On Fri, Jul 27 2018, Boris Brezillon wrote:
> On Fri, 27 Jul 2018 11:33:13 -0700
> Brian Norris <computersforpeace@xxxxxxxxx> 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@xxxxxxxxx>
>> ---
>> 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.
Thanks,
NeilBrown
>
> Thanks,
>
> Boris
>
>> ---
>> .../devicetree/bindings/mtd/jedec,spi-nor.txt | 9 +++++++++
>> drivers/mtd/spi-nor/spi-nor.c | 18 ++++++++++++++++--
>> include/linux/mtd/spi-nor.h | 1 +
>> 3 files changed, 26 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
>> index 956bb046e599..f03be904d3c2 100644
>> --- a/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
>> +++ b/Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt
>> @@ -69,6 +69,15 @@ Optional properties:
>> all chips and support for it can not be detected at runtime.
>> Refer to your chips' datasheet to check if this is supported
>> by your chip.
>> +- broken-flash-reset : Some flash devices utilize stateful addressing modes
>> + (e.g., for 32-bit addressing) which need to be managed
>> + carefully by a system. Because these sorts of flash don't
>> + have a standardized software reset command, and because some
>> + systems don't toggle the flash RESET# pin upon system reset
>> + (if the pin even exists at all), there are systems which
>> + cannot reboot properly if the flash is left in the "wrong"
>> + state. This boolean flag can be used on such systems, to
>> + denote the absence of a reliable reset mechanism.
>>
>> Example:
>>
>> diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
>> index d9c368c44194..f028277fb1ce 100644
>> --- a/drivers/mtd/spi-nor/spi-nor.c
>> +++ b/drivers/mtd/spi-nor/spi-nor.c
>> @@ -2757,8 +2757,18 @@ static int spi_nor_init(struct spi_nor *nor)
>>
>> if ((nor->addr_width == 4) &&
>> (JEDEC_MFR(nor->info) != SNOR_MFR_SPANSION) &&
>> - !(nor->info->flags & SPI_NOR_4B_OPCODES))
>> + !(nor->info->flags & SPI_NOR_4B_OPCODES)) {
>> + /*
>> + * If the RESET# pin isn't hooked up properly, or the system
>> + * otherwise doesn't perform a reset command in the boot
>> + * sequence, it's impossible to 100% protect against unexpected
>> + * reboots (e.g., crashes). Warn the user (or hopefully, system
>> + * designer) that this is bad.
>> + */
>> + WARN_ONCE(nor->flags & SNOR_F_BROKEN_RESET,
>> + "enabling reset hack; may not recover from unexpected reboots\n");
>> set_4byte(nor, nor->info, 1);
>> + }
>>
>> return 0;
>> }
>> @@ -2781,7 +2791,8 @@ void spi_nor_restore(struct spi_nor *nor)
>> /* restore the addressing mode */
>> if ((nor->addr_width == 4) &&
>> (JEDEC_MFR(nor->info) != SNOR_MFR_SPANSION) &&
>> - !(nor->info->flags & SPI_NOR_4B_OPCODES))
>> + !(nor->info->flags & SPI_NOR_4B_OPCODES) &&
>> + (nor->flags & SNOR_F_BROKEN_RESET))
>> set_4byte(nor, nor->info, 0);
>> }
>> EXPORT_SYMBOL_GPL(spi_nor_restore);
>> @@ -2911,6 +2922,9 @@ int spi_nor_scan(struct spi_nor *nor, const char *name,
>> params.hwcaps.mask |= SNOR_HWCAPS_READ_FAST;
>> }
>>
>> + if (of_property_read_bool(np, "broken-flash-reset"))
>> + nor->flags |= SNOR_F_BROKEN_RESET;
>> +
>> /* Some devices cannot do fast-read, no matter what DT tells us */
>> if (info->flags & SPI_NOR_NO_FR)
>> params.hwcaps.mask &= ~SNOR_HWCAPS_READ_FAST;
>> diff --git a/include/linux/mtd/spi-nor.h b/include/linux/mtd/spi-nor.h
>> index e60da0d34cc1..c922e97f205a 100644
>> --- a/include/linux/mtd/spi-nor.h
>> +++ b/include/linux/mtd/spi-nor.h
>> @@ -235,6 +235,7 @@ enum spi_nor_option_flags {
>> SNOR_F_S3AN_ADDR_DEFAULT = BIT(3),
>> SNOR_F_READY_XSR_RDY = BIT(4),
>> SNOR_F_USE_CLSR = BIT(5),
>> + SNOR_F_BROKEN_RESET = BIT(6),
>> };
>>
>> /**
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