linux-kernel - Re: [PATCH 14/14 v5] drm/i915/intel_i2c: remove POSTING

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Message-ID: <20120328144644.GB15551@phenom.ffwll.local>
Date:	Wed, 28 Mar 2012 16:46:44 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	Daniel Kurtz <djkurtz@...omium.org>,
	Daniel Vetter <daniel@...ll.ch>,
	Keith Packard <keithp@...thp.com>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Benson Leung <bleung@...omium.org>,
	Yufeng Shen <miletus@...omium.org>
Subject: Re: [PATCH 14/14 v5] drm/i915/intel_i2c: remove POSTING_READ() from
 gmbus transfers

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@...omium.org> wrote:
> > On Wed, Mar 28, 2012 at 9:05 PM, Chris Wilson <chris@...is-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@...ll.ch
Mobile: +41 (0)79 365 57 48
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