On Thu, Dec 03, 2015 at 07:41:59PM -0700, Eric Blake wrote:
> On 12/03/2015 04:54 PM, David Gibson wrote:
> > On Thu, Dec 03, 2015 at 05:37:39PM +0100, Markus Armbruster wrote:
> >> prop_get_fdt() misuses the visitor API: when fdt is null, it doesn't
> >> visit anything. object_property_get_qobject() happily
> >> object_property_get_qobject(). Amazingly, the latter survives the
> >> misuse. Turns out we've papered over it long before prop_get_fdt()
> >> existed, in commit 1d10b44.
> >>
> >> However, commit 6c2f9a1 changed how we paper over it, and as a side
> >> effect changed qom-get's value from {} to null. Change it right back
> >> by fixing the visitor misuse.
> >>
> >> Signed-off-by: Markus Armbruster <arm...@redhat.com>
> >> ---
> >> hw/ppc/spapr_drc.c | 5 +++++
> >> 1 file changed, 5 insertions(+)
> >>
>
> > I'm not entirely convinced by this. IIUC, this makes the output in
> > the case of NULL (i.e. missing) fdt identical to the output in the
> > case of an empty, valid fdt - in dtc syntax, this:
> > / {
> > };
> >
> > Those are different cases from the point of view of the code which
> > actually uses the fdt, and for purposes of debugging it, I suspect we
> > want to expose that difference.
>
> Expressing null may be the right thing, but it should be a conscious
> decision, and not a side-effect of an unrelated patch. This patch is
> just about avoiding a regression for 2.5, because outputting {} for both
> a missing fdt and an empty one was the behavior we had back in 2.4 (that
> is, we've already returned {} in at least one release, so it won't hurt
> to do it for one more). For 2.6 we can revisit things to actually
> express what is wanted.
>
> >
> > I don't know what the QOMishly correct way of doing that is, though.
> > Can we somehow make the "fdt" property disappear entirely if fdt is
> > NULL?
>
> In qapi terms, if a variable is marked optional and has_FOO is false,
> then the variable disappears completely. But I'm not sure if that maps
> over to qom. Maybe you do it by setting errp if drc->fdt is NULL, so
> that prop_get_fdt() only succeeds when there is something for it to
> return. Or maybe returning qnull() is right after all, but in that
> case, explicitly calling 'QObject *n = qnull(); visit_type_any(v, &n,
> NULL, &err) seems like a nicer way than relying on side effects of how
> the qmp output visitor behaves when nothing was visited.
Ok, thank makes sense.
Acked-by: David Gibson <da...@gibson.dropbear.id.au>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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