Re: [Qemu-devel] [PATCH 0/5] Platform device support

[Alexander Graf <agraf@suse.de>]

On 20.06.14 08:43, Peter Crosthwaite wrote:
> On Wed, Jun 4, 2014 at 10:28 PM, Alexander Graf <agraf@suse.de> wrote:
>> Platforms without ISA and/or PCI have had a seriously hard time in the dynamic
>> device creation world of QEMU. Devices on these were modeled as SysBus devices
>> which can only be instantiated in machine files, not through -device.
>>
>> Why is that so?
>>
>> Well, SysBus is trying to be incredibly generic. It allows you to plug any
>> interrupt sender into any other interrupt receiver. It allows you to map
>> a device's memory regions into any other random memory region. All of that
>> only works from C code or via really complicated command line arguments under
>> discussion upstream right now.
>>
> What you are doing seem to me to be an extension of SysBus - you are
> defining the same interfaces as sysbus but also adding some machine
> specifics wiring info. I think it's a candidate for QOM inheritance to
> avoid having to dup all the sysbus device models for both regular
> sysbus and platform bus. I think your functionality should be added as
> one of
>
> 1: and interface that can be added to sysbus devices
> 2: a new abstraction that inherits from SYS_BUS_DEVICE
> 3: just new features to the sysbus core.
>
> Then both of us are using the same suite of device models and the
> differences between our approaches are limited to machine level
> instantiation method. My gut says #2 is the cleanest.

The more I think about it the more I believe #3 would be the cleanest. 
The only thing my platform devices do in addition to sysbus devices is 
that it exposes qdev properties to give mapping code hints where a 
device wants to be mapped.

If we just add qdev properties for all the possible hints in generic 
sysbus core code, we should be able to automatically convert all devices 
into dynamically allocatable devices. Whether they actually do get 
mapped and the generation of device tree chunks still stays in the the 
machine file's court.


Alex