mtd: spi-nor: keep lock bits if they are non-volatile

Traditionally, Linux unlocks the whole flash because there are legacy
devices which has the write protection bits set by default at startup.
If you actually want to use the flash protection bits, eg. because there
is a read-only part for a bootloader, this automatic unlocking is
harmful. If there is no hardware write protection in place (usually
called WP#), a startup of the kernel just discards this protection.

I've gone through the datasheets of all the flashes (except the Intel
ones where I could not find any datasheet nor reference) which supports
the unlocking feature and looked how the sector protection was
implemented. The currently supported flashes can be divided into the
following two categories:
 (1) block protection bits are non-volatile. Thus they keep their values
     at reset and power-cycle
 (2) flashes where these bits are volatile. After reset or power-cycle,
     the whole memory array is protected.
     (a) some devices needs a special "Global Unprotect" command, eg.
         the Atmel AT25DF041A.
     (b) some devices require to clear the BPn bits in the status
         register.

Due to the reasons above, we do not want to clear the bits for flashes
which belong to category (1). Fortunately for us, only Atmel flashes
fall into category (2a). Implement the "Global Protect" and "Global
Unprotect" commands for these. For (2b) we can use normal block
protection locking scheme.

This patch adds a new flag to indicate the case (2). Only if we have
such a flash we unlock the whole flash array. To be backwards compatible
it also introduces a kernel configuration option which restores the
complete legacy behavior ("Disable write protection on any flashes").
Hopefully, this will clean up "unlock the entire flash for legacy
devices" once and for all.

For reference here are the actually commits which introduced the legacy
behavior (and extended the behavior to other chip manufacturers):

commit f80e521c916cb ("mtd: m25p80: add support for the Intel/Numonyx {16,32,64}0S33B SPI flash chips")
commit ea60658a08f8f ("mtd: m25p80: disable SST software protection bits by default")
commit 7228982442365 ("[MTD] m25p80: fix bug - ATmel spi flash fails to be copied to")

Actually, this might also fix handling of the Atmel AT25DF flashes,
because the original commit 7228982442365 ("[MTD] m25p80: fix bug -
ATmel spi flash fails to be copied to") was writing a 0 to the status
register, which is a "Global Unprotect". This might not be the case in
the current code which only handles the block protection bits BP2, BP1
and BP0. Thus, it depends on the current contents of the status register
if this unlock actually corresponds to a "Global Unprotect" command. In
the worst case, the current code might leave the AT25DF flashes in a
write protected state.

The commit 191f5c2ed4b6f ("mtd: spi-nor: use 16-bit WRR command when QE
is set on spansion flashes") changed that behavior by just clearing BP2
to BP0 instead of writing a 0 to the status register.

Further, the commit 3e0930f109e76 ("mtd: spi-nor: Rework the disabling
of block write protection") expanded the unlock_all() feature to ANY
flash which supports locking.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20201203162959.29589-8-michael@walle.cc

1 files changed, 44 insertions, 0 deletions

diff --git a/drivers/mtd/spi-nor/Kconfig b/drivers/mtd/spi-nor/Kconfig
index ffc4b380f2b1..24cd25de2b8b 100644
--- a/drivers/mtd/spi-nor/Kconfig
+++ b/drivers/mtd/spi-nor/Kconfig
@@ -24,6 +24,50 @@ config MTD_SPI_NOR_USE_4K_SECTORS
 	  Please note that some tools/drivers/filesystems may not work with
 	  4096 B erase size (e.g. UBIFS requires 15 KiB as a minimum).
 
+choice
+	prompt "Software write protection at boot"
+	default MTD_SPI_NOR_SWP_DISABLE_ON_VOLATILE
+
+config MTD_SPI_NOR_SWP_DISABLE
+	bool "Disable SWP on any flashes (legacy behavior)"
+	help
+	  This option disables the software write protection on any SPI
+	  flashes at boot-up.
+
+	  Depending on the flash chip this either clears the block protection
+	  bits or does a "Global Unprotect" command.
+
+	  Don't use this if you intent to use the software write protection
+	  of your SPI flash. This is only to keep backwards compatibility.
+
+config MTD_SPI_NOR_SWP_DISABLE_ON_VOLATILE
+	bool "Disable SWP on flashes w/ volatile protection bits"
+	help
+	  Some SPI flashes have volatile block protection bits, ie. after a
+	  power-up or a reset the flash is software write protected by
+	  default.
+
+	  This option disables the software write protection for these kind
+	  of flashes while keeping it enabled for any other SPI flashes
+	  which have non-volatile write protection bits.
+
+	  If the software write protection will be disabled depending on
+	  the flash either the block protection bits are cleared or a
+	  "Global Unprotect" command is issued.
+
+	  If you are unsure, select this option.
+
+config MTD_SPI_NOR_SWP_KEEP
+	bool "Keep software write protection as is"
+	help
+	  If you select this option the software write protection of any
+	  SPI flashes will not be changed. If your flash is software write
+	  protected or will be automatically software write protected after
+	  power-up you have to manually unlock it before you are able to
+	  write to it.
+
+endchoice
+
 source "drivers/mtd/spi-nor/controllers/Kconfig"
 
 endif # MTD_SPI_NOR