2/5] perf: Allow a PMU to have a parent.

On Wed, Mar 29, 2023 at 09:55:04AM +0100, Jonathan Cameron wrote:
> On Tue, 28 Mar 2023 13:01:08 +0200
> Greg KH <gregkh@...uxfoundation.org> wrote:
> 
> > On Tue, Mar 28, 2023 at 11:54:44AM +0100, Jonathan Cameron wrote:
> > > On Mon, 27 Mar 2023 13:04:08 -0400
> > > "Liang, Kan" <kan.liang@...ux.intel.com> wrote:
> > >   
> > > > On 2023-03-24 1:13 p.m., Jonathan Cameron wrote:  
> > > > > Some PMUs have well defined parents such as PCI devices.
> > > > > As the device_initialize() and device_add() are all within
> > > > > pmu_dev_alloc() which is called from perf_pmu_register()
> > > > > there is no opportunity to set the parent from within a driver.
> > > > > 
> > > > > Add a struct device *parent field to struct pmu and use that
> > > > > to set the parent.    
> > > > 
> > > > Why we want a PMU parent? Maybe I missed it. I don't see that the parent
> > > > is used anywhere.  
> > > 
> > > This allows you to identify the association between PMU and the hardware related
> > > device that is providing it by looking at the directory structure in sysfs rather
> > > than putting them directly under /sys/devices.
> > > 
> > > ls -l /sys/bus/event_sources/devices/
> > > 
> > > ... armv8_pmuv3_0 -> ../../../devices/arm8_pmuv3_0
> > > ... breakpoint -> ../../../devices/breakpoint
> > > ... cpmu0 -> ../../../devices/pci0000:0c/0000:0c:00.0/0000:0d:00.0/cpmu0/cpmu0
> > > etc
> > > 
> > > (the first cpmu0 is the parent registered as a child of the PCI EP and used for
> > >  driver binding).  So it's of use to userspace rather than in the kernel driver
> > > itself.
> > > 
> > > Note that almost nothing is normally in the top level /sys/devices other than
> > > event_sources - because nearly all other struct device instances created by
> > > subsystems have parents assigned.
> > > 
> > > On my system
> > > 
> > > ls /sys/devices
> > > 
> > > armv8_pmuv3_0	LNXSYSTEM:00	pci0000:0c	pnp0		system		uprobe
> > > breakpoint	pci0000:00	platform	software	tracepoint	virtual
> > > 
> > > +CC Greg KH for input on whether / why this make sense.  
> > 
> > That doesn't make sense, nothing should be in /sys/devices/ EXCEPT the
> > root device of busses.  Everything else is wrong and should have their
> > code fixed up (i.e. "breakpoint", "software", etc.)
> 
> Thanks Greg.
> 
> I was thinking to cycle back round to that once I'd got agreement on 'some' devices,
> but great to have clarity from the start that these should all have
> parents.
> 
> For a few cases the parent is not immediately obvious but we'll figure it out.

If there is no "real" parent, then it is a virtual device and belongs
under the "virtual" directory.  Otherwise why are these "devices" at all
with such generic names?

thanks,

greg k-h