Re: [RFC] devicetree: new FDT format version

[Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 01/25/18 04:29, David Gibson wrote:
> On Wed, Jan 24, 2018 at 04:22:15PM -0800, Frank Rowand wrote:
>> On 01/24/18 13:16, Frank Rowand wrote:
>>> On 01/24/18 07:47, Rob Herring wrote:
>>>> On Tue, Jan 23, 2018 at 3:17 PM, Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>>>> On 01/23/18 04:42, David Gibson wrote:
>>>>>> On Mon, Jan 22, 2018 at 03:01:52PM -0600, Rob Herring wrote:
>>>>>>> On Mon, Jan 22, 2018 at 2:09 AM, Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>>>>>>> Hi All,
>>>>>>>>
>>>>>>>> I've tried to create a decent distribution list, but I'm sure I've missed
>>>>>>>> someone or some important list.  Please share this with anyone you think
>>>>>>>> will be affected.
>>>>>>>>
>>>>>>>> I have been playing around with some thoughts for some additions to
>>>>>>>> the devicetree FDT aka blob format.
>>>>>>>>
>>>>>>>> I would like to get the affected parties thinking about how additions to
>>>>>>>> the format could improve whichever pieces of FDT related technology you
>>>>>>>> work on or care about.  In my opinion, the FDT format should change
>>>>>>>> very infrequently because of the impact on so many projects that have
>>>>>>>> to work together to create a final solution, plus the many many users
>>>>>>>> of those projects.
>>>>>>>
>>>>>>> A few things discussed before:
>>>>>>> - Adding type information Even just tagging phandles would be good.
>>>>>>
>>>>>> I'm a bit dubious about this.  It would have to be "hints" only -
>>>>>> there's not really anyway we can put in authoritative type
>>>>>> information, since dtc itself doesn't really know that.  It's also
>>>>>> hard to see how that could be done in a way which wouldn't either a)
>>>>>> require very awkward parallel lookup of the data and type information
>>>>>> or b) not be backwards compatible (even read only).
>>>>
>>>> I never said it was possible. :) I'm just trying to enumerate possible
>>>> FDT format changes because I don't think we want to continuously
>>>> trickle out FDT changes even if they are backwards compatible.
>>>
>>> Yes, I'm trying to capture any pending changes in a single version change.
>>>
>>>
>>>>>>> - Allow applying overlays by just appending to the blob. The need for
>>>>>>> this is somewhat gone now that libfdt can just fully apply overlays.
>>>>>>
>>>>>> I'm not really sure what you want here.  I mean you could easily allow
>>>>>> the format to allow multiple appended overlays, and define that to
>>>>>> mean the overlaid result.  At some point *something* is going to have
>>>>>> to really do the application, so I'm not sure that it really buys you
>>>>>> that much.  It also makes nearly every operation on the tree in libfdt
>>>>>> horrible to implement, at least within the other constraints the
>>>>>> interface was designed around; you'll continually have to scan the
>>>>>> entire tree just in case some other overlay fragment has some bearing
>>>>>> on the thing you're looking at.  It confuses the interface too: what
>>>>>> does "node offset" mean if the same node could be built up from
>>>>>> overlay fragments at multiple offsets.
>>>>
>>>> The idea was to avoid applying overlays to flattened trees at all.
>>>> You're just passing the problem to the next stage (typically the
>>>> kernel). But we have applying overlays to flattened trees now, so
>>>> maybe there's no need anymore.
>>>>
>>>>> Somewhat echoing David's comment, I'm also not sure what you mean.
>>>>> And trying to not overly influence this conversation with preconceptions
>>>>> from what I'm going to propose.
>>>>>
>>>>> My first shot at the new format added a field to the FDT to indicate
>>>>> that an overlay FDT was concatenated to the FDT (and the overlay FDT
>>>>> in turn could set it's field to concatenate another overlay FDT).
>>>>
>>>> Yes, something like this is what I meant. This was something Grant had
>>>> talked about.
>>>>
>>>>> But in the end I decided that information belonged outside the FDT,
>>>>> and it was sufficient to require that all FDTs be padded out so that
>>>>> if an overlay FDT was concatenated to the FDT, the overlay FDT would
>>>>> be properly aligned.
>>>>
>>>> I can't think of why this wouldn't work either.
>>>>
>>>>> For ease of typing, I'll call this "chaining" or "chained".  For
>>>>> Linux, I am envisioning no kernel use of data from a chained FDT
>>>>> until after the tree is unflattened.  I haven't done an exhaustive
>>>>> search to determine all of the uses of data directly from the
>>>>> flattened FDT, but I am optimistic that there will not be any such
>>>>> access that would require data from a chained FDT (and we could
>>>>> make this a rule).
>>>>
>>>> This would be a major downside to leaving it up to the kernel because
>>>> what can't be touched is hard to enumerate and could change. For
>>>> example, we added earlycon and now the uart node can't be modified.
>>>
>>> What you say makes sense.  So I'll reverse myself and say that for
>>> Linux, we should update the FDT read code to scan all chained
>>> overlay FDT(s) as well as the base FDT.
>>
>> < snip >
>>
>> What I wrote was somewhat ambiguous.  What I meant by "FDT read
>> code" was functions that check for existence of nodes in the
>> FDT or read property values from the FDT.
> 
> Oh.. not just FDT unflattening code.
> 
> The trouble with this is that scanning for a specific node or property
> in a set of chained overlays is highly nontrivial.  Even if you set
> aside the arguably self-imposed design constraints in libfdt, you
> can't just do the same lookup in each fragment: along the way you need
> to resolve the path at which each fragment applies.  That in itself is
> non-trivial.  If you have overlays applying on top of other overlays,
> that could involve recursive lookups of things from previous overlays.
> It's spectacularly complicated and we have to do it on *every single*
> read operation.

I totally overlooked having to resolve each fragment.  You are right,
that makes the problem very complex instead of trivial.


> Either fully applying the overlay in flattened form, or (even
> temporarily) unflattening the tree are better solutions.
> 
>> The other way one could read what I wrote was that when Linux
>> unflattens the FDT, it would unflatten the FDT and all of the
>> chained FDTs.  Obviously also true, but not what I was trying
>> to say here.
>>
>> -Frank
>