Re: [PATCH v9 3/4] dtc: Plugin and fixup support

[Phil Elwell <phil-FnsA7b+Nu9XbIbC87yuRow@public.gmane.org>]

On 24/11/2016 13:58, Pantelis Antoniou wrote:
> Hi Phil,
>
>> On Nov 24, 2016, at 15:49 , Phil Elwell <phil-FnsA7b+Nu9XbIbC87yuRow@public.gmane.org> wrote:
>>
>> On 24/11/2016 12:31, Pantelis Antoniou wrote:
>>> This patch enable the generation of symbols & local fixup information
>>> for trees compiled with the -@ (--symbols) option.
>>>
>>> Using this patch labels in the tree and their users emit information
>>> in __symbols__ and __local_fixups__ nodes.
>>>
>>> The __fixups__ node make possible the dynamic resolution of phandle
>>> references which are present in the plugin tree but lie in the
>>> tree that are applying the overlay against.
>>>
>>> While there is a new magic number for dynamic device tree/overlays blobs
>>> it is by default disabled. This is in order to give time for DT blob
>>> methods to be updated.
>>>
>>> Signed-off-by: Pantelis Antoniou <pantelis.antoniou-OWPKS81ov/FWk0Htik3J/w@public.gmane.org>
>>> Signed-off-by: Sascha Hauer <s.hauer-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>
>>> Signed-off-by: Jan Luebbe <jlu-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>
>> It's great to see this work about to come to fruition, but I have a
>> reservation about this patch. Like previous versions, this
>> implementation generates __fixups__, __local_fixups__  and __symbols__
>> whenever the "-@" command line parameter is given; without it you get
>> none. In my opinion, this logic causes the DTBs to be unnecessarily
>> large with no obvious benefit.
>>
>> You can divide the compiled outputs from DTC into two categories - fully
>> resolved DTBs (what I would call base DTBs), and overlays. Base DTBs
>> should include __symbols__ to allow overlays to be applied, and it is
>> right that this should be controlled by the "-@" flag, but since base
>> DTBs are fully resolved I think there is no reason to include either
>> __fixups__ or __local_fixups__. Therefore I think both kinds of fixups
>> should only be omitted when the "/plugin/ " directive is used. This was
>> the purpose of one of the patches I provided you with.
>>
> Yes, I’m quite aware of that. There is a reason for generating every
> resolve node for the base tree. It contains information about the
> dependencies of every hardware device due to phandle references.
> We can utilize that to re-order the device probe order to eliminate
> -EPROBE_DEFER cycles upon boot.
>
> It is important we get the core support in and then you can add extra
> switches for every special platform case.
>
> What kind of problems do you have with larger device tree blobs?
> I do carry a hashed phandle patch on my mainline tracking tree which
> should help with larger DTBs. 
Early Raspberry Pi DT support used to load the DTBs to 0x100, Although
the ARMv6/7 kernel starts at 0x8000, something (the decompressor?) would
use the space between 0x4000 and the 0x8000, which gave us a practical
limit of 16KB on the DTB size. This used to be sufficient for a base DTB
and a few small overlays, but with the patch all Pi DTBs are over 16KB.
The practical limit was overcome a long time ago, but it made me aware
of the DTB contents, and there are may be some situations where it is
desirable to reduce the footprint as much as possible.

I would have thought that all DTBs already contain enough dependency
information in the form of the phandles themselves. One of the first
things the kernel does is to unflatten the DTB, and that is the obvious
point to resolve the phandles and generate the necessary dependencies.
Can you explain how both __fixups__ and __local_fixups__ aid this
process? Ideally they wouldn't duplicate any information already in the
tree, since then you have to cope with the possibility of malformed DTBs
where the two don't actually match.

Phil