• Tao Ren's avatar
    clocksource/drivers/fttmr010: Fix set_next_event handler · 4f5dbf26
    Tao Ren authored
    [ Upstream commit 4451d3f5
    Currently, the aspeed MATCH1 register is updated to <current_count -
    cycles> in set_next_event handler, with the assumption that COUNT
    register value is preserved when the timer is disabled and it continues
    decrementing after the timer is enabled. But the assumption is wrong:
    RELOAD register is loaded into COUNT register when the aspeed timer is
    enabled, which means the next event may be delayed because timer
    interrupt won't be generated until <0xFFFFFFFF - current_count +
    The problem can be fixed by updating RELOAD register to <cycles>, and
    COUNT register will be re-loaded when the timer is enabled and interrupt
    is generated when COUNT register overflows.
    The test result on Facebook Backpack-CMM BMC hardware (AST2500) shows
    the issue is fixed: without the patch, usleep(100) suspends the process
    for several milliseconds (and sometimes even over 40 milliseconds);
    after applying the fix, usleep(100) takes averagely 240 microseconds to
    return under the same workload level.
    Signed-off-by: default avatarTao Ren <taoren@fb.com>
    Reviewed-by: default avatarLinus Walleij <linus.walleij@linaro.org>
    Tested-by: default avatarLei YU <mine260309@gmail.com>
    Signed-off-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
    Signed-off-by: default avatarSasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>