linux-kernel - Re: [PATCH] clocksource/drivers/exynos_mct: Support using only local timer

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Message-ID: <08992f48-6cb6-8dc0-33d2-f18f942d2bee@canonical.com>
Date:   Mon, 7 Mar 2022 10:06:26 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     kernel@...s.com, Alim Akhtar <alim.akhtar@...sung.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH] clocksource/drivers/exynos_mct: Support using only local
 timer

On 07/03/2022 09:32, Vincent Whitchurch wrote:
> The ARTPEC-8 SoC has a quad-core Cortex-A53 and a single-core Cortex-A5
> which share one MCT with one global and eight local timers.
> 
> The Cortex-A53 boots first and starts the global FRC and also registers
> a clock events device using the global timer.  (This global timer clock
> events is usually replaced by arch timer clock events for each of the
> cores.)
> 
> When the A5 boots, we should not use the global timer interrupts or
> write to the global timer registers.  This is because even if there are
> four global comparators, the control bits for all four are in the same
> registers, and we would need to synchronize between the cpus.  Instead,
> the global timer FRC (already started by the A53) should be used as the
> clock source, and one of the local timers which are not used by the A53
> can be used for clock events on the A5.
> 
> To support this, add a module param to set the local timer starting
> index.  If this parameter is non-zero, the global timer clock events
> device is not registered and we don't write to the global FRC if it is
> already started.
> 
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> ---
>  drivers/clocksource/exynos_mct.c | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)

This should not be send separately from the previous patch.

> 
> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
> index f29c812b70c9..7ea2919b1808 100644
> --- a/drivers/clocksource/exynos_mct.c
> +++ b/drivers/clocksource/exynos_mct.c
> @@ -33,7 +33,7 @@
>  #define EXYNOS4_MCT_G_INT_ENB		EXYNOS4_MCTREG(0x248)
>  #define EXYNOS4_MCT_G_WSTAT		EXYNOS4_MCTREG(0x24C)
>  #define _EXYNOS4_MCT_L_BASE		EXYNOS4_MCTREG(0x300)
> -#define EXYNOS4_MCT_L_BASE(x)		(_EXYNOS4_MCT_L_BASE + (0x100 * x))
> +#define EXYNOS4_MCT_L_BASE(x)		(_EXYNOS4_MCT_L_BASE + (0x100 * (x)))
>  #define EXYNOS4_MCT_L_MASK		(0xffffff00)
>  
>  #define MCT_L_TCNTB_OFFSET		(0x00)
> @@ -77,6 +77,13 @@ static unsigned long clk_rate;
>  static unsigned int mct_int_type;
>  static int mct_irqs[MCT_NR_IRQS];
>  
> +/*
> + * First local timer index to use.  If non-zero, global
> + * timer is not written to.
> + */
> +static unsigned int mct_local_idx;
> +module_param_named(local_idx, mct_local_idx, int, 0);

No, it's a no go. Please use dedicated compatible if you need specific
quirks.

> +
>  struct mct_clock_event_device {
>  	struct clock_event_device evt;
>  	unsigned long base;
> @@ -157,6 +164,17 @@ static void exynos4_mct_frc_start(void)
>  	u32 reg;
>  
>  	reg = readl_relaxed(reg_base + EXYNOS4_MCT_G_TCON);
> +
> +	/*
> +	 * If the FRC is already running, we don't need to start it again.  We
> +	 * could probably just do this on all systems, but, to avoid any risk
> +	 * for regressions, we only do it on systems where it's absolutely
> +	 * necessary (i.e., on systems where writes to the global registers
> +	 * need to be avoided).
> +	 */
> +	if (mct_local_idx && (reg & MCT_G_TCON_START))
> +		return;

I don't get it. exynos4_mct_frc_start() is called exactly only once in
the system - during init. Not once per every CPU or cluster (I
understood you have two clusters, right?).

Best regards,
Krzysztof