On Fri, Aug 30, 2019 at 10:11:55AM +0100, Andrew Murray wrote:

On Fri, Aug 30, 2019 at 08:52:20AM +0100, Will Deacon wrote:

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On Fri, Aug 30, 2019 at 01:08:03AM +0100, Andrew Murray wrote:

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On Thu, Aug 29, 2019 at 05:54:58PM +0100, Will Deacon wrote:

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On Thu, Aug 29, 2019 at 04:48:34PM +0100, Will Deacon wrote:

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diff --git a/arch/arm64/include/asm/atomic_ll_sc.h b/arch/arm64/include/asm/atomic_ll_sc.h
index 95091f72228b..7fa042f5444e 100644
--- a/arch/arm64/include/asm/atomic_ll_sc.h
+++ b/arch/arm64/include/asm/atomic_ll_sc.h

@@ -23,6 +23,10 @@ asm_ops "\n"								\
 #define __LL_SC_FALLBACK(asm_ops) asm_ops
 #endif

I downloaded your original patches and tried them, and also got the
build error. After playing with this I think something isn't quite right...

Can you post the error you see?

This is your current test:

 echo 'int main(void) {asm volatile("and w0, w0, %w0" :: "K" (4294967295)); return 0; }' |  aarch64-linux-gnu-gcc -S -x c  - ; echo $?

But on my machine this returns 0, i.e. no error. 

IIUC that's expected, as this is testing if the compiler erroneously
accepts the invalid immediate.

Note that try-run takes (option,option-ok,otherwise), so:

| cc_has_k_constraint := $(call try-run,echo                             \
|        'int main(void) {                                               \
|                asm volatile("and w0, w0, %w0" :: "K" (4294967295));    \
|                return 0;                                               \
|        }' | $(CC) -S -x c -o "$$TMP" -,,-DCONFIG_CC_HAS_K_CONSTRAINT=1)

... means we do nothing when the compile is successful (i.e. when the compiler
is broken), and we set -DCONFIG_CC_HAS_K_CONSTRAINT=1 when the compiler
correctly rejects the invalid immediate.

If we drop the -S, we'll get an error in all cases, as either:

* GCC silently accepts the immediate, GAS aborts
* GCC aborts as it can't satisfy the constraint

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+#ifndef CONFIG_CC_HAS_K_CONSTRAINT
+#define K
+#endif

Here we define K to nothing if the compiler accepts the broken immediate.

If the compiler rejects invalid immediates we don't define K to anything, so
it's treated as a literal later on, and gets added as a constaint.

Thanks,
Mark.

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