RFC on writel and writel_relaxed

Sinan Kaya okaya at codeaurora.org
Tue Mar 27 09:08:47 AEDT 2018

On 3/26/2018 6:01 PM, Benjamin Herrenschmidt wrote:
> 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.

If writel() provides such a guarantee, why do I see code sequences like


all over the place.

>> 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.

I'm still reserving my doubts here. I was told about an explicit
wmb() requirement last week.

> Otherwise almost all drivers out there are broken which I very much
> doubt :-)
> Cheers,
> Ben.

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.

More information about the Linuxppc-dev mailing list