On Wed, Aug 14, 2013 at 10:57:36PM -0400, Nicolas Pitre wrote: > On Wed, 14 Aug 2013, Tom Rini wrote: > > > On 08/14/2013 08:37 PM, Nicolas Pitre wrote: > > > On Wed, 14 Aug 2013, Tom Rini wrote: > > > > > >> On Wed, Aug 14, 2013 at 10:44:22AM -0600, Stephen Warren wrote: > > >>> On 08/14/2013 09:13 AM, Tom Rini wrote: > > >>>> Hey all, > > >>>> > > >>>> Do we have a document yet talking about the best practices for how we > > >>>> would like a hardware vendor to ship, store and possibly update a device > > >>>> tree, on the hardware? "However they like" seems likely to invite > > >>>> problems down the line with everyone trying their own thing. Thanks! > > >>> > > >>> I don't believe there's any written guidance, no. > > >>> > > >>> The initial guidance Grant gave (IIRC at the first Linaro Connect last > > >>> year, or perhaps the ARM workshop in Prague, or perhaps also in various > > >>> ARM kernel list threads?) was that the DTBs should be stored/accessed in > > >>> exactly the same way as the kernel, which on many systems implies it's a > > >>> file in /boot (although MTD partitions, ... are also possible kernel > > >>> locations). The idea here was to explicitly make upgrading the DTB as > > >>> easy as upgrading the kernel, and explicitly without having to upgrade > > >>> any firmware, since that's a more dangerous process in most cases. > > >>> > > >>> Now perhaps that advice was only intended to apply very early on when DT > > >>> was really new on ARM, and has "aged out" by now? If so, I don't recall > > >>> that every being explicitly mentioned or communicated later. > > >> [snip out a bit more of Stephen's answer] > > >> > > >> Yes, this notion certainly is the opposite of what was suggested on the > > >> cross-distro list, both as part of a "what should a bootloader provide > > >> to get commodity distros to support the board" thread and the "where > > >> should a device tree live in the filesystem" thread. Cc'ing them now > > >> because this is one of those things that feels like it needs solving, > > >> solving soon, and done in a way the least number of folks get surprised > > >> about it. > > > > > > I don't think the "however they like" approach is that bad. It is > > > certainly less of a problem than "exactly the same way as the kernel" is. > > > > > > Using /boot implies a distro filesystem and we'd rather wish for DT to > > > be independent from a distro or even Linux. The DTB should therefore be > > > stored and accessed in a way which is hardware/bootloader specific. The > > > distro being installed on top shouldn't have to care. > > > > Right, the expectation has been set that the DT isn't supposed to be > > tied to the kernel and I don't wish to re-hash that. I'm asking, should > > we provide some expectations / guidance on how to store the DT? Should > > quirks be expected to be worked around at the DT consumer level (kernel, > > bootloader, whatever) or in updating the stored copy? That there rules > > out, or not, certain choices. > > Well, the hard guideline should require that the DTB be updateable and > not linked with nor generated by the bootloader or firmware. That > implies some storage separate from the bootloader but this doesn't need > to be a filesystem. I assume generated doesn't mean not updated. Do we want to limit the nodes that can be touched here? For example, a number of platforms have incorrect memory size (and on PowerPC, intentionally zero) so that the bootloader can correct them. And not linked with the bootloader may (or may not, Stephen, can you comment on what Tegra is doing at the moment?) make it harder to use in those cases. > And the DTB should be updated far less often than the kernel. So that > might be acceptable to have a special bootloader-specific procedure to > update it. Right. But you wouldn't want it to be a hardware mod to make things be writable again (thinking the EEPROM I mentioned before). > I expect that the kernel will have to deal with quirks from a bad DTB > that has already been deployed. But those quirks workarounds only have > to make things minimally work while the optimal state would come from an > updated DTB. Same story as with bad BIOSes on a X86 system. > > More importantly, a DTB update is likely to be useful in providing new > data the previous DTB didn't include e.g. if some bindings weren't > finalized when the initial deployment took place. And what bindings it should use in the interm depends on the general outcome of when bindings get locked in and how all of that. But does this mean that the expectation, on the kernel side, would be that a developer should use the latest published bindings for a platform, or the bindings that shipped the very first time? For bootloaders the rule is "whatever shipped on the hardware shouldn't be touched". -- Tom -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
