Re: [PATCH 3/3] sam460ex: Use type cast macro instead of simple cast

[BALATON Zoltan]

On Thu, 7 Jan 2021, Greg Kurz wrote:
> On Wed, 6 Jan 2021 16:24:01 +0100
> BALATON Zoltan via <qemu-ppc@nongnu.org> wrote:
>
> > Use the PCI_BUS type cast macro to convert result of
> > qdev_get_child_bus(). Also remove the check for NULL afterwards which
> > should not be needed because sysbus_create_simple() uses error_abort
>
> It seems to me that sysbus_create_simple() doesn't return NULL because
> it ends up calling object_new_with_type(). This allocates the object
> with either g_malloc() or qemu_memalign(), both of which abort on
> failure.
>
> > and PCI_BUS macro also checks its argument by default so this
>
> AFAICT, PCI_BUS() and all other instance type checking macros are
> happy with a NULL argument. They simply return NULL in this case.

This wasn't my experience when I've got an error in code and got a NULL 
pointer here (on pegasos2 board but same situation). At least with 
qom-debug enabled (which I think is on by default unless explicitly 
disabled in configure) this will abort if the object is not the right 
type.

> > shouldn't fail here.
> >
> > Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
> > ---
> >  hw/ppc/sam460ex.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> >
> > diff --git a/hw/ppc/sam460ex.c b/hw/ppc/sam460ex.c
> > index 14e6583eb0..cc67e9c39b 100644
> > --- a/hw/ppc/sam460ex.c
> > +++ b/hw/ppc/sam460ex.c
> > @@ -384,11 +384,8 @@ static void sam460ex_init(MachineState *machine)
> >      ppc460ex_pcie_init(env);
> >      /* All PCI irqs are connected to the same UIC pin (cf. UBoot source) */
> >      dev = sysbus_create_simple("ppc440-pcix-host", 0xc0ec00000, uic[1][0]);
> > -    pci_bus = (PCIBus *)qdev_get_child_bus(dev, "pci.0");
> > -    if (!pci_bus) {
> > -        error_report("couldn't create PCI controller!");
> > -        exit(1);
> > -    }
> > +    pci_bus = PCI_BUS(qdev_get_child_bus(dev, "pci.0"));
> > +
>
> But PCI_BUS() is being passed qdev_get_child_bus(dev, "pci.0"), not
> dev... so the real question here is whether this can return NULL
> or not. And if this happens, is this a (1) user or (2) programming
> error ?
>
> If (1) then the "if (!pci_bus) { }" should be kept. If (2) then
> it should be converted to an assert().

I think it can only fail if the ppc440-pcix-host type is changed to not 
have a pci.0 child any more which is a programming error that's very 
unlikely to happen but if needed an assert could be added but I don't 
think that's really necessary. The error_report was definitely not needed 
as it's not a user error in any case.

Regards,
BALATON Zoltan

> >      memory_region_init_alias(isa, NULL, "isa_mmio", get_system_io(),
> >                               0, 0x10000);
> >      memory_region_add_subregion(get_system_memory(), 0xc08000000, isa);