Re: [PATCHSET 3/4] sysfs: divorce sysfs from kobject and driver model

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Fri, 05 Oct 2007 17:00:48 +0900,
Tejun Heo <htejun@gmail.com> wrote:

> I think this definitely needs more discussion, so here we go...

I agree, so I'll give my 0.02 € here...
> 
> Greg KH wrote:
> >> 1. What is a kobject?
> [--snip--]
> >> The functionality served by kobject can be broken down into the
> >> following two.
> >>
> >> F-a. To serve as an entity both subsystems can share lifespan
> >> management.  ie. both subsystems reference count on kobject.
> >>
> >> F-b. To serve as an entity both subsystems can base their internal
> >> representation on.  (base object in OO term).
> > 
> > Yes, those are two functions, I can agree with.

I think that's the heart of the question: We first need to agree what
use the different components should have.

(a) The driver model

The driver model serves as a unified layer for all devices managed by
the kernel, organized in trees, and the drivers handling them. This
includes busses, matching of devices and drivers, attributes and so on.
Userspace expects to see these devices, drivers, busses and attributes
by looking under /sys/devices/. /sys/class/ and /sys/bus/ provide
additional views on this data.

(b) kobjects, ktypes, ksets

kobjects provide a mechanism to arrange kernel objects into a tree-like
structure. ktypes and ksets are mechanisms to further order these
objects. Changes in the kobject hierarchy are communicated to userspace
via uevents.

The driver core is implemented using this infrastructure.

(c) krefs

krefs provide a generic reference counting mechanism.

The kobject infrastructure uses krefs for its reference counting needs.

(d) sysfs

sysfs is a virtual filesystem. It exports information on kernel objects
to user space. (IMO, that's the key: sysfs is userspace representation.)

That said, it is logical that kobjects are made visible to userspace
via sysfs. If someone is trying to make things show up in sysfs and has
to jump through hoops to cook up kobjects, they're probably using the
wrong infrastructure.

There are two big problems with the tight coupling of sysfs and kobjects:

- lifetime rules; but this fortunately hugely improved with the
previous patches :)

- relaying implementation details to userspace so that they cannot be
easily changed. We would need to allow kobjects not showing up in sysfs
and making symlinks a sysfs facility not relying on kobjects to help
there.

> > 
> [--snip--]
> >> 3. Where does that leave kobject?
> >>
> >> If you combine #1 and #2, both functionalities become questionable.
> >>
> >> F-a. sysfs no longer plays role in lifespan management of driver model
> >> object.  This functionality is exactly what's killed by #2.
> > 
> > Yes, but a kobject is still needed internally for the lifespan
> > management.
> 
> Yes, exactly - "internally" to the driver model (or drivers which ride
> along).  To sysfs, it has no function other than being an opaque token.

But krefs are used for kobject reference counting, or am I
misunderstanding here?

> 
> [--snip--]
> >> IMHO, both L-a and L-b contribute only to obfuscation of the driver
> >> model and sysfs.
> > 
> > No.  I think you are missing a number of things that kobjects provide
> > and allow:
> > 	- a structurual heirachy of devices.  Combine kobjects with
> > 	  ksets and ktypes, and you have a very powerful system to
> > 	  categorize objects and their representation to each other.
> 
> Yes, which only needs to be used _inside_ the driver model
> implementation proper.

There are use cases outside of the driver model prober where you may
want to use kobject for hierarchy.

> 
> > 	- a consistant and easy interface to userspace through
> > 	  uevents/hotplug of the creation and removal of these objects.
> > 	  This keeps the different parts of the kernel that need this
> > 	  interface from having to create it every time on their own.
> 
> Things can be much easier than now.  Also, the above paragraph is
> inconsistent with the rest of your argument or am I misunderstanding
> what you mean by the above paragraph?

I see uevents as a notifier for changes in the kobject hierarchy, so
they belong to that layer. However, the layering between kobjects and
sysfs (path names etc.) could probably be made cleaner.

> 
> > 	- a way to easily create and export attributes in sysfs
> > 	  automatically.
> 
> This is and should be the function of the driver model not kobjects.

I agree, attributes should belong to the driver model.

> Removing kobject from the interface doesn't change anything about this.

Hm. Currently you have a file<->attribute correlation. This would
change if you allow non-attribute files.

> 
> > 	- a way to provide working reference counting for a variety of
> > 	  different objects.
> 
> To me, this just feels wrong and does more harm than it helps.  I really
> think we shouldn't have multi-role flash light at the core of our driver
> model (inside driver model proper, no problem, but not as exported
> interface).

And I still think that this is the purpose of krefs :)

> 
> > All of those are still needed for the kernel.
> 
> For #1 and #3, I agree if you limit the scope to driver model proper but
> I am not arguing kobject and all its friends should be abolished.  I'm
> arguing that there is no reason to export it as API because it doesn't
> add any value exported.

