Re: [PATCH 4/7] ppc/pnv: Simplify pnv_bmc_create()

[Greg Kurz <groug@kaod.org>]

On Thu, 28 Jan 2021 08:46:01 +0100
Cédric Le Goater <clg@kaod.org> wrote:

> On 1/28/21 1:46 AM, Joel Stanley wrote:
> > On Tue, 26 Jan 2021 at 17:14, Cédric Le Goater <clg@kaod.org> wrote:
> >>
> >> and reuse pnv_bmc_set_pnor() to share the setting of the PNOR.
> >>
> >> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> >> ---
> >>  hw/ppc/pnv_bmc.c | 7 +------
> >>  1 file changed, 1 insertion(+), 6 deletions(-)
> >>
> >> diff --git a/hw/ppc/pnv_bmc.c b/hw/ppc/pnv_bmc.c
> >> index 67ebb16c4d5f..86d16b493539 100644
> >> --- a/hw/ppc/pnv_bmc.c
> >> +++ b/hw/ppc/pnv_bmc.c
> >> @@ -260,13 +260,8 @@ IPMIBmc *pnv_bmc_create(PnvPnor *pnor)
> >>      Object *obj;
> >>
> >>      obj = object_new(TYPE_IPMI_BMC_SIMULATOR);
> >> -    object_ref(OBJECT(pnor));
> >> -    object_property_add_const_link(obj, "pnor", OBJECT(pnor));
> > 
> > I assume it's ok to move the link set to after the realise of the BMC object?
>  
> 
> When 2 objects need to be linked, one has to be realized first. 

Realize isn't a QOM concept in the first place...

> I suppose this is why it is allowed but I am not expert in that area. 
> 

... so no surprise object_property_add_const_link() doesn't care
about it.

> Greg  ?
> 

What is important though is that a property with a given name
can only be added *once* to an object during its lifetime.
Doing the contrary is a bug and QEMU aborts. So, with this
in mind, it seems to me that adding QOM properties to a
device object should only be done from some init path
that is only called once.

> That was the case already when defining a "ipmi-bmc-sim" device on the 
> command line. 
> 

Yeah and the property is added during machine reset... which
is typically a path that can be taken several times during
the machine lifetime. The potential crash is avoided because
pnv_reset() doesn't call pnv_bmc_set_pnor() if pnv->bmc is
already set, but this is a fragile workaround...

A QOM link doesn't look like the correct way to model the BMC
accesses to the PNOR. Since the only user is hiomap_cmd(),
it seems you could reach the same result with a pointer to
the PNOR object being passed to ipmi_sim_register_netfn()
and later passed to hiomap_cmd().

> C. 
> 
> 
> >>      qdev_realize(DEVICE(obj), NULL, &error_fatal);
> >> -
> >> -    /* Install the HIOMAP protocol handlers to access the PNOR */
> >> -    ipmi_sim_register_netfn(IPMI_BMC_SIMULATOR(obj), IPMI_NETFN_OEM,
> >> -                            &hiomap_netfn);
> >> +    pnv_bmc_set_pnor(IPMI_BMC(obj), pnor);
> >>
> >>      return IPMI_BMC(obj);
> >>  }
> >> --
> >> 2.26.2
> >>
> >>
>