Philippe Mathieu-Daudé <phi...@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!
