On Thu, Oct 01, 2020 at 06:29:58PM +0000, Ertman, David M wrote:

> > I would like to move those PCI MFDs away from that, and into this bus
> > instead.

> > If there needs to have a way to pass/share resources, great, let's add
> > it, there's no objection from me.

OK, if we can add resource passing then no huge problem from my end
since there'd be no case where you couldn't use an ancillairy device.
It's still a bit confusing but there's no cases where we're supposed to
use an ancilliary device but it doesn't have required features.

> The sharing of information is done by having the parent driver declare
> the ancillary_device as an element in a parent_struct that also contains a
> pointer to the shared information.  This way, when the ancillary_driver
> is probed, and a pointer to ancillary_device is passed, it can perform a
> container_of on the ancillary_device and gain access to the shared data.

> This keeps all requirements out of the ancillary bus code and it can be
> as flexible as the implementer wants it to be.

This means that every device has to reinvent the wheel for common
resources like interrupts, including things like looking them up by name
and so on.  That doesn't seem ideal.