• Paul Osmialowski's avatar
    i2c: s3c2410: fix ABBA deadlock by keeping clock prepared · 6b07589d
    Paul Osmialowski authored
    commit 34e81ad5 upstream.
    
    This patch solves deadlock between clock prepare mutex and regmap mutex reported
    by Tomasz Figa in [1] by implementing solution from [2]: "always leave the clock
    of the i2c controller in a prepared state".
    
    [1] https://lkml.org/lkml/2014/7/2/171
    [2] https://lkml.org/lkml/2014/7/2/207
    
    On each i2c transfer handled by s3c24xx_i2c_xfer(), clk_prepare_enable() was
    called, which calls clk_prepare() then clk_enable(). clk_prepare() takes
    prepare_lock mutex before proceeding. Note that i2c transfer functions are
    invoked from many places in kernel, typically with some other additional lock
    held.
    
    It may happen that function on CPU1 (e.g. regmap_update_bits()) has taken a
    mutex (i.e. regmap lock mutex) then it attempts i2c communication in order to
    proceed (so it needs to obtain clock related prepare_lock mutex during transfer
    preparation stage due to clk_prepare() call). At the same time other task on
    CPU0 wants to operate on clock (e.g. to (un)prepare clock for some other reason)
    so it has taken prepare_lock mutex.
    
    CPU0:                        CPU1:
    clk_disable_unused()         regulator_disable()
      clk_prepare_lock()           map->lock(map->lock_arg)
      regmap_read()                s3c24xx_i2c_xfer()
        map->lock(map->lock_arg)     clk_prepare_lock()
    
    Implemented solution from [2] leaves i2c clock prepared. Preparation is done in
    s3c24xx_i2c_probe() function. Without this patch, it is immediately unprepared
    by clk_disable_unprepare() call. I've replaced this call with clk_disable() and
    I've added clk_unprepare() call in s3c24xx_i2c_remove().
    
    The s3c24xx_i2c_xfer() function now uses clk_enable() instead of
    clk_prepare_enable() (and clk_disable() instead of clk_unprepare_disable()).
    Signed-off-by: default avatarPaul Osmialowski <p.osmialowsk@samsung.com>
    Reviewed-by: default avatarKrzysztof Kozlowski <k.kozlowski@samsung.com>
    Signed-off-by: default avatarWolfram Sang <wsa@the-dreams.de>
    [ luis: backported to 3.16: adjusted context ]
    Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
    6b07589d
i2c-s3c2410.c 32.1 KB