[OpenBIOS] Beginners How-To for DTS and OpenFirmware?

[BALATON Zoltan]

On Mon, 21 Nov 2016, G 3 wrote:
> On Nov 21, 2016, at 9:34 AM, Michael wrote:
>> But is there a "beginners how-to" somewhere? What I would find most 
>> usefull:

For forth commands I've found this page useful (not sure if this is 
linked from somewhere):
http://www.firmworks.com/QuickRef.html

(Not everything is implemented in OpenBIOS but mostly should work.)

>> 1) Best source-code to use? There is a version-1.1.zip, a svn repo and
>> a git repo.
>
> The git repo is what we are using now. I would chose that.
>
>> 2) The names are also a bit confusing when you see it first. I know about
>> the IEEE standard and a few things about the history (from wikipedia),
>> but if you for example visit the website and click on the "openfirmware"
>> link, the you see a page with "OpenBIOS" heading. So even if one knows
>> about history, there is confusion about actual used names.
>
> We use OpenBIOS as the project's name. Open Firmware is a standard OpenBIOS 
> follows.

This is a bit confusing. There are multiple implementations of the IEEE 
standard (commonly referred to as Open Firmware) and one of these is also 
called Open Firmware. These are listed under Implementations on the left 
of the page at www.openfirmware.info. Another implementation used by QEMU 
for PPC Macintosh models (and thus being active lately and most discussed 
on this list) is OpenBIOS. It may be useful to make it clear what you mean 
when talking about Open Firmware: the standard or the implementation with 
the same name.

Actually running OpenBIOS on real hardware is not something that is well 
tested (I'm not sure if it was ever tried but I think it wasn't done 
recently) so if you try to do that be prepared for likely needing some 
fixing. This is true for any implementation that does not support the 
hardware you want to run it on, then you likely need to port it.

>> 3) How is the device-tree given to OF? And how does OF handle it over
>> to a linux kernel?
>
> I don't think Linux touches the OpenBIOS device tree. It likes to discover 
> things for itself.

It discovers most things it can but still starts from the device tree but 
it's enough to have the basics in there. Not sure how much up to date is 
this but this is discussed in the kernel documentation here: 
https://www.kernel.org/doc/Documentation/devicetree/booting-without-of.txt

As for giving the device tree to OpenBIOS I think it should just know it 
or construct it from discovering the hardware as the point of the device 
tree is to describe the hardware for the operating system so the firmware 
should know the hardware and provide the device tree. It does not get it 
from anywhere as this it the first link that should have this knowledge. 
There are some more info about this here for example:
http://www.informit.com/articles/article.aspx?p=1647051&seqNum=5

>> 4) Minimal prerequisites on a new hardware, so that OF is able to run?
>> E.g. which type of RAM does it need? Does it need a stack or .text segment
>> or other things to run (like c-code)? ...
>
> Being a BIOS, I would think about anything could run OpenBIOS. I think any
> computer made in the last 20 years should work.

I don't know how do you get that idea. Being a BIOS (or more correctly 
firmware) means that it is the lowest level and first program run on a 
computer that has to have detailed knowledge on how to initialise and 
operate hardware components so it very much depends on having low level 
drivers and code for the parts of a computer which is very much dependent 
on the specific harware. It can't just run on any computer for which it 
has no drivers.

In fact OpenBIOS does not have that many drivers but maybe the basics are 
there, only the platform specific init code might need to be adapted for a 
new board. For ppc these would be in arch/ppc. Running on QEMU is quite a 
bit simpler as we can skip a lot of init code (such as memory controller 
or other hardware) so maybe these are not well implemented or buggy as 
they were not tested with real hardware for a while.

I don't know anything about implementations other than OpenBIOS though so 
not sure if they work better or worse but if it's a harware not already 
supported, porting it would be similar.

Regards,
BALATON Zoltan