Re: [Qemu-devel] [RFC] Machine description as data

[David Gibson <dwg@au1.ibm.com>]

On Thu, Feb 12, 2009 at 09:33:59AM -1000, Mitch Bradley wrote:
>>
>> We want a machine configuration: a tree describing configurable devices
>> and their configurable properties.
>
> Regarding configurable devices in Open Firmware:
>
> The baseline usage model for configurable devices was that the firmware  
> is responsible for establishing a consistent system configuration,  
> possibly based on user-modifiable variables in non-volatile storage.  It  
> reports the actual configuration to the OS via the device tree.
>
> For cases where the choice needs to deferred until later, or perhaps  
> changed dynamically, a device tree property reports the set of  
> possibilities.  In cases where the firmware has already set up the  
> devices, it reports the current choice via another property.
>
> The device tree hierarchy serves as the "name space" framework for these  
> properties.  Obviously, you need to specify the device for which the  
> choice set applies.  The device tree is a coherent naming model for that  
> purpose.
>
> Obviously the hierarchical model has problems for highly-configurable  
> chipsets in which a setting can result in a wholesale rearrangement of  
> the overall connectivity, but it seems to me that board-design  
> constraints usually make that a non-problem. The wiring on a given board  
> generally forces the choice at that level, so the firmware for that  
> board need not report that as a configurable choice.
>> For PowerPC, we also want a machine description: a tree describing those
>> devices and properties that the kernel can't easily and safely probe.
>
> The gist of the above sentence seems to presuppose that, if the kernel  
> can probe, it should.  That's not the only way of thinking about the  
> problem.  As a practical matter, the firmware usually needs to do a fair  
> amount of probing too, in order to locate the console display and the  
> boot devices.  In the process, the firmware usually discovers pretty  
> much the entire machine configuration.  If the OS has to repeat the  
> process from scratch, it slows down the boot process.  So the IEEE1275  
> design supports the model where the firmware can do all the probing,  
> handing off a complete system description to the OS.  The OS startup  
> code can walk the tree and attach device drivers for what it finds, then  
> arrange to handle insert/remove events from hot-pluggable buses.

In the context of a full IEEE1275 implementation that makes sense.
However, in the context of flat trees - which were designed with
embedded machines in mind particularly - cutting down the device tree
to only things which the kernel can't probe is normal practice.  In
this context firmware is often really minimal and doesn't actually
need to probe much.  Basic IO devices like console are often on chip
and easily accessible so in particular the firmware has no need to
probe complex bus structures like PCI.

But more importantly, this is a design decision driven by the
prevalence of buggy-as-hell firmwares.  There are many instances where
to work around broken firmware probing, the kernel has to do almost as
much work as probing from scratch (and the code to do it is generally
more confusing).  And while it might be nice to imagine a world with
good firmware supporting standard interfaces, I really can't see it
ever happening.

So, we've taken the approach of moving as much as possible of the
probing and device-discovery logic into the kernel which is usually a
more easily replacable / fixable component of the system.  The less
the firmware has to put into the tree, the less it can get wrong.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson