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@xxxxxxxxx> 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@xxxxxxxxx> 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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