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Re: [Qemu-discuss] Guidance requested creating a QEMU MMIO device

From: trasmussen
Subject: Re: [Qemu-discuss] Guidance requested creating a QEMU MMIO device
Date: Tue, 30 Apr 2019 16:09:01 +0200

I made some good progress and has been able to create the code for my 
MMIO-device borrowing from MPS2, compile and link it.
When I start qemu with gdb to debug MPS2, I get to mps2_machine_init(), 
then to mps2_scc_register_types(), but not to mps2_scc_init() nor to 

I know that the cpu is cortex-a53 and it may not have any UARTs at all.
I was hoping there was a way to introduce a new device on any cpu that is 
not 'born' with it from the manufacturer, like I attempt to introduce the 
MPS2 on the cortex-a53.

I get to introduce it, but no instance is ever made of it, so it could 
become used.

In reality I 'just' need to create a device which is at some  physical 
memory address (e.g. 0x09070000) and of size 0x1000, and I want to catch 
reads and writes to this physical address range in my device and respond 
there. My new device is unknown to a real cortex-a53 board.

Is there a way to add such a device, provided that the chosen physical 
address range is free?
Since I may not have created my MMIO-device correctly, I had hoped I could 
use MPS2 as debugging subject, so I could verify what is done in 

I hope it is possible to augment a qemu-cpu with phony devices in some 
I fear that the device-tree is built-in and unique to each cpu and cannot 
be modified in a simple way.


        Thorkil B. Rasmussen

From:   "Peter Maydell" <address@hidden>
To:     address@hidden
Cc:     "Qemu-discuss" 
<address@hidden>, "qemu-discuss" 
Date:   16-04-2019 15:37
Subject:        Re: [Qemu-discuss] Guidance requested creating a QEMU MMIO 
Sent by:        "Qemu-discuss" 

On Tue, 16 Apr 2019 at 14:18, <address@hidden> wrote:
> Thanks, Peter.
> I can see how to use parts of the mps2-scc code to set up my wanted 
> I would basically need an array of 32-bit words and would not need to 
> between control and data registers. That's what I hope at least.
> You wrote;
> > You would then need to modify the board code for whatever
> > machine you're trying to use this with to create the device
> > at whatever physical address you want it to live at.
> You didn't specify what you meant by 'board code'. Since a qemu is 
supposed to emulate a
> specific processor/cpu, e.g. an arm cortex-a53 or a ppc mpc8544ds.

The board code is the code that models a particular board or
machine (which you specify on the command line with -M machinename).
It's under hw/${ARCH}/, usually, eg the versatilepb and versatileab
boards are handled by hw/arm/versatilepb.c.

For mps2-scc, you can see where the device is created by
looking for TYPE_MPS2_SCC -- it's used in the boards
modelled in hw/arm/mps2-tz.c and hw/arm/mps2.c.

All that code in roms/ that you found is just the source
code for a copy of u-boot which we ship as a ROM blob
to run on certain guest boards -- it is irrelevant to you.

-- PMM

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