linux-kernel - Re: [PATCH] net: emac: emac gigabit ethernet controller driver

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Date:	Tue, 15 Dec 2015 22:55:27 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Timur Tabi <timur@...eaurora.org>
Cc:	Christopher Covington <cov@...eaurora.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Gilad Avidov <gavidov@...eaurora.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, sdharia@...eaurora.org,
	shankerd@...eaurora.org, gregkh@...uxfoundation.org,
	vikrams@...eaurora.org
Subject: Re: [PATCH] net: emac: emac gigabit ethernet controller driver

On Tuesday 15 December 2015 15:09:23 Timur Tabi wrote:
> Arnd Bergmann wrote:
> > If that's in the probe() called from it function, just use writel() everywhere,
> > a few extra microseconds won't kill the boot time. In general, if a user would
> > notice the difference, use the relaxed version and add a comment to explain
> > how you proved it's correct, otherwise stay with the default accessors.
> 
> What about adding a wmb() after the last writel()?  This driver does 
> that a lot.  Is that something we want to discourage?  I can understand 
> how we would want to make sure that the last write is posted before the 
> function exits.

Please explain in a comment specifically which race you are closing by
ensuring that the write gets posted. What does it race against?

As I said earlier, guaranteeing that a write gets posted does not mean
it has arrived at the device, we only get that behavior after a subsequent
read from the same device, but you don't need a wmb() between the
write and the read to guarantee this.

If you have an odd bus that does not follow those rules, it may in fact be
best to have a separate set of I/O accessors and not use readl/writel at all.

	Arnd
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