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

[Greg KH <gregkh@linuxfoundation.org>]

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@linux.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 k-h