2018-02-21 18:56 GMT+09:00 Arnd Bergmann <arnd@xxxxxxxx>: > On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada > <yamada.masahiro@xxxxxxxxxxxxx> wrote: >> 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@xxxxxxxxx>: >> >>>> >>>> I'm not happy that we in one context can reference CONFIG variables >>>> directly, but inside the $(call ...) and $(shell ...) needs the $ prefix. >>>> But I could not come up with something un-ambigious where this could be avoided. >>> >>> I think we should be careful about allowing references to config >>> symbols. It mixes up the parsing and evaluation phases, since $() is >>> expanded during parsing (which I consider a feature and think is >>> needed to retain sanity). >>> >>> Patch 06/23 removes the last existing instance of symbol references in >>> strings by getting rid of 'option env'. That's an improvement to me. >>> We shouldn't add it back. >> >> >> This is really important design decision, >> so I'd like to hear a little more from experts. >> >> >> For example, x86 allows users to choose sub-arch, either 'i386' or 'x86_64'. >> >> https://github.com/torvalds/linux/blob/v4.16-rc2/arch/x86/Kconfig#L4 >> >> >> >> If the user toggles CONFIG_64BIT, >> the bi-arch compiler will work in a slightly different mode >> (at least, back-end parts) >> >> So, my question is, is there a case, >> >> $(cc-option, -m32 -foo) is y, but >> $(cc-option, -m64 -foo) is n ? >> (or vice versa) >> >> >> If the answer is yes, $(cc-option -foo) would have to be re-calculated >> every time CONFIG_64BIT is toggled. >> >> This is what I'd like to avoid, though. > > The -m32/-m64 trick (and -mbig-endian/-mlittle-endian on other architectures > as well as a couple of other flags) only works if the compiler is configured to > support it. In other cases (e.g. big-endian xtensa), the kernel always > detects what the compiler does and silently configures itself to match > using Makefile logic. > > On x86, compilers are usually built as bi-arch, but you can build one that > only allows one of them. > > I can see two reasonable ways out: > > - we don't use $(cc-option -foo) in a case like this, and instead require the > user to have a matching toolchain. > - we could make the 32/64 selection on x86 a 'choice' statement where > each option depends on both the ARCH= variable and the > $(cc-option, -m32)/ $(cc-option, -m64) output. > > Arnd Let me clarify my concern. When we test the compiler flag, is there a case where a particular flag depends on -m{32,64} ? For example, is there a compiler that supports -fstack-protector for 64bit mode, but unsupports it for 32bit mode? $(cc-option -m32) -> y $(cc-option -m64) -> y $(cc-option -fstack-protector) -> y $(cc-option -m32 -fstack-protector) -> n $(cc-option -m64 -fstack-protector) -> y I guess this is unlikely to happen, but I am not whether it is zero possibility. If this could happen, $(cc-option ) must be evaluated together with correct bi-arch option (either -m32 or -m64). Currently, -m32/-m64 is specified in Makefile, but we are moving compiler tests to Kconfig and, CONFIG_64BIT can be dynamically toggled in Kconfig. -- Best Regards Masahiro Yamada -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
