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Message-ID: <20180307233057.54bd35c3@bbrezillon>
Date: Wed, 7 Mar 2018 23:30:57 +0100
From: Boris Brezillon <boris.brezillon@...tlin.com>
To: Pavel Machek <pavel@....cz>
Cc: David Woodhouse <dwmw2@...radead.org>, Greg KH <greg@...ah.com>,
Steve deRosier <derosier@...il.com>,
Richard Weinberger <richard@....at>,
Boris Brezillon <boris.brezillon@...e-electrons.com>,
dedekind1@...il.com, tharvey@...eworks.com,
linux-kernel@...r.kernel.org, stable@...r.kernel.org,
marek.vasut@...il.com, linux-mtd@...ts.infradead.org,
cyrille.pitchen@...ev4u.fr, computersforpeace@...il.com
Subject: Re: [PATCH] ubi: Reject MLC NAND
Hi Pavel,
On Wed, 7 Mar 2018 23:17:33 +0100
Pavel Machek <pavel@....cz> wrote:
> On Wed 2018-03-07 22:11:13, David Woodhouse wrote:
> >
> >
> > On Wed, 2018-03-07 at 14:08 -0800, Steve deRosier wrote:
> > >
> > > To clarify one thing: the reason for this is MLC has actually never
> > > been supported, nor worked properly. The fact that it kinda worked was
> > > incidental and the cause of major problems for people due to that not
> > > being clear. This patch only makes it explicit and avoids people
> > > mistakenly trying to use UBIFS on MLC flash and risking their data and
> > > products. To me, that's what's important.
> > >
> > > This is an important patch, even if all it does is keep people from
> > > loosing data. It also changes the conversation from "I have a
> > > corrupted UBIFS device, BTW it's on MLC..." to "What can we do to get
> > > UBIFS to work on MLC".
>
> Well, for -stable I'd suggest printk(KERN_ALERT ...) but keep the
> system running.
>
> > This is a bug fix.
> >
> > UBI on MLC never worked. It was a bug that we ever permitted it. This
> > is now fixed.
>
> Yeah, well, so lets say I have a working hardware (maybe using
> read-only UBI on MLC), update to next stable kernel, and now kernel
> refuses to see the partition.
Read-only does not save you from the read-disturb issue, and you even
have to take care of programming the full erase-block on some MLC
NANDs, which AFAIR is not done when updating a static volume.
I have one simple question: did you ever play with MLC NANDs or are you
just trolling? If you had, like Richard and I did when working on MLC
support, I'm pretty sure you wouldn't play this "don't backport to
stable" card.
Now, if you volunteer to add reliable MLC support, I can send you a
few boards to play with. I even have a "working but not so tested PoC"
here [1] if you want to finish the job, but please don't do the mistake
of thinking the fix is that simple.
>
> I'll certainly not consider this patch a bug fix.
And apparently a lot of people disagree with you on this point, and I
guess all of them had problems with MLC NANDs.
>
> Removing support for hardware that "only works by mistake" may be good
> idea, but maybe it is slightly too surprising for a -stable.
I wouldn't say "work by mistake" but "seems to work at first but in the
end breaks", so definitely a candidate for -stable IMO.
Regards,
Boris
[1]https://github.com/bbrezillon/linux/tree/nand/mlc
--
Boris Brezillon, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com
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