linux-kernel - [PATCH AUTOSEL 5.4 4/5] clocksource/drivers/timer-atmel-tcb: Fix initialization on SAM9 hardware

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Message-ID: <20231106231655.3735872-4-sashal@kernel.org>
Date:   Mon,  6 Nov 2023 18:16:51 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Ronald Wahl <ronald.wahl@...itan.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Sasha Levin <sashal@...nel.org>, tglx@...utronix.de,
        nicolas.ferre@...rochip.com, claudiu.beznea@...on.dev,
        linux-arm-kernel@...ts.infradead.org
Subject: [PATCH AUTOSEL 5.4 4/5] clocksource/drivers/timer-atmel-tcb: Fix initialization on SAM9 hardware

From: Ronald Wahl <ronald.wahl@...itan.com>

[ 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@...itan.com>
Acked-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
Link: https://lore.kernel.org/r/20231007161803.31342-1-rwahl@gmx.de
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 7427b07495a89..906c1bfdccad3 100644
--- a/drivers/clocksource/timer-atmel-tcb.c
+++ b/drivers/clocksource/timer-atmel-tcb.c
@@ -310,6 +310,7 @@ 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