Re: [PATCH 1/1] iommu/arm-smmu-v3: replace writel with writel_relaxed in queue_inc_prod

From: Will Deacon
Date: Wed Jun 21 2017 - 05:08:03 EST

On Wed, Jun 21, 2017 at 09:28:23AM +0800, Leizhen (ThunderTown) wrote:
> On 2017/6/20 19:35, Robin Murphy wrote:
> > On 20/06/17 12:04, Zhen Lei wrote:
> >> This function is protected by spinlock, and the latter will do memory
> >> barrier implicitly. So that we can safely use writel_relaxed. In fact, the
> >> dmb operation will lengthen the time protected by lock, which indirectly
> >> increase the locking confliction in the stress scene.
> >
> > If you remove the DSB between writing the commands (to Normal memory)
> > and writing the pointer (to Device memory), how can you guarantee that
> > the complete command is visible to the SMMU and it isn't going to try to
> > consume stale memory contents? The spinlock is irrelevant since it's
> > taken *before* the command is written.
> OK, I see, thanks. Let's me see if there are any other methods. And I think
> that this may should be done well by hardware.

FWIW, I did use the _relaxed variants wherever I could when I wrote the
driver. There might, of course, be bugs, but it's not like the normal case
for drivers where the author didn't consider the _relaxed accessors