• Trent Piepho's avatar
    rtc: snvs: Add timeouts to avoid kernel lockups · f1e1eb5c
    Trent Piepho authored
    [ Upstream commit cd7f3a24 ]
    
    In order to read correctly from asynchronously updated RTC registers,
    it's necessary to read repeatedly until their values do not change from
    read to read.  It's also necessary to wait for three RTC clock ticks for
    certain operations.  There are no timeouts in this code and these
    operations could possibly loop forever.
    
    To avoid kernel hangs, put in timeouts.
    
    The iMX7d can be configured to stop the SRTC on a tamper event, which
    will lockup the kernel inside this driver as described above.
    
    These hangs can happen when running under qemu, which doesn't emulate
    the SNVS RTC, though currently the driver will refuse to load on qemu
    due to a timeout in the driver probe method.
    
    It could also happen if the SRTC block where somehow placed into reset
    or the slow speed clock that drives the SRTC counter (but not the CPU)
    were to stop.
    
    The symptoms on a two core iMX7d are a work queue hang on
    rtc_timer_do_work(), which eventually blocks a systemd fsnotify
    operation that triggers a work queue flush, causing systemd to hang and
    thus causing all services that should be started by systemd, like a
    console getty, to fail to start or stop.
    
    Also optimize the wait code to wait less.  It only needs to wait for the
    clock to advance three ticks, not to see it change three times.
    
    Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
    Cc: Alessandro Zummo <a.zummo@towertech.it>
    Cc: Fabio Estevam <fabio.estevam@nxp.com>
    Cc: Shawn Guo <shawn.guo@linaro.org>
    Cc: Bryan O'Donoghue <pure.logic@nexus-software.ie>
    Signed-off-by: default avatarTrent Piepho <tpiepho@impinj.com>
    Signed-off-by: default avatarAlexandre Belloni <alexandre.belloni@bootlin.com>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
    f1e1eb5c
rtc-snvs.c 10.5 KB