mtd: spi-nor: only apply reset hacks to broken hardware

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 <[email protected]>

---
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

---
 .../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(-)

Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt +9 -0

index 956bb046e599..f03be904d3c2 100644
@@ -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:
 

drivers/mtd/spi-nor/spi-nor.c +16 -2

index d9c368c44194..f028277fb1ce 100644
@@ -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;

include/linux/mtd/spi-nor.h +0 -0

index e60da0d34cc1..c922e97f205a 100644

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 <[email protected]>

---
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

---
 .../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(-)

Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt +9 -0

index 956bb046e599..f03be904d3c2 100644
@@ -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:
 

drivers/mtd/spi-nor/spi-nor.c +16 -2

index d9c368c44194..f028277fb1ce 100644
@@ -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;

include/linux/mtd/spi-nor.h +0 -0

index e60da0d34cc1..c922e97f205a 100644

Quoted 47 lines · On Fri, 27 Jul 2018 11:33:13 -0700 Brian Norris <[email protected]>…

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.

Quoted 5 lines

Signed-off-by Brian Norris <[email protected]>

---
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,

Boris

Quoted 81 lines

---
 .../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(-)

Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt +9 -0

index 956bb046e599..f03be904d3c2 100644
@@ -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:
 

drivers/mtd/spi-nor/spi-nor.c +16 -2

index d9c368c44194..f028277fb1ce 100644
@@ -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;

include/linux/mtd/spi-nor.h +1 -0

index e60da0d34cc1..c922e97f205a 100644
@@ -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),
 };
 

include/linux/mtd/spi-nor.h enum spi_nor_option_flags { · line 241

Quoted 1 line

include/linux/mtd/spi-nor.h +0 -0

 /**

Quoted 56 lines · Boris Brezillon

Quoted 47 lines · On Fri, 27 Jul 2018 11:33:13 -0700 Brian Norris <[email protected]>…

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.

Quoted 5 lines

Signed-off-by Brian Norris <[email protected]>

---
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

Quoted 4 lines

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.

Quoted 82 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 56 lines · Boris Brezillon

Quoted 47 lines · On Fri, 27 Jul 2018 11:33:13 -0700 Brian Norris <[email protected]>…

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.

Quoted 5 lines

Signed-off-by Brian Norris <[email protected]>

---
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

Quoted 4 lines

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.

Quoted 5 lines

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.

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.

Quoted 94 lines · Boris Brezillon

Quoted 82 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 56 lines · Boris Brezillon

Quoted 47 lines · On Fri, 27 Jul 2018 11:33:13 -0700 Brian Norris <[email protected]>…

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.

Quoted 5 lines

Signed-off-by Brian Norris <[email protected]>

---
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

Quoted 4 lines

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.

Quoted 5 lines

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?

Quoted 6 lines

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.

Quoted 7 lines

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.

Signature · 2 lines

Best regards,
Marek Vasut

Quoted 102 lines · Marek Vasut

Quoted 100 lines · Boris Brezillon

Quoted 82 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 56 lines · Boris Brezillon

Quoted 47 lines · On Fri, 27 Jul 2018 11:33:13 -0700 Brian Norris <[email protected]>…

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.

Quoted 5 lines

Signed-off-by Brian Norris <[email protected]>

---
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

Quoted 4 lines

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.

Quoted 5 lines

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.

Quoted 2 lines

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

And you think that printing a loud warning would be likely to get vendor to make fewer crappy boards?
I think it would just annoy people who aren't in a position to do anything about it.

Quoted 11 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 3 lines · Boris Brezillon

On Fri, 27 Jul 2018 11:33:13 -0700
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;

Quoted 7 lines

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.

Quoted 8 lines

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.

Quoted 1 line

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.
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.

Hi Neil, Boris,

Quoted 13 lines · Boris Brezillon

Quoted 11 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 3 lines · Boris Brezillon

On Fri, 27 Jul 2018 11:33:13 -0700
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;

Quoted 7 lines

Quoted 7 lines

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.

One reason I didn't specifically say something like "not connected", is because IIUC it's actually *possible* to have a robust boot sequence without the RESET# pin -- e.g., if your boot ROM hardcoded a software reset command (just because it's not really standardized doesn't mean one can't do it).

Quoted 9 lines

Quoted 9 lines

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.

Did you read my last response on the original thread? In my understanding, there's always a way to, e.g., b0rk your exception handlers, etc., such that you cannot guarantee your software fallbacks will work. Normally, one would rely on a (hardware) watchdog to do your last resort reset for you, but if said reset cannot also reset your boot flash, then...you're stuck.

IOW, it's impossible.

Is that not an accurate description?

Quoted 5 lines

Quoted 1 line

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?

Quoted 8 lines

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.
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

How else would we help someone figure out what's wrong with their design? My best attempt is to make it quite obvious, as long as they're using vanilla mainline: if their system hangs on reboot (without this property), then it's probably a bad design.

And if instead, someone stuck in this DT property already, the loud warning might suggest the reader look at the DT binding doc or code comments, where I elaborated.

Quoted 4 lines

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.

Brian

Hi Neil, Boris,

Quoted 13 lines · Boris Brezillon

Quoted 11 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 3 lines · Boris Brezillon

On Fri, 27 Jul 2018 11:33:13 -0700
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;

Quoted 7 lines

Quoted 7 lines

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.

One reason I didn't specifically say something like "not connected", is because IIUC it's actually *possible* to have a robust boot sequence without the RESET# pin -- e.g., if your boot ROM hardcoded a software reset command (just because it's not really standardized doesn't mean one can't do it).

Quoted 5 lines · Brian Norris

One reason I didn't specifically say something like "not connected", is because IIUC it's actually *possible* to have a robust boot sequence without the RESET# pin -- e.g., if your boot ROM hardcoded a software reset command (just because it's not really standardized doesn't mean one can't do it).

Based on that, then it sounds like you need a specific compatible string so you too can know how to s/w reset the device.

I guess you are assuming a bootloader didn't leave the flash in an unknown addressing state?

Quoted 82 lines · On Tue, 31 Jul 2018 11:05:11 +1000 NeilBrown <[email protected]> wrote:

Quoted 56 lines · Boris Brezillon

Quoted 47 lines · On Fri, 27 Jul 2018 11:33:13 -0700 Brian Norris <[email protected]>…

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.

Quoted 5 lines

Signed-off-by Brian Norris <[email protected]>

---
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

Quoted 4 lines

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.

Quoted 5 lines

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.
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.

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 <[email protected]>

---
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

---
 .../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(-)

Documentation/devicetree/bindings/mtd/jedec,spi-nor.txt +9 -0

index 956bb046e599..f03be904d3c2 100644
@@ -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:
 

drivers/mtd/spi-nor/spi-nor.c +16 -2

index d9c368c44194..f028277fb1ce 100644
@@ -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;

include/linux/mtd/spi-nor.h +0 -0

index e60da0d34cc1..c922e97f205a 100644

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 <[email protected]>

---
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

---
 .../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(-)