clocksource/drivers/fttmr010: Fix set_next_event handler

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>

1 files changed, 11 insertions, 7 deletions

diff --git a/drivers/clocksource/timer-fttmr010.c b/drivers/clocksource/timer-fttmr010.c
index c020038ebfab..cf93f6419b51 100644
--- a/drivers/clocksource/timer-fttmr010.c
+++ b/drivers/clocksource/timer-fttmr010.c
@@ -130,13 +130,17 @@ static int fttmr010_timer_set_next_event(unsigned long cycles,
         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);