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