• Peter Rosin's avatar
    i2c: mux: relax locking of the top i2c adapter during mux-locked muxing · 6ef91fcc
    Peter Rosin authored
    With a i2c topology like the following
    
                           GPIO ---|  ------ BAT1
                            |      v /
       I2C  -----+----------+---- MUX
                 |                   \
               EEPROM                 ------ BAT2
    
    there is a locking problem with the GPIO controller since it is a client
    on the same i2c bus that it muxes. Transfers to the mux clients (e.g. BAT1)
    will lock the whole i2c bus prior to attempting to switch the mux to the
    correct i2c segment. In the above case, the GPIO device is an I/O expander
    with an i2c interface, and since the GPIO subsystem knows nothing (and
    rightfully so) about the lockless needs of the i2c mux code, this results
    in a deadlock when the GPIO driver issues i2c transfers to modify the
    mux.
    
    So, observing that while it is needed to have the i2c bus locked during the
    actual MUX update in order to avoid random garbage on the slave side, it
    is not strictly a must to have it locked over the whole sequence of a full
    select-transfer-deselect mux client operation. The mux itself needs to be
    locked, so transfers to clients behind the mux are serialized, and the mux
    needs to be stable during all i2c traffic (otherwise individual mux slave
    segments might see garbage, or worse).
    
    Introduce this new locking concept as "mux-locked" muxes, and call the
    pre-existing mux locking scheme "parent-locked".
    
    Modify the i2c mux locking so that muxes that are "mux-locked" locks only
    the muxes on the parent adapter instead of the whole i2c bus when there is
    a transfer to the slave side of the mux. This lock serializes transfers to
    the slave side of the muxes on the parent adapter.
    
    Add code to i2c-mux-gpio and i2c-mux-pinctrl that checks if all involved
    gpio/pinctrl devices have a parent that is an i2c adapter in the same
    adapter tree that is muxed, and request a "mux-locked mux" if that is the
    case.
    
    Modify the select-transfer-deselect code for "mux-locked" muxes so
    that each of the select-transfer-deselect ops locks the mux parent
    adapter individually.
    Signed-off-by: 's avatarPeter Rosin <peda@axentia.se>
    Signed-off-by: 's avatarWolfram Sang <wsa@the-dreams.de>
    6ef91fcc
i2c-mux.h 2.16 KB