Re: [PATCH v3 04/10] usb/xhci: fixup xhci kconfig deps

[Paolo Bonzini]

On 20/11/20 01:27, Bruce Rogers wrote:
> >   config USB_XHCI_SYSBUS
> >       bool
> > -    default y if USB_XHCI
> > -    select USB
> > +    default y
> > +    select USB_XHCI
> >   
> >   config USB_MUSB
> >       bool
> I was reviewing what device changes are happening between v5.1.0 and
> v5.2.0 for the QEMU arch's we support and noticed for s390x system
> emulation that usb devices have appeared in the info qdm list in the
> monitor.
>
> I bisected the change to this patch, now commit 7114f6eac333d99b, which
> does a 'default y' without any conditionals. I'm fairly sure that was
> not the intent, though I do know what the proper conditionals should
> be.
>
> Can you take a look at it?

Generally, SYSBUS devices should never be "default y" because they're 
not user creatable.  Also kconfig.rst says

  Devices are usually ``default y`` if and only if they have at least
  one ``depends on``; the default could be conditional on a device
  group.

In this case, the right thing to do is to remove "default y" here; 
microvm already has a "select USB_XHCI_SYSBUS" so it would still work. 
That said, I would also change

    select PCI_EXPRESS_GENERIC_BRIDGE
    select USB_XHCI_SYSBUS

for microvm to "imply".  Again, according to kconfig.rst

  Boards specify their constituent devices using ``imply`` and
  ``select`` directives.  A device should be listed under ``select`` if
  the board cannot be started at all without it.  It should be listed
  under ``imply`` if (depending on the QEMU command line) the board may
  or may not be started without it.

Thanks,

Paolo