Re: [PATCH v2 cmpxchg 12/13] sh: Emulate one-byte cmpxchg

Arnd Bergmann
2024-05-02T11:31:21Z

On Thu, May 2, 2024, at 07:42, D. Jeff Dionne wrote:
> On May 2, 2024, at 14:07, Paul E. McKenney <paulmck@kernel.org> wrote:
>
>> That would be 8-bit xchg() rather than 8-byte cmpxchg(), correct?
>>
>> Or am I missing something subtle here that makes sh also support one-byte
>> (8-bit) cmpxchg()?
>
> The native SH atomic operation is test and set TAS.B. J2 adds a
> compare and swap CAS.L instruction, carefully chosen for patent free
> prior art (s360, IIRC).
>
> The (relatively expensive) encoding space we allocated for CAS.L does
> not contain size bits.
>
> Not all SH4 patents had expired when J2 was under development, but now
> have (watch this space). Not sure (me myself) if there are more atomic
> operations in sh4.

SH4A supports MIPS R4000 style LL/SC instructions, but it looks like
the older SH4 does not.

Arnd