Re: [PATCH] mtd: spi-nor: only apply reset hacks to broken hardware

[Marek Vasut <marek.vasut@gmail.com>]

On 07/31/2018 10:12 PM, Boris Brezillon wrote:
> On Tue, 31 Jul 2018 11:05:11 +1000
> NeilBrown <neilb@suse.com> wrote:
> 
>> On Fri, Jul 27 2018, Boris Brezillon wrote:
>>
>>> On Fri, 27 Jul 2018 11:33:13 -0700
>>> Brian Norris <computersforpeace@gmail.com> wrote:
>>>  
>>>> Commit 59b356ffd0b0 ("mtd: m25p80: restore the status of SPI flash when
>>>> exiting") is the latest from a long history of attempts to add reboot
>>>> handling to handle stateful addressing modes on SPI flash. Some prior
>>>> mostly-related discussions:
>>>>
>>>> http://lists.infradead.org/pipermail/linux-mtd/2013-March/046343.html
>>>> [PATCH 1/3] mtd: m25p80: utilize dedicated 4-byte addressing commands
>>>>
>>>> http://lists.infradead.org/pipermail/barebox/2014-September/020682.html
>>>> [RFC] MTD m25p80 3-byte addressing and boot problem
>>>>
>>>> http://lists.infradead.org/pipermail/linux-mtd/2015-February/057683.html
>>>> [PATCH 2/2] m25p80: if supported put chip to deep power down if not used
>>>>
>>>> Previously, attempts to add reboot-time software reset handling were
>>>> rejected, but the latest attempt was not.
>>>>
>>>> Quick summary of the problem:
>>>> Some systems (e.g., boot ROM or bootloader) assume that they can read
>>>> initial boot code from their SPI flash using 3-byte addressing. If the
>>>> flash is left in 4-byte mode after reset, these systems won't boot. The
>>>> above patch provided a shutdown/remove hook to attempt to reset the
>>>> addressing mode before we reboot. Notably, this patch misses out on
>>>> huge classes of unexpected reboots (e.g., crashes, watchdog resets).
>>>>
>>>> Unfortunately, it is essentially impossible to solve this problem 100%:
>>>> if your system doesn't know how to reset the SPI flash to power-on
>>>> defaults at initialization time, no amount of software can really rescue
>>>> you -- there will always be a chance of some unexpected reset that
>>>> leaves your flash in an addressing mode that your boot sequence didn't
>>>> expect.
>>>>
>>>> While it is not directly harmful to perform hacks like the
>>>> aforementioned commit on all 4-byte addressing flash, a
>>>> properly-designed system should not need the hack -- and in fact,
>>>> providing this hack may mask the fact that a given system is indeed
>>>> broken. So this patch attempts to apply this unsound hack more narrowly,
>>>> providing a strong suggestion to developers and system designers that
>>>> this is truly a hack. With luck, system designers can catch their errors
>>>> early on in their development cycle, rather than applying this hack long
>>>> term. But apparently enough systems are out in the wild that we still
>>>> have to provide this hack.
>>>>
>>>> Document a new device tree property to denote systems that do not have a
>>>> proper hardware (or software) reset mechanism, and apply the hack (with
>>>> a loud warning) only in this case.
>>>>
>>>> Signed-off-by: Brian Norris <computersforpeace@gmail.com>
>>>> ---
>>>> Note that I intentionall didn't split the documentation patch. It seems
>>>> clearer to do these together IMO, but if it's *really* important to
>>>> someone...I can resend  
>>>
>>> I'm fine with that.
>>>
>>> I'll leave Neil some time to review/test/comment on the patch before
>>> queuing it, but it looks good to me.  
>>
>> Thanks.
>> I can confirm that if I apply this patch, my system won't reboot
>> properly (as expected), and if I then add
>>
>> 		broken-flash-reset;
>>
>> to the jedec,spi-nor device, it starts functioning correctly again.
>>
>> I don't like the pejorative "broken", and it also suggests that a thing
>> used to work, but something happened to break it - this is not
>> accurate.
>> I would prefer something like "reset-not-connected" which is an accurate
>> description of the state of the hardware.
>>
>> I also think that having a WARN_ON is an over-reaction.  Certainly a
>> warning could be appropriate, but just one pr_warn() should be enough.
>> The "problem" is unlikely in practice, and loudly warning people that an
>> asteroid might kill them isn't particularly helpful.
>>
>> I genuinely think that if the system fails to reboot, then Linux is at
>> fault. I accept that changing Linux to be completely robust might be
>> more trouble than it is worth, but I don't accept that it is impossible.
>>
>> But I don't intend to fight either of these battles.
> 
> Does that mean you're accepting this change? Brian, any comment on what
> Neil said?
> 
> To be honest, I hate being in the middle of this discussion without
> having been involved in the first decision to accept such workarounds.
> I keep thinking that making boards that do not have reset properly
> wired less likely to fail rebooting is a wise decision, but I also
> agree with Brian when he says we should inform people that their design
> is unreliable.

Hiding the issue in most cases only leads to vendors making more such
crippled boards and never learning.

> The main problem I see here, is that adding this prop won't help people
> figuring out what is wrong with their design, it will just help them
> workaround the problem when they find out, and it might already be to
> late to fix the HW design. But maybe it's not what we're trying to do
> here. Maybe we just want to warn users that rebooting such boards is a
> risky procedure.

The thing is, this is not a workaround, it's just a way of hiding the
problem because the problem does not go away completely. There are still
scenarios in which the system will fail.

-- 
Best regards,
Marek Vasut

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