Date:	Wed, 10 Nov 2010 12:10:23 +1100
From:	Michael Ellerman <michael@...erman.id.au>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] perf: sysfs type id

On Wed, 2010-11-10 at 01:57 +0100, Kay Sievers wrote:
> Stay on the list please, with any possible reply. Thanks!

You dropped the CC when you replied, or is my mailer being weird?

> On Wed, Nov 10, 2010 at 01:52, Michael Ellerman <michael@...erman.id.au> wrote:
> > On Wed, 2010-11-10 at 00:45 +0100, Kay Sievers wrote:
> >> On Wed, Nov 10, 2010 at 00:36, Michael Ellerman <michael@...erman.id.au> wrote:
> >> > On Tue, 2010-11-09 at 14:13 -0800, Greg KH wrote:
> >> >> On Tue, Nov 09, 2010 at 10:45:19PM +0100, Peter Zijlstra wrote:
> >> >> > The below is a RFC patch adding dynamic type ids to perf.
> >> >> >
> >> >> > We need to represent PMUs in sysfs because we want to allow multiple
> >> >> > (loadable) PMUs and need a way to identify them.
> >> >> >
> >> >> > This patch creates a new device class "pmu" and adds a single attribute
> >> >> > "type" to it. This device attribute will expose the dynamic type id as
> >> >> > required by perf_event_attr::type.
> >> >> >
> >> >> > The sysfs layout looks like:
> >> >> >
> >> >> > [root@...tmere ~]# cd /sys/class/pmu/
> >> >>
> >> >> You missed the embedded track at Plumbers where we talked about never
> >> >> adding another class to the kernel.  Please use bus_id instead for this.
> >> >
> >> > At least in the examples I've seen creating a bus requires a lot more
> >> > code than a class. Or is there a shortcut I don't know about when it's a
> >> > virtual bus?
> >>
> >> What do you mean? Instead of registering a class you register a bus?
> >
> > Yeah. From what I've seen doing the latter is a lot more involved.
> >
> >> Instead of assigning a class to the device, you assign the bus before
> >> you add it. Other than this you do the same with the device.
> >
> > Sure. My point is creating a bus (from what I've seen) is not as easy as
> > creating a bus. No one said kernel hacking should be easy, but if the
> > advice is "use a bus not a class" it'd be good if they were
> > approximately equivalent in effort.
> >
> > My code to use a class is ~=:
> >
> > struct class foo_class;
> > foo_class = class_create(THIS_MODULE, "foo");
> > device = device_create(foo_class, NULL, dev, "foo0");
> 
> device_create must not be used for devices without a dev_t.
> device_destroy, which is the counterpart can not work.

It has a dev_t, that's the third argument?

> If you use device_del/device_unregister, you need to use
> device_add/device_register.

I don't use them.

> The pseudo-convenience device_register/device_unregister should also
> not be used.

Why are they in the tree if they shouldn't be used?

So far you are failing to dispel my notion that sysfs is a place where
mortals dare not tread ;)

> > And I'm looking at eg. drivers/usb/serial/bus.c as an example bus.
> >
> > But in my case (and I think perf too), we don't need a bus that probes
> > etc. it's just a virtual bus that groups things, so it seems like it
> > should be simple.
> >
> > Anyway I feel like I'm missing something, so hopefully you can clue me
> > in :)
> 
> Buses without drivers do not probe at all, they behave like classes.

OK, good, that would seem to be a prerequisite for replacing the latter
with the former.

I'm just not clear on how that actually works in the code. For example I
have a device which is on a bus (that's how it got probed), how do I
also put it on another bus (my virtual bus replacing my class) ?

cheers



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