Re: [PATCH 51/55] qdev: Make qdev_realize() support bus-less devices

[Markus Armbruster]

Paolo Bonzini <address@hidden> writes:

> On 19/05/20 16:55, Markus Armbruster wrote:
>> So far, qdev_realize() supports only devices that plug into a bus:
>> argument @bus cannot be null.  Extend it to support bus-less devices,
>> too.
>> 
>> qdev_realize_and_unref() remains restricted, because its reference
>> counting would become rather confusing for bus-less devices.
>
> I think it would be fine, you would just rely on the reference held by
> the QOM parent (via the child property).

I took one look at the contract I wrote for it, and balked :)

qdev_realize()'s contract before this patch:

    /*
     * Realize @dev.
     * @dev must not be plugged into a bus.
     * Plug @dev into @bus.  This takes a reference to @dev.
     * If @dev has no QOM parent, make one up, taking another reference.
     * On success, return true.
     * On failure, store an error through @errp and return false.
     */
    bool qdev_realize(DeviceState *dev, BusState *bus, Error **errp)

Simple enough.

This patch merely adds "If @bus, " before "plug".  Still simple enough.

qdev_realize_and_unref()'s contract:

    /*
     * Realize @dev and drop a reference.
     * This is like qdev_realize(), except it steals a reference rather
     * than take one to plug @dev into @bus.  On failure, it drops that
     * reference instead.  @bus must not be null.  Intended use:
     *     dev = qdev_new();
     *     [...]
     *     qdev_realize_and_unref(dev, bus, errp);
     * Now @dev can go away without further ado.
     */
    bool qdev_realize_and_unref(DeviceState *dev, BusState *bus, Error **errp)

If @bus is null, who gets to hold the stolen reference?

You seem to suggest the QOM parent.  What if @dev already has a parent?

Oh, sod it, let's go shopping.