linux-kernel - Re: [PATCH v3 2/2] MIPS: CPS: Optimise delay CPU calibration for SMP

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Message-Id: <cd148cc2-bc18-43f0-9b6d-1b59b45cb400@app.fastmail.com>
Date: Tue, 08 Jul 2025 22:42:37 +0100
From: "Jiaxun Yang" <jiaxun.yang@...goat.com>
To: "Gregory CLEMENT" <gregory.clement@...tlin.com>,
 "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>
Cc: "Vladimir Kondratiev" <vladimir.kondratiev@...ileye.com>,
 Théo Lebrun <theo.lebrun@...tlin.com>,
 "Tawfik Bayouk" <tawfik.bayouk@...ileye.com>,
 "Thomas Petazzoni" <thomas.petazzoni@...tlin.com>,
 "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] MIPS: CPS: Optimise delay CPU calibration for SMP



在2025年7月8日周二 下午2:46，Gregory CLEMENT写道：
> On MIPS architecture with CPS-based SMP support, all CPU cores in the
> same cluster run at the same frequency since they share the same L2
> cache, requiring a fixed CPU/L2 cache ratio.
>
> This allows to implement calibrate_delay_is_known(), which will return
> 0 (triggering calibration) only for the primary CPU of each
> cluster. For other CPUs, we can simply reuse the value from their
> cluster's primary CPU core.
>
> With the introduction of this patch, a configuration running 32 cores
> spread across two clusters sees a significant reduction in boot time
> by approximately 600 milliseconds.
>
> Signed-off-by: Gregory CLEMENT <gregory.clement@...tlin.com>

Reviewed-by: Jiaxun Yang <jiaxun.yang@...goat.com>

> ---
>  arch/mips/kernel/smp-cps.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/arch/mips/kernel/smp-cps.c b/arch/mips/kernel/smp-cps.c
> index 
> 6c5f15293a8e58a701601b242f43ba19a6814f06..22d4f9ff3ae2671b07da5bb149154c686e07b209 
> 100644
> --- a/arch/mips/kernel/smp-cps.c
> +++ b/arch/mips/kernel/smp-cps.c
> @@ -281,6 +281,17 @@ static void __init cps_smp_setup(void)
>  #endif /* CONFIG_MIPS_MT_FPAFF */
>  }
> 
> +unsigned long calibrate_delay_is_known(void)
> +{
> +	int first_cpu_cluster = 0;
> +
> +	/* The calibration has to be done on the primary CPU of the cluster */
> +	if (mips_cps_first_online_in_cluster(&first_cpu_cluster))
> +		return 0;
> +
> +	return cpu_data[first_cpu_cluster].udelay_val;
> +}
> +
>  static void __init cps_prepare_cpus(unsigned int max_cpus)
>  {
>  	unsigned int nclusters, ncores, core_vpes, nvpe = 0, c, cl, cca;
>
> -- 
> 2.47.2

-- 
- Jiaxun