Struct Inheritance in drivers/ata/

[Peter Hamilton]

I see that it works, but I'm not exactly following how.  Maybe I just don't
have enough C experience.  But I can't seem to duplicate that functionality.


I understand the way I've normally seen inheritance, where:

struct Child {
  struct Parent p;
  int somethingOnlyForChildren;
}

Or something similar.  Then the Child struct can be casted as a Parent, and
since there is no padding before the first piece of data in a struct, every
parent attribute is accessed as though the struct were a Parent.

I see how it all lines up in memory, but in the case of the kernel code I
can't quite follow it.  Wouldn't everything assigned to 'inherits' be so
much farther down in memory?  I feel like I'm missing something.  Is there a
special way they all need to be accessed?

- Peter

On Thu, May 26, 2011 at 7:23 PM, Greg KH <greg at kroah.com> wrote:

> On Thu, May 26, 2011 at 02:46:08PM -0600, Peter Hamilton wrote:
> > The code in drivers/ata/ uses an implementation of inheritance that I
> have not
> > seen before.  It's only briefly explained in the header file (
> include/linux/
> > libata.h:885):
> >
> > /*
> >  * ->inherits must be the last field and all the preceding
> >  * fields must be pointers.
> >  */
> >
> > The structs are then initialized with .inherits assigned first:
> >
> > drivers/ata/sata_nv.c:475
> >
> > static struct ata_port_operations nv_nf2_ops = {
>
> >         .inherits               = &nv_generic_ops,
> >         .freeze                 = nv_nf2_freeze,
>
> >         .thaw                   = nv_nf2_thaw,
>
> > };
> >
> >
> > Is this actually implementing inheritance?  Why do all preceding fields
> need to
> > be pointers?
> >
> > As far as I can tell, this style is only found in the ata drivers.
>
> I think you are right, but more subsystems need to emulate it, it is
> very powerful and works very well.  I have been wanting to convert the
> usb-serial layer to use the same thing one of these days.
>
> > Could anyone explain how this works?
>
> The code is all there that shows it, but basically the driver is telling
> the core to "use this type of functions, but if I set any others, use
> them instead."  It's a nicer way of doing inheritance in C than we do in
> other places in the kernel where we are a bit more "verbose" in making
> it happen.
>
> hope this helps,
>
> greg k-h
>
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