• Roman Volkov's avatar
    clocksource/drivers/vt8500: Increase the minimum delta · 142ad71d
    Roman Volkov authored
    commit f9eccf24 upstream.
    
    The vt8500 clocksource driver declares itself as capable to handle the
    minimum delay of 4 cycles by passing the value into
    clockevents_config_and_register(). The vt8500_timer_set_next_event()
    requires the passed cycles value to be at least 16. The impact is that
    userspace hangs in nanosleep() calls with small delay intervals.
    
    This problem is reproducible in Linux 4.2 starting from:
    c6eb3f70 ('hrtimer: Get rid of hrtimer softirq')
    
    From Russell King, more detailed explanation:
    
    "It's a speciality of the StrongARM/PXA hardware. It takes a certain
    number of OSCR cycles for the value written to hit the compare registers.
    So, if a very small delta is written (eg, the compare register is written
    with a value of OSCR + 1), the OSCR will have incremented past this value
    before it hits the underlying hardware. The result is, that you end up
    waiting a very long time for the OSCR to wrap before the event fires.
    
    So, we introduce a check in set_next_event() to detect this and return
    -ETIME if the calculated delta is too small, which causes the generic
    clockevents code to retry after adding the min_delta specified in
    clockevents_config_and_register() to the current time value.
    
    min_delta must be sufficient that we don't re-trip the -ETIME check - if
    we do, we will return -ETIME, forward the next event time, try to set it,
    return -ETIME again, and basically lock the system up. So, min_delta
    must be larger than the check inside set_next_event(). A factor of two
    was chosen to ensure that this situation would never occur.
    
    The PXA code worked on PXA systems for years, and I'd suggest no one
    changes this mechanism without access to a wide range of PXA systems,
    otherwise they're risking breakage."
    
    Cc: Russell King <linux@arm.linux.org.uk>
    Acked-by: default avatarAlexey Charkov <alchark@gmail.com>
    Signed-off-by: default avatarRoman Volkov <rvolkov@v1ros.org>
    Signed-off-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    142ad71d
vt8500_timer.c 4.59 KB