linux-kernel - Re: [PATCH 1/2] clocksource/drivers/exynos_mct: Prioritise Arm arch timer on arm64

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Date:   Mon, 21 Jun 2021 12:24:44 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Will Deacon <will@...nel.org>,
        Chanwoo Choi <cw00.choi@...sung.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] clocksource/drivers/exynos_mct: Prioritise Arm arch
 timer on arm64

On 21/06/2021 12:17, Marek Szyprowski wrote:

[ ... ]

>>>>>> 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 ?
>>>> I think that DT changes is not proper way to keep the order between
>>>> exynos_mct and arch timer.
>>> exynos4_mct_frc_start() is called unconditionally from probe via
>>> exynos4_clocksource_init() so as long as the mct probes first, then the
>>> arch timer should work, no? The rating shouldn't affect that.
>> How do you ensure the exynos mct is probed before the arch timer ?
>>
>> The Makefile provides the right order, but the dependency is implicit.
> 
> In my test systems based on Exynos5433, the bootloader enabled MCT, so 
> kernel can also start with arch_timer then switch to MCT. The probe 
> order also depend on the order of DT nodes, so with the current dts MCT 
> is probed first. This is not the best approach, but so far it works.

Right, generally assuming the firmware is doing the right thing from an
initialization POV is not a good idea :/

But in our case, as pointed by Will, the cpuhotplug ordering gives the
right sequence. So it should be fine.


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