On Mon, Jan 29, 2018 at 06:32:35PM +0000, Grant Likely wrote: > On Thu, Jan 25, 2018 at 8:01 PM, Frank Rowand <frowand.list@xxxxxxxxx> wrote: > > 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@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. > > > > I totally overlooked having to resolve each fragment. You are right, > > that makes the problem very complex instead of trivial. > > It would be possible to do if each fragment was pre-resolved at append > time to the node that it modifies. ie. each fragment node points back > with an offset to the node it modifies Then searching the tree could > be done by walking backwards through the fragments instead of > searching forwards. Would need to research the best way to do that, > and it would also mean that merely appending a DTB fragment isn't an > option. I guess. But if you're going to pre-resolve, why not just apply the overlay. Yes, it's a bit simpler at the time, but you increase overall complexity in that: 1) you split one logical operation into two places, 2) you have to define an intermediate resolved-but-not-applied overlay format. We already have a full overlay application implementation that works on flattened trees. -- 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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