On Thu, Jun 2, 2016 at 11:24 PM, Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > On Wednesday 01 Jun 2016 15:27:59 Rob Herring wrote: >> On Wed, Jun 1, 2016 at 2:50 PM, Geert Uytterhoeven wrote: >> > When moving functionality from C code to DT, we're regularly faced with >> > stable DT issues: old DTBs should keep on working. This requires keeping >> > workaround code in the kernel. >> > >> > An alternative solution to having workaround C code, would be to >> > dynamically modify the DT, to add missing device nodes and phandle links. >> > >> > This has several advantages: >> > - All workarounds are kept together, >> > - Workarounds can be enabled/disabled using a single Kconfig option, >> > - Individual driver code is not polluted by workaround code. >> > >> > Examples of missing support in DT are: >> > - A device node for the R-Car RST (Reset Controller), which a.o. >> > provides access to the Mode Pins (currently handled using an >> > hardcoded address in platform/driver code), cfr. the series >> > "[PATCH/RFC v3 00/22] soc: renesas: Add R-Car RST driver for >> > obtaining mode pin state" I've just sent >> > (http://www.spinics.net/lists/linux-renesas-soc/msg04289.html), >> > >> > - A device node for the R-Car SYSC (System Controller), to link CPUs >> > to their respective power domains (On R-Car Gen2 CPUs can be >> > auto-detected, as there's a register indicating which CPU cores are >> > present), >> > >> > - Add a device node for the R-Car Gen2 APMU (Advanced Power >> > >> > Management Unit), for modern CPU bringup using "enable-method". >> > Note that the method from this RFC doesn't work for >> > "enable-method", as that is parsed in arm_dt_init_cpu_maps(), >> > immediately after unflatten_device_tree(), long before initcalls >> > run. >> > >> > However, there are other possible uses: >> > - Workarounds for hardware bugs: early engineering samples of an SoC >> > >> > may have non-functional devices. This would allow to describe the >> > latest (functional) hardware in the .dtsi, knowing that the fixup >> > code will disable non-functional devices when running on an early >> > engineering sample, based on reading the PRR (Product Revision >> > Register). >> > >> > - Handle other differences between SoC versions, e.g. change >> > >> > compatible values for an early engineering sample that needs special >> > handling, or limit the features of a device. >> > >> > - Add SoC-specific compatible values to all device nodes (e.g. add >> > >> > "renesas,r8a7795-wdt" to a node already having >> > "renesas,rcar-gen3-wdt" when running on r8a7795). This would make >> > it easier to share .dtsi files within the same SoC family, without >> > relying on e.g. C preprocessor tricks. >> > >> > This proof-of-concept implements this for the missing R-Car RST (Reset >> > Controller) node. This poc is not suitable for all of the above, as some >> > DT structures (e.g. the CPU's "enable-method) are parsed long before >> > early_initcall(), and would need a different workaround. >> > >> > What do you think? >> >> I have no objection to this method of dealing with compatibility. >> However your handling is still C code. What I would like to see here >> is using overlays to apply updates. I would like to be able to take 2 >> dts files and create an overlay dts based on their diff (or you could >> do this step manually). Then build the overlay dtb into the kernel and >> apply it on boot based on some match. Then thru the magic of linker >> sections, it becomes a matter of just adding the dtbo into the build >> and a one line declaration: >> >> DT_QUIRK(my_quirk_dtbo, "vendor,board"); >> >> BTW, I'd also like to see tools to apply overlays offline into a new >> dtb or compile dts files and overlays to a dtb. > > We need to keep the use case in mind. The main (and possibly only) reason why > we want to patch DT this way is to support systems whose DTB can't be updated > (otherwise we could just update the DTB) and isn't fully known in advance to > the kernel (otherwise we would just bundle an updated full DTB with the > kernel). We thus need a heuristic-based approach at runtime to identify > missing or outdated DT pieces and patch them, with some level of fuzziness. > I'm not sure we could handle this with overlays. Indeed. While I'm a big fan of DT overlays, I don't think they're suitable for all kinds of fixups we need. Simple things like adding a device node for the RST could be handled with a built-in overlay. More complex things, like adding SYSC and APMU devices nodes need some extra bit of logic, to e.g. add phandles to/from the (existing) CPU nodes. The same is true for fixups that need to check on which revision of the SoC they're running. >> > Should this be handled at another level? E.g. operate on the FDT? >> >> We should try to avoid doing things with the FDT if possible. OK. So I should try to hook up my code immediately after unflatten_device_tree()... Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
