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

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Date:	Mon, 26 Jan 2015 17:11:24 +0100
From:	Sylvain Rochet <sylvain.rochet@...secur.com>
To:	Nicolas Ferre <nicolas.ferre@...el.com>
Cc:	Peter Rosin <peda@...ntia.se>, 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.

Hello Nicolas and Peter,

On Mon, Jan 26, 2015 at 05:04:25PM +0100, Nicolas Ferre wrote:
> Le 26/01/2015 16:58, Peter Rosin a écrit :
> > 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?
> 
> That's the option that I'll take.
> 
> Let's go for it (and anyone remind me if I don't when the errata is
> released).

I fully agree with that, a pointer to a datasheet errata is exactly the 
same thing as explaining the issue.

Sylvain
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