Re: [PATCH 14/14 v5] drm/i915/intel_i2c: remove POSTING_READ() from gmbus transfers

Chris Wilson
2012-03-28T14:30:34Z

On Wed, 28 Mar 2012 21:21:42 +0800, Daniel Kurtz <djkurtz@chromium.org> wrote:
> On Wed, Mar 28, 2012 at 9:05 PM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > We do need the write flush here (and set_data) as the next action is a
> > udelay loop which is not per-se a mb.
>
> Now I am confused. I915_WRITE_NOTRACE() calls writel(), which has an
> explicit mb(); Why do you need another mb?

Nominally writel isn't a memory barrier. I see that x86 does include mb
in its writel define. However, if memory serves, that is only a write
barrier to memory (equivalent of mfence), and not a PCI write flush/barrier
for which we need to an explicit PCI read.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre