Subject: Re: RFC on writel and writel_relaxed

From: Sinan Kaya <[email-masked]>
Date: Mon, 26 Mar 2018 17:46:22 -0400

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.

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.

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.