Re: [PATCH kernel] prom_init: Fetch flatten device tree from the system firmware

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 03/06/2019 09:23, Segher Boessenkool wrote:
> Hi!
> 
> On Fri, May 31, 2019 at 11:03:26AM +1000, Benjamin Herrenschmidt wrote:
>> On Thu, 2019-05-30 at 14:37 -0500, Segher Boessenkool wrote:
>>> On Thu, May 30, 2019 at 05:09:06PM +1000, Alexey Kardashevskiy wrote:
>>>> so, it is sort-of nack from David and sort-of ack from Segher, what
>>>> happens now?
>>>
>>> Maybe what we really need just a CI call to get all properties of a node
>>> at once?  Will that speed up things enough?
>>>
>>> That way you need no change at all in lifetime of properties and how they
>>> are used, etc.; just a client getting the properties is a lot faster.
>>
>> Hrm... if we're going to create a new interface, let's go for what we
>> need.
>>
>> What we need is the FDT. It's a rather ubiquitous thing these days, it
>> makes sense to have a way to fetch an FDT directly from FW.
> 
> That is all you need if you do not want to use OF at all.

? We also need OF drivers to boot grub and the system, and a default
console for early booting, but yes, we do not want to keep using slof
that much.

> If you *do* want to keep having an Open Firmware, what we want or need
> is a faster way to walk huge device trees.

Why? We do not need to _walk_ the tree at all (we _have_ to now and
while walking we do nothing but pack everything together into the fdt
blob) as slof can easily do this already.

>> There is no use for the "fetch all properties" cases other than
>> building an FDT that any of us can think of, and it would create a more
>> complicated interface than just "fetch an FDT".
> 
> It is a simple way to speed up fetching the device tree enormously,
> without needing big changes to either OF or the clients using it -- not
> in the code, but importantly also not conceptually: everything works just
> as before, just a lot faster.

I can safely presume though that this will never ever be used in
practice. And it will be still slower, a tiny bit. And require new code
in both slof and linux.

I can rather see us getting rid of SLOF in the future which in turn will
require the fdt blob pointer in r5 (or whatever it is, forgot) when the
guest starts; so "fetch-all-props" won't be used there either.

>> So I go for the simple one and agree with Alexey's idea.
> 
> When dealing with a whole device tree you have to know about the various
> dynamically generated nodes and props, and handle each appropriately.

The code I am changing fetches the device tree and build an fdt. What is
that special knowledge in this context you are talking about?



-- 
Alexey