Re: [PATCH v3 5/5] xtensa: Emulate two-byte cmpxchg

[Bradley Morgan]

On 17 September 2026 17:38:30 BST, Bradley Morgan <brads@xxxxxxxxxxxxxx>
wrote:
>Xtensa has no two-byte atomic compare and swap, so the __cmpxchg()
>switch lets case 2 fall through to __cmpxchg_called_with_bad_pointer(),
>which is declared but never defined, so a two-byte cmpxchg() fails at
>link time. Route case 2 through the new cmpxchg_emu_u16(), which takes
>the old and new values as unsigned long, so the (unsigned long) casts
>on _o_ and _n_ at the call move into the function and the macro's
>__typeof__(*(ptr)) typing of _o_ and _n_ remains the pointer-integer
>type check.
>
>Signed-off-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>
>---
> arch/xtensa/include/asm/cmpxchg.h | 1 +
> 1 file changed, 1 insertion(+)
>
>diff --git a/arch/xtensa/include/asm/cmpxchg.h b/arch/xtensa/include/asm/cmpxchg.h
>index b6db4838b175..8dea8e357fc0 100644
>--- a/arch/xtensa/include/asm/cmpxchg.h
>+++ b/arch/xtensa/include/asm/cmpxchg.h
>@@ -76,6 +76,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new, int size)
> {
> 	switch (size) {
> 	case 1:  return cmpxchg_emu_u8(ptr, old, new);
>+	case 2:  return cmpxchg_emu_u16(ptr, old, new);
> 	case 4:  return __cmpxchg_u32(ptr, old, new);
> 	default: __cmpxchg_called_with_bad_pointer();
> 		 return old;
>
Oops! Forgot to add R-B tag! Merger, could you add this when it's ready to
merge?
--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@xxxxxxxxx/