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Subject: Re: [PATCH 1/2] clocksource/drivers/exynos_mct: Prioritise Arm arch
 timer on arm64
To:     Chanwoo Choi <cw00.choi@samsung.com>,
        Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org
Cc:     linux-arm-kernel@lists.infradead.org,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20210608154341.10794-1-will@kernel.org>
 <CGME20210608154400epcas1p1b22fd50629611a9475cb4d2b8dd9442d@epcas1p1.samsung.com>
 <20210608154341.10794-2-will@kernel.org>
 <466bfc19-2260-87c6-c458-b43cf23617e3@samsung.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <2a0181ea-a26e-65e9-16f6-cc233b6b296f@linaro.org>
Date:   Wed, 16 Jun 2021 17:25:20 +0200
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On 10/06/2021 03:03, Chanwoo Choi wrote:
> Hi,
> 
> On 6/9/21 12:43 AM, Will Deacon wrote:
>> All arm64 CPUs feature an architected timer, which offers a relatively
>> low-latency interface to a per-cpu clocksource and timer. For the most
>> part, using this interface is a no-brainer, with the exception of SoCs
>> where it cannot be used to wake up from deep idle state (i.e.
>> CLOCK_EVT_FEAT_C3STOP is set).
>>
>> On the contrary, the Exynos MCT is extremely slow to access yet can be
>> used as a wakeup source. In preparation for using the Exynos MCT as a
>> potential wakeup timer for the Arm architected timer, reduce its ratings
>> so that the architected timer is preferred.
>>
>> This effectively reverts the decision made in 6282edb72bed
>> ("clocksource/drivers/exynos_mct: Increase priority over ARM arch timer")
>> for arm64, as the reasoning for the original change was to work around
>> a 32-bit SoC design.
>>
>> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
>> Cc: Krzysztof Kozlowski <krzk@kernel.org>
>> Cc: Chanwoo Choi <cw00.choi@samsung.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Will Deacon <will@kernel.org>
>> ---
>>  drivers/clocksource/exynos_mct.c | 13 +++++++++++--
>>  1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
>> index fabad79baafc..804d3e01c8f4 100644
>> --- a/drivers/clocksource/exynos_mct.c
>> +++ b/drivers/clocksource/exynos_mct.c
>> @@ -51,6 +51,15 @@
>>  
>>  #define TICK_BASE_CNT	1
>>  
>> +#ifdef CONFIG_ARM
>> +/* Use values higher than ARM arch timer. See 6282edb72bed. */
>> +#define MCT_CLKSOURCE_RATING		450
>> +#define MCT_CLKEVENTS_RATING		500
>> +#else
>> +#define MCT_CLKSOURCE_RATING		350
>> +#define MCT_CLKEVENTS_RATING		350
>> +#endif
>> +
>>  enum {
>>  	MCT_INT_SPI,
>>  	MCT_INT_PPI
>> @@ -206,7 +215,7 @@ static void exynos4_frc_resume(struct clocksource *cs)
>>  
>>  static struct clocksource mct_frc = {
>>  	.name		= "mct-frc",
>> -	.rating		= 450,	/* use value higher than ARM arch timer */
>> +	.rating		= MCT_CLKSOURCE_RATING,
>>  	.read		= exynos4_frc_read,
>>  	.mask		= CLOCKSOURCE_MASK(32),
>>  	.flags		= CLOCK_SOURCE_IS_CONTINUOUS,
>> @@ -457,7 +466,7 @@ static int exynos4_mct_starting_cpu(unsigned int cpu)
>>  	evt->set_state_oneshot_stopped = set_state_shutdown;
>>  	evt->tick_resume = set_state_shutdown;
>>  	evt->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT;
>> -	evt->rating = 500;	/* use value higher than ARM arch timer */
>> +	evt->rating = MCT_CLKEVENTS_RATING,
>>  
>>  	exynos4_mct_write(TICK_BASE_CNT, mevt->base + MCT_L_TCNTB_OFFSET);
>>  
>>
> 
> I'm not sure that exynos mct is working without problem
> such as the case of 6282edb72bed.
> As described on On ,6282edb72bed the arch timer on exynos SoC
> depends on Exynos MCT device. the arch timer is not able to work
> without Exynos MCT because of using the common module.

Is it possible to change the DT to have a phandle to the exynos_mct, so
it will be probed before the arch_arm_timer ?


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