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

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Message-ID: <7a3213a4-5f35-17b3-3d05-a5c6f032d1f5@canonical.com>
Date:   Wed, 9 Jun 2021 13:41:33 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH 1/2] clocksource/drivers/exynos_mct: Prioritise Arm arch
 timer on arm64

On 08/06/2021 17:43, 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@...sung.com>
> Cc: Krzysztof Kozlowski <krzk@...nel.org>
> Cc: Chanwoo Choi <cw00.choi@...sung.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Will Deacon <will@...nel.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
> +

Tested on Exynos5422 (32-bit ARM) and it seems to work. I don't have
ARMv8 board to check it. We want Exynos MCT on ARMv8 as well, because
unfortunately it shares the block with Arch timer. This was investigated
by Marek:
https://lore.kernel.org/linux-samsung-soc/20181017123622eucas1p14654c89a8590fd094d638b60ab9af8f0~eZY3j27rs0422004220eucas1p1M@eucas1p1.samsung.com/

Best regards,
Krzysztof