Re: sysbus_create_simple Vs qdev_create

[Pratik Parvati]

Hi Markus and Philippe,

Thanks for your reply. Now I am pretty clear about Qdev and sysbus helper function.

Can you please explain to me in brief on buses and device hierarchies (i.e. BusState and DeviceState) and how they are related to each other? As I can see, the DeviceState class inherits the BusState

struct DeviceState {
    /*< private >*/
    Object parent_obj;
    /*< public >*/

    const char *id;
    char *canonical_path;
    bool realized;
    bool pending_deleted_event;
    QemuOpts *opts;
    int hotplugged;
    bool allow_unplug_during_migration;
    BusState *parent_bus; \\ BusState is inherited here
    QLIST_HEAD(, NamedGPIOList) gpios;
    QLIST_HEAD(, BusState) child_bus;
    int num_child_bus;
    int instance_id_alias;
    int alias_required_for_version;
    ResettableState reset;
};

and BusState, in turn, inherits the DeviceState as

/**
 * BusState:
 * @hotplug_handler: link to a hotplug handler associated with bus.
 * @reset: ResettableState for the bus; handled by Resettable interface.
 */
struct BusState {
    Object obj;
    DeviceState *parent; \\ DeviceState is inherited here
    char *name;
    HotplugHandler *hotplug_handler;
    int max_index;
    bool realized;
    int num_children;
    QTAILQ_HEAD(, BusChild) children;
    QLIST_ENTRY(BusState) sibling;
    ResettableState reset;
};

I am a bit confused. Can you brief me this relation!

Thanks and Regards,
Pratik

On Wed, Jul 15, 2020 at 2:02 PM Markus Armbruster <armbru@redhat.com> wrote:

Philippe Mathieu-Daudé <philmd@redhat.com> writes:

> Hi Pratik,
>
> On 7/14/20 6:17 PM, Pratik Parvati wrote:
>> Here is a brief context that might help you.
>> I am referring hw/arm/versatilepb.c
>>
>> The ARM PrimeCell UART (PL011) device created as follows
>>
>>     dev = qdev_create(NULL, "pl011");
>>     s = SYS_BUS_DEVICE(dev);
>>     qdev_prop_set_chr(dev, "chardev", chr);
>>     qdev_init_nofail(dev);
>>     sysbus_mmio_map(s, 0, addr);
>>     sysbus_connect_irq(s, 0, irq);

This is pl011_create().

Since recent merge commit 6675a653d2e, it's

       dev = qdev_new("pl011");
       s = SYS_BUS_DEVICE(dev);
       qdev_prop_set_chr(dev, "chardev", chr);
       sysbus_realize_and_unref(s, &error_fatal);
       sysbus_mmio_map(s, 0, addr);
       sysbus_connect_irq(s, 0, irq);

>>
>> Whereas the PL031 RTC device is created as
>>
>>     /* Add PL031 Real Time Clock. */
>>     sysbus_create_simple("pl031", 0x101e8000, pic[10]);
>>
>> What is the difference between these two devices creation?
>
> Both devices inherit SysBusDevice, which itself inherits QDev.

Yes: TYPE_SYS_BUS_DEVICE is a subtype of TYPE_DEVICE.

> You can create QDev objects with the qdev API, and
> SysBusDevice objects with the sysbus API.

Yes.

qdev_new(), qdev_realize_and_unref(), ... work with DeviceState * (the C
type of an instance of QOM TYPE_DEVICE).

sysbus_realize_and_unref(), ... work with SysBusDevice * (the C type of
an instance of QOM TYPE_SYS_BUS_DEVICE).

Since TYPE_SYS_BUS_DEVICE is a subtype of TYPE_DEVICE, you can safely
use qdev_ functions with sysbus devices.  Example: pl011_create() uses
qdev_new() to create a sysbus device.  That's fine.

> sysbus_create_simple() is a condensed helper, but only allow you
> to pass qemu_irq objects, not a 'chardev' property. So for this
> case you have to use the qdev API instead.

Yes.  It's a helper that combines creating a sysbus device, wiring up
one MMIO region and one IRQ, and realizing.  If you need to configure or
wire up more than that, you can't use it.

>> How do I know
>> which method to use while creating an object?
>
> SysBusDevice are plugged onto a bus. QDev aren't.
> The sysbus API results in smaller code, easier to review.

The general pattern for a stand-alone device is

    dev = qdev_new(type_name);
    set properties and wire up stuff...
    qdev_realize_and_unref(dev, bus, &err);

When this is to be done in device code, say to create a component
device, the split between .instance_init() and .realize() complicates
things.  If interested, ask and I'll explain.

There are quite a few wrappers around qdev_ functions for various
subtypes of TYPE_DEVICE.  Use them to make your code more concise and
easier to understand.  Example: sysbus_realize_and_unref().

There are also convenience functions that capture special cases of the
entire general pattern.  Example: sysbus_create_simple().

Hope this helps!