Re: [PATCH v2] rtc-snvs: Add timeouts to avoid kernel lockups

[Alexandre Belloni <alexandre.belloni@free-electrons.com>]

On 16/05/2018 16:45:51-0700, Trent Piepho wrote:
> 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: Trent Piepho <tpiepho@impinj.com>
> ---
>  drivers/rtc/rtc-snvs.c | 105 ++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 70 insertions(+), 35 deletions(-)
> 
Applied, thanks.

-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com