Re: [RFC] devicetree: new FDT format version

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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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@gmail.com> 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.

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

-- 
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_!
http://www.ozlabs.org/~dgibson

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