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

[Arnd Bergmann]

On Thu, Sep 15, 2022, at 7:01 AM, Parav Pandit wrote:
> The cited commit [1] describes that when using writel(), explcit wmb()
> is not needed. However, it should have said that dma_wmb() is not
> needed.

Are you sure? As I understand it, the dma_wmb() only serializes
a set of memory accesses, but does not serialized against an MMIO
access, which depending on the CPU architecture may require a
different type of barrier.

E.g. on arm, writel() uses __iowmb(), which like wmb() is defined
as "dsb(x); arm_heavy_mb();", while dma_wmb() is a "dmb(oshst)".

     Arnd