• Brian Norris's avatar
    mtd: spi-nor: only apply reset hacks to broken hardware · bb276262
    Brian Norris authored
    Commit 59b356ff ("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: default avatarBrian Norris <computersforpeace@gmail.com>
    Reviewed-by: default avatarGuenter Roeck <linux@roeck-us.net>
    Signed-off-by: default avatarBoris Brezillon <boris.brezillon@bootlin.com>
    bb276262
jedec,spi-nor.txt 3.15 KB