I see the value for those code paths that want to provide a hierarchy
of kernel objects outside the driver model proper.

> >> The rest isn't great in number and, much more importantly, many of those
> >> suffer from the current interface which is painful to use independently.
> >>  For example, kernel/module.c does all the kobject dances including
> >> defining a subsystem just to ignore everything else and use it as an
> >> opaque token to sysfs (kset_find_obj doesn't count, a generic map or
> >> sysfs with sysfs_dirent interface can do that just as well).
> > 
> > I will not deny that the current use of kobjects/ksets/ktypes (subsystem
> > is now gone) is difficult and extreemly painful.  I am currently working
> > to fix this issue.  But don't think that the reason this is hard to use
> > means that it should be abolished alltogether.
> > 
> > Rather, it means that this interface to using kobjects needs to be fixed
> > and made easier, not circumvented.

Yes, an easier-to-use interface to the kobject stuff would be helpful
for everyone :)

> The thing is that functionality-wise, kobject and its friends don't
> serve anything anymore outside of driver model implementation proper
> (I'll talk about uevent later) and thus there is no reason to use it
> outside of driver model implementation anymore in the long term.

I disagree. A hierarchy of kernel objects has uses beyond the driver
model.

> I can agree to that.  Unfortunately, it also sometimes distorts driver
> implementation because representing the proper picture is so difficult.

Yes, stuff visible to userspace is hard :(

>  libata attributes are under constant pressure to escape to SCSI and
> block nodes (nothing bad has actually happened yet tho), new features
> are being delayed and/or pushed to use different userland interface
> (module parameter being the most common).  I know libata is a corner
> case at the moment but a bit of flexibility would have been very helpful
> for both developers and users.

I'm not familiar with the libata attributes, but shouldn't these
features really be part of the respective block devices or module
parameters?

> >> So, no, I don't agree to keeping kobject based interface to keep sysfs
> >> hierarchy tidy.
> > 
> > I strongly object here.
> > 
> > I think that if your current patches are accepted, we will see a lot of
> > new users of sysfs in ways that are not "standard" to how it is used
> > today.  Things that rely on "close" happening to the sysfs file, or
> > trees created that do not emit uevents.
> 
> Adding policies to prevent such usages to easy interface is the right
> thing to do.  Currently, we don't even have defined policies for sysfs
> outside of driver model.  The only thing is that it's difficult to
> understand and painful to use.

s/outside of driver model/outside of kobjects/

And I don't think something that is not part of the kobject hierarchy
really wants to have a sysfs representation.

> Yes and sysfs restructuring when it's finished won't change that at all.
>  Things will be better toward the same goal.  Remember that I said the
> next step was moving uevent over to sysfs?  Uevent belongs to sysfs
> because it's by design bound to userland visible representation of
> kernel objects.  The current placement is awkward - kobject carries
> uevent related fields whether it's needed or not, uevent suppression is
> in struct device not in kobject and sysfs creation / uevent
> synchronization is done in awkward way.

uevent suppression does not belong to devices, yes. Rather to
kobjects :) And I think uevents as a way to notify users of changes in
the kobject hierarchy is useful outside of sysfs.

> > Now if we can keep that enforcement of sysfs, then I have no objections
> > to cleaning up the internal interface between sysfs and kobjects, and
> > the overall fixing of the kobject/kset/ktype code.  That is all good
> > things overall.

*nod* Cleaning up the layering and the interfaces is a good thing in
itself.

> I think what's missing here is why we need to enforce kobject interface.
>  It certainly isn't for kobject itself's sake, right?  Originally, it
> served a valid purpose for interaction with sysfs.  Also, by the virtue
> of being difficult to use, it limited the usage of sysfs.
> 
> My arguments here are...
> 
> 1. kobject no longer has such valid purpose as far as sysfs is
> concerned, which was its biggest out-of-driver-model functionality.
> And, in the long term, I don't see any reason why kobject needs to be
> visible outside of driver model.
> 
> 2. I'm all for keeping the tree tidy but I think it's better and more
> cleanly done by well defined policies clearly stated in the
> documentation and enforced by code such that changing sysfs hierarchy
> always goes through driver model review chain.
> 
> 3. In this series, all that happened was implementing new interface and
> features and reimplement original interface in terms of them.  As such,
> there is no code clean up out of sysfs.  In fact, sysfs gained
> considerable amount of code.  Considering wide spread use of sysfs, I'm
> pretty sure the net code amount and complexity will drop considerably
> with future API user conversions.
> 
> Hopefully, I stated things clearer this time.  If you disagree, please
> try to convince me.  I'm listening and I think we really need to
> establish consensus on this subject.

I hope I have made my own view clear :) Thanks for reading through this
longish mail.
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