Hi Laurent, On Mon, Jan 15, 2018 at 7:01 PM, Laurent Pinchart <laurent.pinch...@ideasonboard.com> wrote: > On Monday, 15 January 2018 19:09:53 EET Rob Herring wrote: >> On Fri, Jan 12, 2018 at 5:14 PM, Laurent Pinchart wrote: >> > The internal LVDS encoders now have their own DT bindings. Before >> > switching the driver infrastructure to those new bindings, implement >> > backward-compatibility through live DT patching. >> >> Uhh, we just got rid of TI's patching and now adding this one. I guess >> it's necessary, but I'd like to know how long we need to keep this? > > That's a good question. How long are we supposed to keep DT backward > compatibility for ? I don't think there's a one-size-fits-them-all answer to > this question. Geert, any opinion ? How long do you plan to keep the CPG > (clocks) DT backward compatibility for instance ?
Good question indeed... It's not just CPG/MSSR. Over the years we also added or changed APMU (SMP), SYSC (power domains), RST (mode pins), CMT (timers), ..., all of which have some sort of compatibility support in the kernel. Hence to avoid having to remember the kernel versions that dropped backwards compatibility for each of the above components, I was thinking about an R-Car Gen2 DT Flag Day. However, that was before I learned about your plans for LVDS (it seems every kernel release we learn about something new, postponing the flag day ;-). And now I'm quite sure we'll have another change in the future (DU per channel device nodes)... About using DT fixups to implement backwards compatibility: I did my share of thinking and experimenting with DT fixups (see e.g. "[PATCH/RFC 0/1] soc: renesas: Add DT fixup code for backwards compatibility" (https://www.spinics.net/lists/linux-renesas-soc/msg04305.html). DT fixups are hard to implement right, and not everything can be done that way. Hence in the end, none of this was ever used upstream, and everything was handled in C... So I'm wondering if it would be easier if you would implement backwards compatibility in C, using different compatible values for the new bindings? I.e. switch from "renesas,du-r8a77*" to "renesas,r8a77*-du" + "renesas,r8a77*-lvds"? That way it also becomes very clear that there are old and new bindings, and that there is backwards compatibility code for the old binding. I know you're aware (the rest of the audience may not be) that the LVDS part is not the only separate hardware block current unified in the single DU node: each DU channel has its own hardware block. Perhaps you can also bite the bullet and have a single device node per DT channel, allowing Runtime PM for DU channels? Of course you have to tie channels together using a "companion" (cfr. USB) or "renesas,bonding" (cfr. DRIF) property (unfortunately I learned about the former only after the latter was already established). Finally, implementing backwards compatibility support by DT fixup using overlays may complicate backporting to LTSI kernels, due to the dependency on DT overlays, which were reworked lately. >> > --- a/drivers/gpu/drm/rcar-du/Kconfig >> > +++ b/drivers/gpu/drm/rcar-du/Kconfig >> > @@ -22,6 +22,8 @@ config DRM_RCAR_LVDS >> > bool "R-Car DU LVDS Encoder Support" >> > depends on DRM_RCAR_DU >> > select DRM_PANEL >> > + select OF_FLATTREE >> > + select OF_OVERLAY >> >> OF_OVERLAY should probably select OF_FLATTREE. I guess in theory, we >> could have an overlay from a non-FDT source... Currently the select is needed for of_fdt_unflatten_tree() only, which is not used by the core OF_OVERLAY code. So you could build an overlay in memory yourself, and pass that, without using of_fdt_unflatten_tree(). But that is going to change if I read Frank's reponse well? Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org 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
