linux-kernel - RE: [PATCH v2 02/12] pm: at91: Workaround DDRSDRC self-refresh bug with LPDDR1 memories.

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Message-ID: <372b731350484f8ea1d29bd016421dfb@EMAIL.axentia.se>
Date:	Mon, 26 Jan 2015 15:58:44 +0000
From:	Peter Rosin <peda@...ntia.se>
To:	Sylvain Rochet <sylvain.rochet@...secur.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>
CC:	Wenyou Yang <wenyou.yang@...el.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"alexandre.belloni@...e-electrons.com" 
	<alexandre.belloni@...e-electrons.com>
Subject: RE: [PATCH v2 02/12] pm: at91: Workaround DDRSDRC self-refresh bug
 with LPDDR1 memories.

Sylvain Rochet wrote:
> Hello Nicolas,
> 
> On Mon, Jan 26, 2015 at 02:34:38PM +0100, Nicolas Ferre wrote:
> > Le 26/01/2015 11:36, Sylvain Rochet a écrit :
> > >
> > > I think we should explain we are dealing with an errata here, this
> > > is not obvious at first sight, the patch summary may find its place
> > > here :-)
> >
> > True but the problem is that this errata is not public yet, it will be
> > in a couple of weeks.
> >
> > I have the feeling though that the commit message is pretty clear.
> > We'll maybe add that it"s an actual errata.
> 
> Humm, this is not what I meant actually. I only proposed a code source
> comment explaining why this is done this way, the current patch summary
> looked like it will be perfect between /* */ ;-)

I did not want to fill up the source with wordy comments, and settled
for a one-liner. I don't know much about the underlying reasons other
than the fact that LPDDR1 mode of the controller isn't working properly
in self-refresh and that the DDR2 spec is similar enough to work.

The one-liner comment says about the same thing, but not with so
many words. The comment does make it clear that the switch to DDR2
is intentional, and that is all that is needed as protection from some
future cleanup. I mean, anyone seeing that comment and just erasing
the whole thing without further investigation is not doing a very good
job as there is no reason to intentionally switch from LPDDR1 mode to
DDR2 mode, other that the fact that the LPDDR1 mode isn't working for
some reason. That reason is not to be found in the commit message
and I have no information to improve the situation. IMO, the only thing
missing is a pointer to the as yet unreleased errata, which should explain
the situation clearly for any and all interested parties. May I suggest that
someone who cares sends a patch with the comment update when the
errata is released?

If others feel differently, by all means please reword and expand the
comment.

Cheers,
Peter