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

[Alexander Graf <agraf@suse.de>]

On 27.06.14 13:17, Andreas Färber wrote:
> Am 27.06.2014 12:54, schrieb Peter Crosthwaite:
>> On Fri, Jun 27, 2014 at 8:30 PM, Andreas Färber <afaerber@suse.de> wrote:
>>> Am 26.06.2014 14:01, schrieb Alexander Graf:
>>>> 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.
>>> As discussed offline with Alex, one issue I see is that this would be
>>> encouraging people to add more devices to an artificial global bus in
>>> /machine/unassigned that we've been trying to obsolete, rather than
>>> sitting down and please creating an e500 SoC object as a start. Maybe we
>>> should start generating a list of shame for 2.1. ;)
>>> Instantiating a new [Sys/AXI/AMBA/...]Bus inside that SoC object would
>>> make me much happier than using SysBus as is.
>>>
>> Do you mean &address_space_memory (as used by sysbus_mmio_map)?
> No, I mean the QOM composition model. When we think of using -device,
> then they will go to /machine/peripheral/<id> or
> /machine/peripheral-anon/device[n]; in your case that means that you get
> a flat list of devices rather than a structure matching your device
> tree. And like I said above, in both your and Alex' case SysBus is
> something that has no real place in the composition tree unless we go
> from that single unholy qdev-required bus to buses as they exist in the
> hardware, like Anthony suggested long time ago. Alex' problem with that
> is that he doesn't want to implement the same UART logic for 50
> different-but-same buses, so some form of reuse or inheritance would be
> needed.
>
> Disclaimer: I have not yet reviewed this series, I was commenting on
> abstract ideas that Alex requested feedback for.

I think we can all agree that the sysbus bus is not a bus per se. So 
conceptually, what's the difference between a device attached to a 
non-bus and a device not attached to a bus at all? And why can't we 
convert sysbus to not be a bus anymore?


Alex