Re: [patch 0/8] Nesting class_device patches that actually work

[Kay Sievers]

On Tue, Oct 18, 2005 at 03:18:22AM -0400, Adam Belay wrote:
> On Mon, Oct 17, 2005 at 10:26:17PM -0700, Greg KH wrote:
> > On Mon, Oct 17, 2005 at 07:24:30PM -0400, Adam Belay wrote:
> > > 
> > > Sounds good to me.  The changes to driver model internals may be substantial.
> > > For example, because buses and classes will share more code, it's
> > > reasonable to allow drivers to bind to any "device" object, even class
> > > devices.  Of course this would be limited to classes that choose to
> > > implement driver matching etc.  We are doing this now with the pci express
> > > port driver.
> > 
> > That's a bus, not a class device.  Drivers bind to devices through a
> > bus.  That's why we have busses.
> 
> If class devices and devices belong in the same tree, then clearly the original
> distinction is artificial.  "struct bus_type" is a class of "struct device".
> "struct class" is a class of "struct class_dev".  We now know of devices
> in between these two extremes (e.g. pci express port driver).  It's also
> possible that drivers will want to bind to class devices (e.g. a partition
> driver binding to a block device).  Isn't it fair to say that the "bus_type"
> vs. "class" distinction is also artificial?  At the very least they are
> duplicating some code.

I agree and would like to see the "bus" functionality just as set of special
methods of a unified device struture also used for class devices.

> > > > Oh, one tiny problem.  "virtual devices" are not currently represented
> > > > in our device tree, but are in the class tree.  Things like the
> > > > different vc and ttys and misc devices are examples of this.  I'll just
> > > > put them on the "platform" bus if no one minds.
> > > 
> > > I think we should be trying to kill off the platform bus (it's artifical and
> > > doesn't show the real relationships between these devices).  Instead, just
> > > hang them off the root of the tree.
> > 
> > Everything that's currently a platform device go to the root?  No,
> > that's not going to happen, sorry.

But will sticking stuff like "mice" or "tty" into "platform" will really
work? These devices belong to their own primary class like "input" or "tty" and
they can not be part of a "bus" at the same time, right?

I'm dreaming of:
  - merging "struct device" and "struct class_device"

  - provide current "bus" and "class" methodes for _all_ devices

  - every current "bus", just becomes a "class" to indicate a group
    of devices, where "input" has the same "group interface" in
    /sys/class/ as "pci" - buses go away completely

  - every device is a member of one single class, be it a pci-bus-device
    or a virtual class

  - separate strictly between hierarchy and classification, which
    is a bit ugly sometimes cause of the flat class layout, but it
    provides a very nice way to keep the class interface simple and
    therefore stable, while still be able to change the device hierarchy

  - /sys/devices becomes a buch of trees and some parent-less devices
    in the root, its whole reason is just to export the device dependency
    structure, nothing else

That way the "platform bus" would be a "platform class", which will only
contain devices which are not part of any other class. All these
parent-less devices would live in the root of /sys/devices to reflect
their lack of dependency.
Things like "tty" and "mice", would be members of their natural
class, but also live in the root of /sys/devices. That way we have the
technically correct classifcation and hierarchy.

Does that make sense?

Thanks,
Kay
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