linux-kernel - Re: [PATCH mvebu v3 05/10] clk: mvebu: armada-37xx-periph: Fix switching CPU freq from 250 Mhz to 1 GHz

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Date:   Mon, 29 Mar 2021 16:48:04 +0200
From:   Gregory CLEMENT <gregory.clement@...tlin.com>
To:     Pali Rohár <pali@...nel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-clk@...r.kernel.org
Cc:     Marek Behún <kabel@...nel.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Tomasz Maciej Nowak <tmn505@...il.com>,
        Luka Perkov <luka.perkov@...tura.hr>,
        Andre Heider <a.heider@...il.com>,
        Vladimir Vid <vladimir.vid@...tura.hr>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Gérald Kerma <gerald@....net>,
        Konstantin Porotchkin <kostap@...vell.com>
Subject: Re: [PATCH mvebu v3 05/10] clk: mvebu: armada-37xx-periph: Fix
 switching CPU freq from 250 Mhz to 1 GHz

Pali Rohár <pali@...nel.org> writes:

> It was observed that the workaround introduced by commit 61c40f35f5cd
> ("clk: mvebu: armada-37xx-periph: Fix switching CPU rate from 300Mhz to
> 1.2GHz") when base CPU frequency is 1.2 GHz is also required when base
> CPU frequency is 1 GHz. Otherwise switching CPU frequency directly from
> L2 (250 MHz) to L0 (1 GHz) causes a crash.
>
> When base CPU frequency is just 800 MHz no crashed were observed during
> switch from L2 to L0.
>
> Signed-off-by: Pali Rohár <pali@...nel.org>
> Acked-by: Stephen Boyd <sboyd@...nel.org>
> Tested-by: Tomasz Maciej Nowak <tmn505@...il.com>
> Tested-by: Anders Trier Olesen <anders.trier.olesen@...il.com>
> Tested-by: Philip Soares <philips@...isense.com>
> Fixes: 2089dc33ea0e ("clk: mvebu: armada-37xx-periph: add DVFS support for cpu clocks")
> Cc: stable@...r.kernel.org # 61c40f35f5cd ("clk: mvebu: armada-37xx-periph: Fix switching CPU rate from 300Mhz to 1.2GHz")


Acked-by: Gregory CLEMENT <gregory.clement@...tlin.com>

Thanks,

Gregory

> ---
>  drivers/clk/mvebu/armada-37xx-periph.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/clk/mvebu/armada-37xx-periph.c b/drivers/clk/mvebu/armada-37xx-periph.c
> index 6507bd2c5f31..b15e177bea7e 100644
> --- a/drivers/clk/mvebu/armada-37xx-periph.c
> +++ b/drivers/clk/mvebu/armada-37xx-periph.c
> @@ -487,8 +487,10 @@ static long clk_pm_cpu_round_rate(struct clk_hw *hw, unsigned long rate,
>  }
>  
>  /*
> - * Switching the CPU from the L2 or L3 frequencies (300 and 200 Mhz
> - * respectively) to L0 frequency (1.2 Ghz) requires a significant
> + * Workaround when base CPU frequnecy is 1000 or 1200 MHz
> + *
> + * Switching the CPU from the L2 or L3 frequencies (250/300 or 200 MHz
> + * respectively) to L0 frequency (1/1.2 GHz) requires a significant
>   * amount of time to let VDD stabilize to the appropriate
>   * voltage. This amount of time is large enough that it cannot be
>   * covered by the hardware countdown register. Due to this, the CPU
> @@ -498,15 +500,15 @@ static long clk_pm_cpu_round_rate(struct clk_hw *hw, unsigned long rate,
>   * To work around this problem, we prevent switching directly from the
>   * L2/L3 frequencies to the L0 frequency, and instead switch to the L1
>   * frequency in-between. The sequence therefore becomes:
> - * 1. First switch from L2/L3(200/300MHz) to L1(600MHZ)
> + * 1. First switch from L2/L3 (200/250/300 MHz) to L1 (500/600 MHz)
>   * 2. Sleep 20ms for stabling VDD voltage
> - * 3. Then switch from L1(600MHZ) to L0(1200Mhz).
> + * 3. Then switch from L1 (500/600 MHz) to L0 (1000/1200 MHz).
>   */
>  static void clk_pm_cpu_set_rate_wa(unsigned long rate, struct regmap *base)
>  {
>  	unsigned int cur_level;
>  
> -	if (rate != 1200 * 1000 * 1000)
> +	if (rate < 1000 * 1000 * 1000)
>  		return;
>  
>  	regmap_read(base, ARMADA_37XX_NB_CPU_LOAD, &cur_level);
> -- 
> 2.20.1
>

-- 
Gregory Clement, Bootlin
Embedded Linux and Kernel engineering
http://bootlin.com