Re: a quetion about sysbus_realize_and_unref function

[Philippe Mathieu-Daudé]

Hi,

On 10/18/21 14:01, Chan Kim wrote:
> Hi, list members,
> 
> I see for example in hw/arm/virt.c, when it creates a device to attach
> to a system bus, it calls “sysbus_realize_and_unref(SYS_BUS_DEVICE(dev),
> &error_fatal);” function
> 
> .
> 
> static void create_its(VirtMachineState *vms)
> 
> {
> 
>     const char *itsclass = its_class_name();
> 
>     DeviceState *dev;
> 
>  
> 
>     if (!itsclass) {
> 
>         /* Do nothing if not supported */
> 
>         return;
> 
>     }
> 
>  
> 
>     dev = qdev_new(itsclass);
> 
>  
> 
>     object_property_set_link(OBJECT(dev), "parent-gicv3", OBJECT(vms->gic),
> 
>                              &error_abort);
> 
>     sysbus_realize_and_unref(SYS_BUS_DEVICE(dev), &error_fatal);
> 
>     sysbus_mmio_map(SYS_BUS_DEVICE(dev), 0, vms->memmap[VIRT_GIC_ITS].base);
> 
>  
> 
>     fdt_add_its_gic_node(vms);
> 
>     vms->msi_controller = VIRT_MSI_CTRL_ITS;
> 
> }
> 
>  
> 
> The document says it decrements the reference counter and when the ref
> count is non zero, the object can’t be removed.
> 
> Then why does it call sysbus_realize_and_unref function to reduce the
> ref count of the system bus(=parent object) of the device being create?

The qemu-devel@nongnu.org is better for developer questions.

Cc'ing Markus for:

commit 496a8525622d4ac5d276f76840dd30eddb73672d
Author: Markus Armbruster <armbru@redhat.com>
Date:   Wed Jun 10 07:32:33 2020 +0200

    sysbus: New sysbus_realize(), sysbus_realize_and_unref()

    Sysbus devices almost always plug into the main system bus.
    qdev_create() even has a convenience feature to make that easy:
    a null bus argument gets replaced by the main system bus.
    qdev_realize() and qdev_realize_and_unref() do the same.

    We can do better.  Provide convenience wrappers around
    qdev_realize() and qdev_realize_and_unref() that don't take a
    @bus argument.  They always pass the main system bus.