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

Daniel Vetter
2012-03-28T14:46:03Z

On Wed, Mar 28, 2012 at 03:30:13PM +0100, Chris Wilson wrote:
> 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.

Yeah, I think for gmbus here we should only drop the POSTING_READ if we'll
read a register right afterwards anyway. Let's better be slightly paranoid
about this.
-Daniel
--
Daniel Vetter
Mail: daniel@ffwll.ch
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