Re: [Qemu-devel] modelling omap_gpmc with the hierarchical memory API

[Peter Maydell]

On 2 August 2011 20:11, Avi Kivity <address@hidden> wrote:
> On 08/02/2011 09:21 PM, Peter Maydell wrote:
>>
>> On 2 August 2011 19:05, Avi Kivity<address@hidden>  wrote:
>> >  On 08/02/2011 08:21 PM, Peter Maydell wrote:
>> >>  So I think we just need a sysbus_mmio_get_memoryregion()
>> >>  (and convert the devices I need to attach to use memory
>> >>  regions, and live with not being able to attach unconverted
>> >>  devices).
>> >
>> >  I don't follow - why do we need get_memoryregion? who would call it?
>>
>> The machine model would call it. So you do something like
>>  DeviceState *dev = qdev_create(NULL, "whatever");
>>  /* Note the parallel here to the existing
>>   *   sysbus_mmio_map(sysbus_from_qdev(dev), mmio_idx, addr);
>>   */
>>  MemoryRegion *mr =
>> sysbus_mmio_get_memoryregion(sysbus_from_qdev(dev), mmio_idx);
>>  omap_gpmc_attach(gpmc, 7, mr);
>
> This is where the gpmc provides the sysbus.  It doesn't need to call
> get_memoryregion() on itself.

Why should the gpmc provide a sysbus? It doesn't need it,
all we need to pass it is a MemoryRegion *. A bus would
imply multiple different things that could all sit on it
at different addresses, whereas if gpmc provided 8 different
sysbuses they'd each have either 0 or 1 child always at
address 0.

>> ie the machine model is where we wire up the subdevices
>> to the gpmc, and at the machine model level what you have is
>> a pointer to an entire device, so you need to be able to
>> convert the (sysbus*, mmio_index) tuple to a MemoryRegion*.
>
> I believe that it is in general unnecessary.  A device hands its bus a
> memory region, and the bus does with it what it will (generally mapping it
> into a container, and presenting the container to a parent bus).
>  get_memoryregion() implies a third party.

The third party here is the machine model. The machine model
owns and instantiates both gpmc and the subdevice. It wants
to wire them up. In the same way that you can use qdev_get_gpio_in
and qdev_connect_gpio_out to connect a gpio line from one thing
to another, you need to be able to connect a memory region
from one thing to another.

-- PMM