netdev - Re: [PATCH v3 08/10] ARM: mxs: add ocotp read function

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Date:	Thu, 6 Jan 2011 09:45:19 +0800
From:	Shawn Guo <shawn.guo@...escale.com>
To:	Jamie Lokier <jamie@...reable.org>
CC:	Jamie Iles <jamie@...ieiles.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, <gerg@...pgear.com>,
	<B32542@...escale.com>, <netdev@...r.kernel.org>,
	<s.hauer@...gutronix.de>, <baruch@...s.co.il>,
	<w.sang@...gutronix.de>, <r64343@...escale.com>,
	<linux-arm-kernel@...ts.infradead.org>, <eric@...rea.com>,
	<bryan.wu@...onical.com>, <davem@...emloft.net>,
	<lw@...o-electronics.de>
Subject: Re: [PATCH v3 08/10] ARM: mxs: add ocotp read function

On Wed, Jan 05, 2011 at 05:56:17PM +0000, Jamie Lokier wrote:
> Jamie Iles wrote:
> > On Wed, Jan 05, 2011 at 05:44:09PM +0100, Uwe Kleine-König wrote:
> > > Hello Jamie,
> > > On Wed, Jan 05, 2011 at 04:16:46PM +0000, Jamie Iles wrote:
> > > > On Wed, Jan 05, 2011 at 10:07:35PM +0800, Shawn Guo wrote:
> > > > > +	/* check both BUSY and ERROR cleared */
> > > > > +	while ((__raw_readl(ocotp_base) &
> > > > > +		(BM_OCOTP_CTRL_BUSY | BM_OCOTP_CTRL_ERROR)) && --timeout)
> > > > > +		/* nothing */;
> > > > 
> > > > Is it worth using cpu_relax() in these polling loops?
> > > I don't know what cpu_relax does for other platforms, but on ARM it's
> > > just a memory barrier which AFAICT doesn't help here at all (which
> > > doesn't need to be correct).  Why do you think it would be better?
> > 
> > Well I don't see that there's anything broken without cpu_relax() but 
> > using cpu_relax() seems to be the most common way of doing busy polling 
> > loops that I've seen. It's also a bit easier to read than a comment and 
> > semi-colon. Perhaps it's just personal preference.
> 
> cpu_relax() is a hint to the CPU to, for example, save power or be
> less aggressive on the memory bus (to save power or be fairer).
> 
> Currently these architectures do more than just a barrier in cpu_relax():
> x86, IA64, PowerPC, Tile and S390.
> 
> Although it's just a hint on ARM at the moment, it might change in
> future - especially with power mattering on so many ARM systems.
> (Even now, just changing it to a very short udelay might save power
> on existing ARMs without breaking drivers.)
> 
Sounds reasonable.  I would take the suggestion.  Thanks, both Jamie.

-- 
Regards,
Shawn

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