Subject: Re: RFC on writel and writel_relaxed

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 27 Mar 2018 09:01:57 +1100

On Mon, 2018-03-26 at 17:46 -0400, Sinan Kaya wrote:
> On 3/26/2018 5:30 PM, Arnd Bergmann wrote:
> > > But that was never a requirement of writel(),
> > > Documentation/memory-barriers.txt gives an explicit example demanding
> > > the wmb() before writel() for ordering system memory against writel.
> > 
> > Indeed, but it's in an example for when to use dma_wmb(), not wmb().
> > Adding Alexander Duyck to Cc, he added that section as part of
> > 1077fa36f23e ("arch: Add lightweight memory barriers dma_rmb() and
> > dma_wmb()"). Also adding the other people that were involved with that.
> > 
> 
> ARM developers can get away with not including wmb() in their code and use
> writel() to observe memory writes due to implicit barriers.
> 
> However, same code will not work on Intel.

Wrong. It will.

You do NOT need wmb between writes to memory and writel.

> writel() has a compiler barrier in it for x86.
> wmb() has a sync operation in it for x86. 
> 
> Unless wmb() is called, PCIe device won't observe memory updates from the CPU.

This is completely wrong. They will. Intel provides the necessary
ordering guarantees without an explicit wmb.

Otherwise almost all drivers out there are broken which I very much
doubt :-)

Cheers,
Ben.