Messages in this thread | | | Date | Thu, 9 Apr 2020 16:07:46 +0100 (BST) | From | "Maciej W. Rozycki" <> | Subject | Re: [PATCH] MIPS: Limit check_bugs32() under CONFIG_32BIT |
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On Thu, 9 Apr 2020, Jiaxun Yang wrote:
> > > There is no need to build and call check_bugs32() under > > > CONFIG_64BIT, just limit it under CONFIG_32BIT. > > > > Since 32bit is subset of 64bit, and due to the code, I think that the > > initial purpose > > of check_bugs32() is also willing to run even with CONFIG_64BIT. > > > > For example, if we have a CPU which is 64bit, and work well on 64bit > > mode, while has a bug only on 32bit mode, check_bugs32 should be used > > here. > > > > Loongson's 3A 1000 is the example, I cannot support FP32 mode well. > > In this case bugs32 only contains a workaround for MIPS34K, which is a > MIPS32 processor. It's safe to do so.
This is because commit c65a5480ff29 ("[MIPS] Fix potential latency problem due to non-atomic cpu_wait.") moved the other generic workaround elsewhere.
The intent has been since historical commit 450ad16ba0ab ("Get rid of arch/mips64/kernel. 9116 lines of code gone.") that `check_bugs32' is for generic errata affecting both 32-bit and 64-bit operation (e.g. 32-bit instructions, which naturally may occur in both cases) and `check_bugs64' is for errata affecting 64-bit operation only (e.g. 64-bit instructions).
But currently it appears we have no generic errata handled, as surely a 34K erratum cannot affect 64-bit operation. So I think such a change makes sense in principle (if a generic erratum appears in the future we can add a third category, which includes workarounds that are always applied), but I think it has to be made in a cleaner way.
Specifically `check_errata' has to be renamed to `check_errata32', some commentary added as to the intent, and last but not least a proper change description added that not only repeats what the change does (and what everyone sees regardless), but actually justifies why the change is made. Saying: "There is no need[...]" does not tell us *why* there is no need.
> But my suggestion is if you're going to clean-up bugs and workarounds > you'd better establish a file for silicon bugs and provide Kconfig > options to enable & disable them. Manage bug dependencies by Kconfig > will be easier.
Why is using Kconfig supposed to be better? Several configurations support multiple processor types (e.g. swappable CPU daugthercards or FPGA soft-cores) and having to list CPU types across platforms as CPUs are added is going to be a maintenance nightmare. Whereas having workarounds or panics associated with run-time determination of the actual CPU type guarantees they will trigger where necessary. The use of `init' sections assures the reclaim of memory for use after bootstrap.
OTOH I agree splitting off errata handling to a separate file may make sense for structural reasons; we have it already for `check_bugs64'.
Maciej
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