Struct Inheritance in drivers/ata/

[greg@kroah.com (Greg KH)]

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