Re: [PATCH] Open Firmware device tree virtual filesystem

David Miller
2006-12-31T09:52:20Z

From: Mitch Bradley <wmb@firmworks.com>
Date: Sat, 30 Dec 2006 23:36:45 -1000

> The base interface function is callofw(), which is effectively identical
> to call_prom_ret() in arch/powerpc/kernel/prom_init.c . So it seems
> that PowerPC could use it. I suppose I could change the name of
> callofw() to call_prom_ret(), thus making the base interface identical
> to PowerPC's. All it does is argument marshalling, translating between
> C varargs argument lists and the OFW argarray format.

Please create explicit function calls for each operation, this
way the caller is immune to open-firmware implementation details.

> SPARC should be able to use that same base interface function directly.
> It is written to the standard OFW client interface.

Sparc 32-bit predates the OFW specification and does things differently.

Please use a functional interface using a C function for each device
tree operation, then the implementation behind it doesn't matter.

> It wouldn't work on ancient Sun machines with the sunmon romvec
> interface, but Sun stopped making such machines something like 16 years ago.

Yet we still support them in the 32-bit sparc port. And it's so
easy to support this properly, please use the clean stuff we've
created for this.

> I did consider first creating a memory data structure identical to the
> powerpc/sparc one, but that looked like it was going to be essentially
> twice as much code for no extra capability. The code to traverse the
> device tree and create the memory data structure is roughly the same as
> the code to create the filesystem structure. I just didn't see the
> value of an intermediate representation for systems that don't otherwise
> need it.

Since we put it in memory, we have zero reason to call into the
firmware for device tree access and this simplifies things a lot.

But all of that really doesn't matter, if you use a functional
C interface for each device tree access operation, it doesn't
matter what's behind it right?