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

Akira Yokosawa
2022-10-18T07:40:23Z

On Tue, 18 Oct 2022 08:44:09 +0200, Arnd Bergmann wrote:
> On Tue, Oct 18, 2022, at 3:37 AM, Akira Yokosawa wrote:
>> On 2022/10/18 5:55, Arnd Bergmann wrote:
>>> On Mon, Oct 10, 2022, at 12:13 PM, Parav Pandit wrote:
>>
>> "a barrier" can mean "any barrier", which can include a full barrier
>> in theory.
>>
>> So I'd rather make the substituted text read something like:
>>
>> Note that, when using writel(), a prior wmb() or weaker is not
>> needed to guarantee that the cache coherent memory writes have
>> completed before writing to the MMIO region.
>>
>> In my opinion, "or weaker" is redundant for careful readers who are
>> well aware of context of this example, but won't do no harm.
>
> I think that would be more confusing than either of the other variants.
>
> 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?

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

I'm confused...

Thanks, Akira

>
> The current version works because it specifically mentions the correct
> barrier to use, while Parav's version works because it doesn't
> make any attempt to name the specific barrier and just states that
> adding one is a bad idea regardless.
>
> Arnd