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Re: [Qemu-ppc] [PATCH 0/9] Add platform bus

From: Peter Maydell
Subject: Re: [Qemu-ppc] [PATCH 0/9] Add platform bus
Date: Tue, 23 Jul 2013 13:22:43 +0100

On 23 July 2013 13:19, Paolo Bonzini <address@hidden> wrote:
> Il 22/07/2013 20:21, Peter Maydell ha scritto:
>>> > 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?
>> Because you can't as a user of this sort of hardware
>> plug in an extra serial port to a SoC, because there's just nowhere
>> to plug it in. So why should it be possible to plug an extra
>> serial port into the QEMU model of the SoC?
> And why exactly should QEMU be limited to modeling an existing SoC?
> Perhaps the user is not working with an existing SoC.  They are working
> with with IP building blocks that they can combine the way they prefer,
> and they haven't yet made up their mind on the exact set of devices
> they'll have.  (because not all the world is a PC, but then not all the
> non-PC world is ARM either).

This sounds like (a) a good thing (b) something that will
turn into an incredible incomprehensible mess if we try
to specify it on the command line. Why would we want to do that?

That is, you're arguing for a scripting/config language for
putting together board models so you don't have to write them
in C. That's a good thing, but not what this patch series is doing.

> Perhaps the user is working on kernel support for device tree / ACPI,
> wants to test many device combinations, and does not want to touch C
> code in order to do that.
> Perhaps the user can plug daughterboards that connect to the SoC and add
> an extra serial port, visible as yet another MMIO device.

Pluggable daughterboards should be implemented by actually
defining the bus/socket that exists between the mainboard
and the daughterboard, so you could say -device my-daughterboard
and have it plug in to the mainboard.

-- PMM

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