Re: [PATCH 2/2] arm64: Move the LSE gas support detection to Kconfig

From: Catalin Marinas <catalin.marinas@arm.com>
To: Will Deacon <will@kernel.org>
Cc: Vladimir Murzin <vladimir.murzin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/2] arm64: Move the LSE gas support detection to Kconfig
Date: Mon, 13 Jan 2020 11:36:14 +0000	[thread overview]
Message-ID: <20200113113613.GA1876@arrakis.emea.arm.com> (raw)
In-Reply-To: <20200110154542.GB24322@willie-the-truck>

On Fri, Jan 10, 2020 at 03:45:43PM +0000, Will Deacon wrote:
> On Fri, Jan 10, 2020 at 03:30:14PM +0000, Catalin Marinas wrote:
> > On Fri, Jan 10, 2020 at 12:08:26PM +0000, Will Deacon wrote:
> > > On Fri, Jan 10, 2020 at 11:54:38AM +0000, Vladimir Murzin wrote:
> > > > I was not lucky with the similar patch [1], anyway
> > > > 
> > > > Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
> > > > 
> > > > [1] https://www.spinics.net/lists/linux-crypto/msg36059.html
> > > 
> > > It's the loss of the warning that I object to, since I think it's a useful
> > > diagnostic to have. Is there some way we can keep that, but using the new
> > > kbuild logic?
[...]
> The thing I dislike about it is that if somebody sends you a .config with
> LSE enabled, and your compiler doesn't support it, then it silently get
> disabled.

I was thinking the other way. Someone sending me a .config file and I
can't tell whether LSE was built into their kernel or not unless they
also send the build log (I haven't seen anyone do this). At least for my
local compiler, I have some control over it and can compare the
resulting .config file. It would have been nice if oldconfig warned of
features being disabled but I guess it would become too noisy.

To accommodate the two scenarios, what about this diff on top of patch 2
(I can fold it in in v2 and feel free to suggest better config names):

-------8<-------------------------

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 2a0521f0f156..cf3b6d2a67cf 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1362,12 +1362,13 @@ config ARM64_PAN
 	 The feature is detected at runtime, and will remain as a 'nop'
 	 instruction if the cpu does not implement the feature.
 
-config ARM64_AS_HAS_LSE
-	def_bool $(as-instr,.arch_extension lse)
-
 config ARM64_LSE_ATOMICS
+	bool
+	default ARM64_USE_LSE_ATOMICS
+	depends on $(as-instr,.arch_extension lse)
+
+config ARM64_USE_LSE_ATOMICS
 	bool "Atomic instructions"
-	depends on ARM64_AS_HAS_LSE
 	depends on JUMP_LABEL
 	default y
 	help
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index cca6de192d42..6dd8ecacc428 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -30,6 +30,12 @@ LDFLAGS_vmlinux	+= --fix-cortex-a53-843419
   endif
 endif
 
+ifeq ($(CONFIG_ARM64_USE_LSE_ATOMICS), y)
+  ifneq ($(CONFIG_ARM64_LSE_ATOMICS), y)
+$(warning LSE atomics not supported by binutils)
+  endif
+endif
+
 cc_has_k_constraint := $(call try-run,echo				\
 	'int main(void) {						\
 		asm volatile("and w0, w0, %w0" :: "K" (4294967295));	\

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