4.14-stable review patch. If anyone has any objections, please let me know.
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From: Tao Ren taoren@fb.com
[ Upstream commit 4451d3f59f2a6f95e5d205c2d04ea072955d080d ]
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 + cycles>.
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: Tao Ren taoren@fb.com Reviewed-by: Linus Walleij linus.walleij@linaro.org Tested-by: Lei YU mine260309@gmail.com Signed-off-by: Daniel Lezcano daniel.lezcano@linaro.org Signed-off-by: Sasha Levin alexander.levin@microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/clocksource/timer-fttmr010.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-)
--- a/drivers/clocksource/timer-fttmr010.c +++ b/drivers/clocksource/timer-fttmr010.c @@ -130,13 +130,17 @@ static int fttmr010_timer_set_next_event cr &= ~fttmr010->t1_enable_val; writel(cr, fttmr010->base + TIMER_CR);
- /* Setup the match register forward/backward in time */ - cr = readl(fttmr010->base + TIMER1_COUNT); - if (fttmr010->count_down) - cr -= cycles; - else - cr += cycles; - writel(cr, fttmr010->base + TIMER1_MATCH1); + if (fttmr010->count_down) { + /* + * ASPEED Timer Controller will load TIMER1_LOAD register + * into TIMER1_COUNT register when the timer is re-enabled. + */ + writel(cycles, fttmr010->base + TIMER1_LOAD); + } else { + /* Setup the match register forward in time */ + cr = readl(fttmr010->base + TIMER1_COUNT); + writel(cr + cycles, fttmr010->base + TIMER1_MATCH1); + }
/* Start */ cr = readl(fttmr010->base + TIMER_CR);