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

From: Alexander Graf
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH 0/5] Platform device support
Date: Fri, 27 Jun 2014 13:24:48 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

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 <address@hidden> 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 <address@hidden> 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
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

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?


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