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

Arnd Bergmann
2022-10-18T06:44:40Z

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.

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