RFC on writel and writel_relaxed

[Arnd Bergmann]

On Tue, Mar 27, 2018 at 10:56 AM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> On Tue, 2018-03-27 at 09:56 +0200, Arnd Bergmann wrote:
>> On Tue, Mar 27, 2018 at 12:27 AM, Jason Gunthorpe <jgg at ziepe.ca> wrote:
>>
>> I'm pretty sure I've never seen
>> any bug reports pointing to a missing wmb() between memory
>> and MMIO write accesses, but if you remember seeing them in the
>> list, maybe you can look again for some evidence of something going
>> wrong on x86 without it?
>
> The interesting thing is that we do seem to have a whole LOT of these
> spurrious wmb before writel all over the tree, I suspect because of
> that incorrect recommendation in memory-barriers.txt.
>
> We should fix that.

Maybe the problem is just that it's so counter-intuitive that we don't
need that barrier in Linux, when the hardware does need one on some
architectures.

How about we define a barrier type instruction specifically for this
purpose, something like wmb_before_mmio() and have all architectures
define that to an empty macro?

That way, having correct code using wmb_before_mmio() will not
trigger an incorrect review comment that leads to extra wmb(). ;-)

       Arnd