• Johan Hovold's avatar
    Revert "tty_port: register tty ports with serdev bus" · 95a639d1
    Johan Hovold authored
    [ Upstream commit d3ba126a ]
    
    This reverts commit 8ee3fde0.
    
    The new serdev bus hooked into the tty layer in
    tty_port_register_device() by registering a serdev controller instead of
    a tty device whenever a serdev client is present, and by deregistering
    the controller in the tty-port destructor. This is broken in several
    ways:
    
    Firstly, it leads to a NULL-pointer dereference whenever a tty driver
    later deregisters its devices as no corresponding character device will
    exist.
    
    Secondly, far from every tty driver uses tty-port refcounting (e.g.
    serial core) so the serdev devices might never be deregistered or
    deallocated.
    
    Thirdly, deregistering at tty-port destruction is too late as the
    underlying device and structures may be long gone by then. A port is not
    released before an open tty device is closed, something which a
    registered serdev client can prevent from ever happening. A driver
    callback while the device is gone typically also leads to crashes.
    
    Many tty drivers even keep their ports around until the driver is
    unloaded (e.g. serial core), something which even if a late callback
    never happens, leads to leaks if a device is unbound from its driver and
    is later rebound.
    
    The right solution here is to add a new tty_port_unregister_device()
    helper and to never call tty_device_unregister() whenever the port has
    been claimed by serdev, but since this requires modifying just about
    every tty driver (and multiple subsystems) it will need to be done
    incrementally.
    
    Reverting the offending patch is the first step in fixing the broken
    lifetime assumptions. A follow-up patch will add a new pair of
    tty-device registration helpers, which a vetted tty driver can use to
    support serdev (initially serial core). When every tty driver uses the
    serdev helpers (at least for deregistration), we can add serdev
    registration to tty_port_register_device() again.
    
    Note that this also fixes another issue with serdev, which currently
    allocates and registers a serdev controller for every tty device
    registered using tty_port_device_register() only to immediately
    deregister and deallocate it when the corresponding OF node or serdev
    child node is missing. This should be addressed before enabling serdev
    for hot-pluggable buses.
    Signed-off-by: default avatarJohan Hovold <johan@kernel.org>
    Reviewed-by: default avatarRob Herring <robh@kernel.org>
    Cc: stable <stable@vger.kernel.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: default avatarSasha Levin <alexander.levin@verizon.com>
    95a639d1
tty_port.c 16.1 KB