Re: [RFC] [PATCH] Device Tree on ARM platform

From: Scott Wood
Date: Wed May 27 2009 - 12:26:48 EST

Peter Korsgaard wrote:
"Robert" == Robert Schwebel <r.schwebel@xxxxxxxxxxxxxx> writes:


Robert> - The whole concept is based on the assumption that bindings
Robert> are defined *once*, then never to be changed again. As this
Robert> is not true (check MPC5200 to find out what I mean), oftree
Robert> wreckage is *the* main cause of new kernels not working on
Robert> old bootloaders any more. Is there a solution of this
Robert> problem? I have not seen a good idea how to avoid the
Robert> constant change in definitions.

Just bundle the .dtb with the kernel and they'll always be in sync. I
know this isn't really in the spirit of OF, but currently imho the
only realistic solution.

That removes the ability to use the device tree to pass information from the bootloader, such as MAC addresses and clock frequencies. On the u-boot list, you'll find people trying such hacks (which were rightly NACKed) as passing the information in the device's volatile registers (which the Linux driver must then not reset) to deal with ARM Linux's lack of this ability.

Robert> - The oftree layering is fundamentally broken. We already
Robert> have a sane abstraction for arbitrary hardware in the kernel:
Robert> platform devices. Why not instanciate platform devices from
Robert> a generic oftree core?

You can, if you want. But you'll need extra glue code that understands the individual bindings. IMHO that logic is usually better off in the driver itself, but if you really need platform code to involve itself in some way (such as providing callbacks), then exceptions can be made.


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