5.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Ronald Wahl <ronald.wahl@xxxxxxxxxxx> [ Upstream commit 6d3bc4c02d59996d1d3180d8ed409a9d7d5900e0 ] On SAM9 hardware two cascaded 16 bit timers are used to form a 32 bit high resolution timer that is used as scheduler clock when the kernel has been configured that way (CONFIG_ATMEL_CLOCKSOURCE_TCB). The driver initially triggers a reset-to-zero of the two timers but this reset is only performed on the next rising clock. For the first timer this is ok - it will be in the next 60ns (16MHz clock). For the chained second timer this will only happen after the first timer overflows, i.e. after 2^16 clocks (~4ms with a 16MHz clock). So with other words the scheduler clock resets to 0 after the first 2^16 clock cycles. It looks like that the scheduler does not like this and behaves wrongly over its lifetime, e.g. some tasks are scheduled with a long delay. Why that is and if there are additional requirements for this behaviour has not been further analysed. There is a simple fix for resetting the second timer as well when the first timer is reset and this is to set the ATMEL_TC_ASWTRG_SET bit in the Channel Mode register (CMR) of the first timer. This will also rise the TIOA line (clock input of the second timer) when a software trigger respective SYNC is issued. Signed-off-by: Ronald Wahl <ronald.wahl@xxxxxxxxxxx> Acked-by: Alexandre Belloni <alexandre.belloni@xxxxxxxxxxx> Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx> Link: https://lore.kernel.org/r/20231007161803.31342-1-rwahl@xxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- drivers/clocksource/timer-atmel-tcb.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/clocksource/timer-atmel-tcb.c b/drivers/clocksource/timer-atmel-tcb.c index 27af17c995900..2a90c92a9182a 100644 --- a/drivers/clocksource/timer-atmel-tcb.c +++ b/drivers/clocksource/timer-atmel-tcb.c @@ -315,6 +315,7 @@ static void __init tcb_setup_dual_chan(struct atmel_tc *tc, int mck_divisor_idx) writel(mck_divisor_idx /* likely divide-by-8 */ | ATMEL_TC_WAVE | ATMEL_TC_WAVESEL_UP /* free-run */ + | ATMEL_TC_ASWTRG_SET /* TIOA0 rises at software trigger */ | ATMEL_TC_ACPA_SET /* TIOA0 rises at 0 */ | ATMEL_TC_ACPC_CLEAR, /* (duty cycle 50%) */ tcaddr + ATMEL_TC_REG(0, CMR)); -- 2.42.0
