RFC on writel and writel_relaxed

[Benjamin Herrenschmidt]

On Tue, 2018-03-27 at 10:42 +0100, Will Deacon wrote:
> > 
> > This example adds a wmb() between two writes to a coherent DMA
> > area, it is definitely required there. 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?
> 
> If this is just about ordering accesses to coherent DMA, then using
> dma_wmb() instead will be much better performance on arm/arm64.

Ah, something we should look into for powerpc as well, as we could use
an lwsync for that which is also cheaper than a full sync wmb does.

dma_wmb() is basically the same as smp_wmb() without the CONFIG_SMP
conditional right ?

Cheers,
Ben.