Of the device tree binary format endianness on little-endian platform

[David Gibson]

On Wed, Feb 25, 2009 at 05:16:49PM +0100, Laurent Gregoire wrote:
> Hi Timur,
> 
> On Tue, 2009-02-24 at 15:26 -0600, Timur Tabi wrote:
> > Laurent,
> > 
> > I'm very interested in the work you are planning on doing.  How are
> > you planning on integrating device-tree support into the kernel?  That
> > is, are you going to be adding the code so that's local to your ARM
> > platforms, or are you looking to make the device tree API available
> > for all ARM?
> > 
> 
> We would like to use the device-tree for three purposes:
> 
> 1. Device-specific parameters like device ID, bluetooth mac address...
> 2. Platform-device activation and parametrization
> 3. GPIO pins configuration
> 
> We're in research phase, but the current plan is to focus first on point
> 1 and 2, point 2 being only used for *some* of the platform-devices. We
> will use the code only in our platform-specific code for now, ie. our
> specific machine_init() routine.
> 
> We plan also to stick to the use of the flattened device-tree format,
> not using or building any expanded version, and once platform-device
> initialization is done to forget about the device-tree data.
> 
> We ported some part of the fdt parsing routine from the ppc arch branch,
> but we modified the parsing library interface to use external iterators
> instead of callbacks. Once validated internally, this library could be
> easily moved if needed to some more generic place like
> arch/arm/kernel.

If you're working exclusively with the flattened tree, then please use
libfdt as your parsing library.  It's not used in the powerpc kernel
proper, because that's based on an expanded tree, but it is used in
the powerpc bootwrapper, see arch/powerpc/boot/libfdt.  That's just a
copy of the upstream version that's part of the dtc tree.  If ARM
starts using this as well we can move it to lib/ or some other
non-arch-specific location.

If you need to extend libfdt, that's fine - but talk to me about
getting your extensions upstream.  The whole idea is that it gets
extended to be the flat tree parsing library of choice.

> The loading of the platform-device is done using a callback function,
> setup via an arch_init function, with a keyword argument. This keyword
> is used as a key to a specific node in the device-tree. If the keyword
> is present in the device-tree, the callback is called, with the
> responsibility of setting-up specific parameters (through the use of the
> fdt library) and registering any needed platform-device. Thus we do not
> enforce any one-to-one mapping between device-tree node and
> platform-devices, even if it will be probably most of the time the case.

I don't entirely follow what you're doing here, but it sounds like it
might not be quite correct.  The device tree should always (well,
nearly always) describe the hardware, *not* Linux or driver specific
information.  Generally drivers will use the "compatible" property to
determine if a device tree node is relevant to them.

> The platform-device initialization order is given by device-tree order.

This is not usually a good idea.  Flattened device trees do have an
order, but for real OF trees, the tree order is not well defined, so
it's generally best not to use the order to control initialization,
but instead to use explicit content to control order, if it's
important.

> Also we noticed the lack of type information in device-tree binary
> format (could be needed for exporting values to user-land through
> something like sysfs, to have human-readable integer values in scripts -
> also big-endian integers have to be converted).

Yes, the device tree structure treats property values as "bag of
bytes".  It's assumed that you need to know the binding for a given
node/property - which describes the format of the content - in order
to correctly interpret it.

-- 
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_!
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