Re: [PATCH v4] locking/memory-barriers.txt: Improve documentation for writel() example

Arnd Bergmann
2022-10-18T07:50:12Z

On Tue, Oct 18, 2022, at 9:40 AM, Akira Yokosawa wrote:
> On Tue, 18 Oct 2022 08:44:09 +0200, Arnd Bergmann wrote:
>>
>> Anything weaker than a full "wmb()" probably makes the driver calling
>> the writel() non-portable, so that is both vague and incorrect.
>
> Do you mean there is a writel() implementation somewhere in the kernel
> which doesn't guarantee an implicit wmb() before MMIO write?

There are lots of those, but that's not what I meant. E.g. on x86,
writel() does not imply a full wmb() but still guarantees serialization
between DMA and the register access.

> Or do you mean my version is confusing because it can imply a weaker
> write barrier is sufficient before writel_relaxed()?

That's what I meant, yes. On a lot of architectures, it is sufficient
to have something weaker than wmb() before writel_relaxed(), especially
on anything that defines writel_relaxed() to be the same as writel(),
any barrier would technically work. On arm32, using __iowmb() would be
sufficient, and this can be less than a full wmb() but again it's
obviously not portable. These details should not be needed in the
documentation.

Arnd