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Date:   Fri, 13 Oct 2023 12:23:11 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Ronald Wahl <rwahl@gmx.de>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Claudiu Beznea <claudiu.beznea@tuxon.dev>,
        Ronald Wahl <ronald.wahl@raritan.com>
Subject: Re: [RESEND][PATCH] clocksource/drivers/timer-atmel-tcb: Fix
 initialization on SAM9 hardware
Message-ID: <20231013102311929c3fab@mail.local>
References: <20231007161803.31342-1-rwahl@gmx.de>
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On 07/10/2023 18:17:13+0200, Ronald Wahl wrote:
> From: Ronald Wahl <ronald.wahl@raritan.com>
> 
> 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@raritan.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

> ---
>  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 27af17c99590..2a90c92a9182 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));
> --
> 2.41.0
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com