linux-kernel - Re: [PATCH v2] mtd: spi-nor: clear Winbond Extended Address Reg on switch to 3-byte addressing.

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Message-ID: <87r2n9o5ir.fsf@notabene.neil.brown.name>
Date:   Sat, 21 Apr 2018 08:51:56 +1000
From:   NeilBrown <neil@...wn.name>
To:     Boris Brezillon <boris.brezillon@...tlin.com>
Cc:     Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        Marek Vasut <marek.vasut@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "Brian Norris" <computersforpeace@...il.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Richard Weinberger <richard@....at>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mtd: spi-nor: clear Winbond Extended Address Reg on switch to 3-byte addressing.

On Sat, Apr 21 2018, Boris Brezillon wrote:

> On Sat, 21 Apr 2018 07:28:19 +1000
> NeilBrown <neil@...wn.name> wrote:
>
>> On Fri, Apr 20 2018, Boris Brezillon wrote:
>> 
>> > Hi Neil,
>> >
>> > On Mon, 16 Apr 2018 09:42:30 +1000
>> > NeilBrown <neil@...wn.name> wrote:
>> >  
>> >> Winbond spi-nor flash 32MB and larger have an 'Extended Address
>> >> Register' as one option for addressing beyond 16MB (Macronix
>> >> has the same concept, Spansion has EXTADD bits in the Bank Address
>> >> Register).
>> >> 
>> >> According to section
>> >>    8.2.7 Write Extended Address Register (C5h)
>> >> 
>> >> of the Winbond W25Q256FV data sheet (256M-BIT SPI flash)
>> >> 
>> >>    The Extended Address Register is only effective when the device is
>> >>    in the 3-Byte Address Mode.  When the device operates in the 4-Byte
>> >>    Address Mode (ADS=1), any command with address input of A31-A24
>> >>    will replace the Extended Address Register values. It is
>> >>    recommended to check and update the Extended Address Register if
>> >>    necessary when the device is switched from 4-Byte to 3-Byte Address
>> >>    Mode.
>> >> 
>> >> So the documentation suggests clearing the EAR after switching to
>> >> 3-byte mode.  Experimentation shows that the EAR is *always* one after
>> >> the switch to 3-byte mode, so clearing the EAR is mandatory at
>> >> shutdown for a subsequent 3-byte-addressed reboot to work.
>> >> 
>> >> Note that some SOCs (e.g. MT7621) do not assert a reset line at normal
>> >> reboot, so we cannot rely on hardware reset.  The MT7621 does assert a
>> >> reset line at watchdog-reset.
>> >> 
>> >> Signed-off-by: NeilBrown <neil@...wn.name>  
>> >
>> > We should probably backport the fix. Can you add a Fixes and Cc-stable
>> > tag?  
>> 
>> It's a bit weird having Fixes when this isn't a regression, but I guess
>> it doesn't hurt.
>
> Well, I thought you wanted this patch to be backported to stable
> releases, hence my suggestion. Of course it's not a regression, since
> the bug is here from the beginning, but nonetheless it's fixing a bug.

I have not interest in this patch going to stable.  I'll be perfectly
happy once it lands upstream.

>
>> I chose
>>   Fixes: 59b356ffd0b0 ("mtd: m25p80: restore the status of SPI flash when exiting")
>> as this patch it useless without that one.
>
> Not sure that's how Fixes should be used. IIUC, it should point to the
> first commit where the bug was introduced, so I guess it's 0aa87b7563f1
> ("mtd: m25p80: add support for the windbond w25q256 chip") in this case.

That does make a certain sort of sense, but if you tried applying this
patch to that kernel, it wouldn't apply at all.  So it is hard to say
that it fixes anything there.

I think it is best to drop the fixes/stable tags.  It isn't a
regression, doesn't cause data corruption, doesn't make it possible to
crash the kernel, so it isn't really "stable" material in my mind.

Thanks,
NeilBrown


>
> Now, if you want to restrict the kernel releases this patch should be
> backported to, you can use the '# vX.Y+' suffix in your Cc-stable tag.
>
>> 
>> I also fixed the comment and have resent.
>
> Thanks.

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