On Tue, Nov 29, 2016 at 10:32:28AM +0000, Phil Elwell wrote: > On 29/11/2016 02:11, David Gibson wrote: > > On Mon, Nov 28, 2016 at 12:24:20PM +0000, Phil Elwell wrote: > >> On 28/11/2016 12:10, Pantelis Antoniou wrote: > >>> For plugins we need the __symbols__ node to support stacked overlays, i.e. > >>> overlays referring label that were introduced by a previous overlay. > >> Although it is arguably useful to be able to refer to symbols created by > >> one overlay from within another, do we really want all symbols to be > >> global? Isn't there a call for a new syntax or usage pattern to indicate > >> either that a symbol should be local to the overlay or, my preferred > >> option, global? > > So, this is back to a design question about the overlay format. As > > noted in the initial discussions about possible "connector" formats, I > > think we will want some sort of local symbols. But the current > > overlay format with all global symbols is out there and we need to > > support it. > The overlay format we have does not dictate the scope of the symbols. > > In all implementations I know of - the Raspberry Pi loader, the current > Linux kernel, the latest dtc patch set - there is a completely > asymmetric relationship between the base DTB and an overlay: > * the base DTB exports __symbols__ to resolve the overlays unresolved > label references, as recorded by the __fixups__ node > * the overlay's phandles are renumbered so as not to clash with the base > tree using the __local_fixups__ > * the contents of the __overlay__ nodes are applied to the base tree, as > directed by the "target" or "target-path" properties > > The __symbols__ node of the overlay is ignored and discarded. The > __fixups__ and __local_fixups__ in the base DTB (if present - the RPi > dtc only generates them for /plugins/) are ignored. > > In the set of RPi overlays only one exports a global symbol, which it > achieves with an overlay aimed at target-path = "/__symbols__" that adds > a new symbol (in this case "i2c_gpio"). > > If the __symbols__ in an overlay are automatically merged with the base > symbols, that is a significant change in semantics which needs to be > discussed. Symbols are global, they really can't be anything else in this format. This is why I think the connector approach - though substantially more complex - is a better idea in the wrong run. -- 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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