On Wed, Sep 9, 2020 at 2:58 AM Matthias Schiffer <matthias.schiffer@xxxxxxxxxxxxxxx> wrote: > > On Thu, 2020-08-27 at 09:10 +0200, Matthias Schiffer wrote: > > On Wed, 2020-08-26 at 13:26 -0600, Rob Herring wrote: > > > On Wed, Aug 26, 2020 at 8:47 AM Frank Rowand < > > > frowand.list@xxxxxxxxx> > > > wrote: > > > > > > > > Hi Rob, > > > > > > > > On 2020-08-26 08:54, Matthias Schiffer wrote: > [snip] > > > > > > > > > > Hmm, I see. This difference in behaviour is quite unfortunate, > > > > > as > > > > > I'm > > > > > currently looking for a way to *really* disable a CPU core. > > > > > > > > > > In arch/arm64/boot/dts/freescale/imx8mn.dtsi (and other > > > > > variants > > > > > of the > > > > > i.MX8M), there are 4 CPU nodes for the full-featured quad-core > > > > > version. > > > > > The reduced single- and dual-core versions are currently > > > > > handled > > > > > in > > > > > NXP's U-Boot fork by deleting the additional nodes. > > > > > > > > > > Not doing so causes the kernel to hang for a while when trying > > > > > to > > > > > online the non-existent cores during boot (at least in linux- > > > > > imx > > > > > 5.4 - > > > > > I have not checked a more recent mainline kernel yet), but the > > > > > deletion > > > > > is non-trivial to do without leaving dangling phandle > > > > > references. > > > > > > > > Any thoughts on implementing another universal property that > > > > means > > > > something like "the hardware described by this node does not > > > > exist > > > > or is so broken that you better not use it". > > > > > > There's a couple of options: > > > > > > The DT spec defines 'fail' value for status. We could use that > > > instead > > > of 'disabled'. > > > > > > The spec behavior with cpu 'disabled' is only on PPC AFAIK. On > > > arm/arm64 (probably riscv now too) we've never followed it where we > > > online 'disabled' CPUs. So we could just make the check conditional > > > on > > > !IS_ENABLED(CONFIG_PPC). This would need some spec update. > > > > On ARM(64), the "disabled" status on CPUs doesn't have any effect. I > > assume this changed with the mentioned commit c961cb3be906 "of: Fix > > cpu > > node iterator to not ignore disabled cpu nodes", as reverting it > > gives > > me the desired behaviour of considering the disabled CPUs non- > > existent. > > > > So it seems that we already changed the interpretation in a non- > > compatible way once (back in v4.20), and somehow PPC has yet another > > different behaviour? > > > > How do we get out of this mess? Is going back to the v4.19 logic for > > non-PPC platforms an acceptable regression fix, or would this be > > considered another breaking change? Yes, this is my 2nd option above. Need to double check MIPS and Sparc though. > Any new insights on this? I'll gladly provide patches or proposals for > a spec update if we can decide where we want to go with this. I gave 2 options. I'm fine with either one (or both). Using 'fail' is the simplest. Rob
