Messages in this thread | | | From | Arnd Bergmann <> | Date | Wed, 22 Aug 2018 17:03:56 +0200 | Subject | Re: [PATCH] ARM: use choice for kernel unwinders |
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On Wed, Aug 22, 2018 at 4:38 PM Stefan Agner <stefan@agner.ch> wrote: > On 22.08.2018 12:02, Arnd Bergmann wrote: > > On Wed, Aug 22, 2018 at 12:24 AM Stefan Agner <stefan@agner.ch> wrote: > > > > Looks ok to me. I've added it to my randconfig test environment, you > > will hear from me within a day if I run into build regressions. > > > > We may still want to clean up these three lines: > > > > lib/Kconfig.debug: select FRAME_POINTER if !MIPS && !PPC && > > !ARM_UNWIND && !S390 && !MICROBLAZE && !ARC && !X86 > > lib/Kconfig.debug: select FRAME_POINTER if !MIPS && !PPC && !S390 && > > !MICROBLAZE && !ARM_UNWIND && !ARC && !X86 > > lib/Kconfig.debug: select FRAME_POINTER if !MIPS && !PPC && !S390 && > > !MICROBLAZE && !ARM_UNWIND && !ARC && !X86 > > > > in which ARM is the odd case that currently depends on an architecture > > specific rather than the architecture itself. > > I guess we would just follow X86 lead by saying ARM is guaranteed to > have unwinding support, and hence we can add !ARM.
Right, that was the idea.
> > We could introduce a 'config ARCH_HAS_UNWINDER' symbol that gets > > selected by mips, ppc, s390, microblaze, arm and x86 unconditionally, > > and then simplify the 'select' statements here. > > Yeah I was thinking about something like that too. > > It seems to be a bit weird to me that lib/Kconfig.debug selects a > specific stack unwinding technique...
This must be a historic artifact from the time when FRAME_POINTER was the only unwinding method that existed. We may also have some architectures that don't support any unwinding.
> Ideally other config symbol should just ask arch to make sure a > unwinding technique is available (NEED_STACK_UNWINDING?) and arch then > makes sure to provide a reasonable default. > > This then also would make it possible to select no stack unwinding in > case arch supports that and all the users of stack unwinding are > disabled too. Not sure how that exactly would look like in Kconfig, I > was thinking like: > > choice > prompt "Choose kernel unwinder" > optional if !NEED_STACK_UNWINDING > default UNWINDER_ARM if AEABI && !FUNCTION_GRAPH_TRACER > default UNWINDER_FRAME_POINTER if !AEABI || FUNCTION_GRAPH_TRACER > > But "optional if" does not exist yet :-)
You can write that as
choice prompt "Choose kernel unwinder" if NEED_STACK_UNWINDING
This will hide the prompt when NEED_STACK_UNWINDING is disabled, making it impossible to pick one of the two unwinders.
> Reading the comments in arch/arm/Kconfig.debug seems to suggest that > once upon a time it was possible to disable stack unwinding on ARM. > > But then, maybe we don't really want to go there? Might be interesting > for tinification efforts.
I'd leave that for another day ;-)
Arnd
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