On 09/06/2013 12:53 AM, Tero Kristo wrote: > On 09/05/2013 11:30 PM, Stephen Warren wrote: ... >> 1) >> >> At least for large SoCs (rather than e.g. a simple clock generator >> chip/crystal with 1 or 2 outputs), clock drivers need a *lot* of data. >> We can either: >> >> a) Put that data into the clock driver, so it's "just there" for the >> clock driver to use. >> >> b) Represent that data in DT, and write code in the clock driver to >> parse DT when the driver probe()s. >> >> Option (a) is very simple. > > How can you claim option (a) to be very simple compared to (b)? I think > both are as easy / or hard to implement. Well, the work required for (b) is a pure super-set of the work require for (a), so clearly (a) is less work (perhaps the issue you're debating is harder/easier rather than more/less work?) >> Option (b) entails writing (and executing) a whole bunch of DT parsing >> code.It's a lot of effort to define the DT binding for the data, >> convert the data into DT format, and write the parsing code. It wastes >> execution time at boot. In the end, the result of the parsing is exactly >> the same data structures that could have been embedded into DT in the >> first place. This seems like a futile effort. > > Not really, consider a new SoC where you don't have any kind of data at > all. You need to write the data tables anyway, whether they are under DT > or some kernel data struct. Yes. But beyond writing the data tables, you also don't/do have to write all the DT parsing code based on choosing (a) or (b), etc. > The execution time remain in place for > parsing DT data though, but I wouldn't think this to be a problem. Also, > you should consider multiarch ARM kernel, where same kernel binary > should support multiple SoCs, this would entail having clock data for > all built in to the kernel, which can be a problem also. There's no reason that the clock data has to be built into the kernel at all; we should support even SoC clock drivers as modules in an initrd. Alternatively, drop the unused data from the kernel after boot via __init or similar. Alternatively, "pre-link" the clock driver module into the kernel in a way that allows it to be unloaded after boot even though it was built-in. ... > You can just as easily claim that anything internal to SoC should be > left out from DT, as this is cast in stone (or rather, silicon) also. We > should only use it to describe board layout. Everything else, the kernel > can 'know' by compile time. I did, immediately below:-) And then I went on to explain why that's necessary in many cases. ... > I can turn this around, as you went to this road. Why have DT at all? I believe (at least for ARM) the idea was to avoid churn to the kernel for supporting the numerous different *boards*. The kernel needs and contains drivers for HW blocks, and so since they're there, they may as well encode everything about the HW block. However, in most cases, the kernel shouldn't contain drivers for boards; boards are built from various common components which have drivers. DT is used to describe how those components are inter-connected. Hence, we can hopefully remove all board-related churn from the kernel (once the DT files are moved out of the kernel). > Personally I hate the whole idea of a devicetree, however am forced to > use it as somebody decided it is a good idea to have one. It doesn't > really solve any problems, it just creates new ones in a way of > political BS where everybody claims they know how DT should be used, and > this just prevents people from actually using it at all. Also, it > creates just one new unnecessary dependency for boot, previously we had > bootloader and kernel images which had to be in sync, now we have > bootloader + DT + kernel. What next? Maybe we should move the clock data > into a firmware file of its own? Well, I can sympathize, but I think the time is past for debating that. > Why do you care so much what actually goes into the devicetree? To get DT right. Even if we went back to board files and mach-xxx specific code rather than cleanly separated drivers, it would still be beneficial to have much more oversight of board/mach-xxx code than there was previously. Board files made it very easy to do SoC-specific hacks. To avoid that, in either DT or board files, we're trying to impose standards so that we pick correct, generic, appropriate solutions, rather than letting everyone run of with isolated ad-hoc solutions. > Shouldn't people be let use it how they see fit? For the clock bindings > business this is the same, even if the bindings are there, you are free > to use them if you like, and if you don't like them, you can do things > differently. We'd be better of creating as much standardization as possible, so that all SoCs/boards/.. work as similarly as possible, and we achieve maximal code reuse, design-reuse, and allow people to understand everything rather than just one particular SoC's/board's solution. If we don't get some re-use and standardization out of DT, we really may as well just use board files. -- 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
