Re: Purpose of QOM properties registered at realize time?

[BALATON Zoltan]

On Tue, 6 Oct 2020, Eduardo Habkost wrote:
> Hi,
>
> While trying to understand how QOM properties are used in QEMU, I
> stumbled upon multiple cases where alias properties are added at
> realize time.
>
> Now, I don't understand why those properties exist.  As the
> properties are added at realize time, I assume they aren't
> supposed to be touched by the user at all.  If they are not
> supposed to be touched by the user, what exactly is the purpose
> of those QOM properties?
>
> For reference, these are the cases I've found:
>
> $ git grep -nwpE 
> 'object_property_add_alias|qdev_pass_gpios|qdev_alias_all_properties' | grep 
> -A1 realize
> hw/arm/allwinner-a10.c=71=static void aw_a10_realize(DeviceState *dev, Error 
> **errp)
> hw/arm/allwinner-a10.c:89:    qdev_pass_gpios(DEVICE(&s->intc), dev, NULL);
> --
> hw/arm/allwinner-h3.c=232=static void allwinner_h3_realize(DeviceState *dev, 
> Error **errp)
> hw/arm/allwinner-h3.c:359:    object_property_add_alias(OBJECT(s), "sd-bus", 
> OBJECT(&s->mmc0),
> --
> hw/arm/armsse.c=429=static void armsse_realize(DeviceState *dev, Error **errp)
> hw/arm/armsse.c:1092:    qdev_pass_gpios(dev_secctl, dev, "mscexp_status");
> --
> hw/arm/armv7m.c=149=static void armv7m_realize(DeviceState *dev, Error **errp)
> hw/arm/armv7m.c:219:    qdev_pass_gpios(DEVICE(&s->nvic), dev, NULL);
> --
> hw/arm/bcm2835_peripherals.c=128=static void 
> bcm2835_peripherals_realize(DeviceState *dev, Error **errp)
> hw/arm/bcm2835_peripherals.c:322:    object_property_add_alias(OBJECT(s), "sd-bus", 
> OBJECT(&s->gpio), "sd-bus");
> --
> hw/arm/bcm2836.c=69=static void bcm2836_realize(DeviceState *dev, Error **errp)
> hw/arm/bcm2836.c:87:    object_property_add_alias(OBJECT(s), "sd-bus", 
> OBJECT(&s->peripherals),
> hw/arm/msf2-soc.c=79=static void m2sxxx_soc_realize(DeviceState *dev_soc, Error 
> **errp)
> hw/arm/msf2-soc.c:170:        object_property_add_alias(OBJECT(s), bus_name,
> --
> hw/arm/nrf51_soc.c=58=static void nrf51_soc_realize(DeviceState *dev_soc, Error 
> **errp)
> hw/arm/nrf51_soc.c:138:    qdev_pass_gpios(DEVICE(&s->gpio), dev_soc, NULL);
> --
> hw/arm/xlnx-zynqmp.c=276=static void xlnx_zynqmp_realize(DeviceState *dev, 
> Error **errp)
> hw/arm/xlnx-zynqmp.c:522:        object_property_add_alias(OBJECT(s), bus_name, sdhci, 
> "sd-bus");
> --
> hw/misc/mac_via.c=1011=static void mac_via_realize(DeviceState *dev, Error 
> **errp)
> hw/misc/mac_via.c:1028:    object_property_add_alias(OBJECT(dev), "irq[0]", 
> OBJECT(ms),

Mark likely knows this one. I think he mentioned once that it may be used 
for storing reference to some object that needs to be accessed later but I 
could be wrong, haven't checked actual code only recalling from memory.

Regards,
BALATON Zoltan

> --
> hw/ppc/spapr_drc.c=511=static void realize(DeviceState *d, Error **errp)
> hw/ppc/spapr_drc.c:530:    object_property_add_alias(root_container, link_name,
> hw/riscv/sifive_e.c=186=static void sifive_e_soc_realize(DeviceState *dev, 
> Error **errp)
> hw/riscv/sifive_e.c:233:    qdev_pass_gpios(DEVICE(&s->gpio), dev, NULL);
> hw/riscv/sifive_u.c=651=static void sifive_u_soc_realize(DeviceState *dev, 
> Error **errp)
> hw/riscv/sifive_u.c:743:    qdev_pass_gpios(DEVICE(&s->gpio), dev, NULL);