[PATCH] Open Firmware device tree virtual filesystem

[Mitch Bradley]

David Miller wrote:
> ...
> Can we please not have N different interfaces to the open-firmware
> calls so that perhaps powerpc and Sparc have a chance of using this
> code too?
>   
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.

SPARC should be able to use that same base interface function directly.  
It is written to the standard OFW client interface.  The x86 client 
interface that I tested it on is essentially the same code that is in 
OBP.  It wouldn't work on ancient Sun machines with the sunmon romvec 
interface, but Sun stopped making such machines something like 16 years ago.
> On sparc and powerpc, we even build an in-kernel data structure of the
> entire open-firmware device tree that code like your's could use if
> you make a simple setup layer for i386 as well.  We have interfaces for
> modifying property values at run time too.
>
> I would strongly suggest looking at things like
> arch/{sparc,sparc64,powerpc}/kernel/prom.c and
> include/asm-{sparc,sparc64,powerpc}/prom.h and
> arch/{sparc,sparc64,powerpc}/kernel/of_device.c and
> include/asm-{sparc,sparc64,powerpc}/of_device.h
> since we've already invested a lot of thought and
> infrastructure into providing interfaces to this information
> on powerpc and the two sparc platforms.
>
>
>   
I did look at those files, until my eyes glazed over.  In powerpc land, 
the files that are the underlayer for proc_devtree.c comprise 4700 lines 
of code (the files you list plus prom_init.c).  In sparc land, it is 
only 3200 lines (the files you list plus the prom interface library).  
On top of that, proc_devtree.c is 233 lines.

In contrast, ofw_fs.c is 261 lines, and the base interface function 
callofw() is 97 lines (half of them comments).

Admittedly, this is something of an apples-to-oranges comparison, 
because ofw_fs only exports a read-only device tree and nothing else.  
But in the case where that is all you need, a direct interface to OFW 
that avoids the middleman seems like a good choice.

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.  (A setup layer would have let me use proc_devtree.c directly, 
so the total amount of new code would have been the same, but many 
people told me that if I even suggested using procfs the kernel gurus 
would blow me out of the water without bothering to blink.)

In the SPARC and PowerPC spaces, Open Firmware is widespread, so it 
makes sense for those kernels to use OFW extensively.  In x86 land, OFW 
is far from being the dominant firmware, so the x86 kernel is unlikely 
to depend on OFW services at a deep level.  That being the case, the 
deep-integration features of the sparc and powerpc OFW interfaces are 
not needed in x86 land.   But a lightweight interface to the device tree 
is certainly useful for the platforms that do have OFW.  It might be 
useful for other processors as well, especially on platforms that don't 
need the deep configurability that drove the OFW design.