linux-kernel - Re: [RFT PATCH] clk: ls1c: Fix PLL rate calculation

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Message-ID: <d92dd996-6961-5291-9504-1fe284b40dd6@gmail.com>
Date:   Wed, 3 Aug 2022 19:28:41 -0400
From:   Sean Anderson <seanga2@...il.com>
To:     linux-mips@...r.kernel.org, linux-clk@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Keguang Zhang <keguang.zhang@...il.com>,
        Du Huanpeng <dhu@...carrier.org>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Yang Ling <gnaygnil@...il.com>
Subject: Re: [RFT PATCH] clk: ls1c: Fix PLL rate calculation

On 4/19/22 1:11 AM, Sean Anderson wrote:
> While reviewing Dhu's patch adding ls1c300 clock support to U-Boot [1], I
> noticed the following calculation, which is copied from
> drivers/clk/loongson1/clk-loongson1c.c:
> 
> ulong ls1c300_pll_get_rate(struct clk *clk)
> {
> 	unsigned int mult;
> 	long long parent_rate;
> 	void *base;
> 	unsigned int val;
> 
> 	parent_rate = clk_get_parent_rate(clk);
> 	base = (void *)clk->data;
> 
> 	val = readl(base + START_FREQ);
> 	mult = FIELD_GET(FRAC_N, val) + FIELD_GET(M_PLL, val);
> 	return (mult * parent_rate) / 4;
> }
> 
> I would like to examine the use of M_PLL and FRAC_N to calculate the multiplier
> for the PLL. The datasheet has the following to say:
> 
> START_FREQ 位    缺省值      描述
> ========== ===== =========== ====================================
> FRAC_N     23:16 0           PLL 倍频系数的小数部分
> 
>                   由          PLL 倍频系数的整数部分
> M_PLL      15:8  NAND_D[3:0] (理论可以达到 255，建议不要超过 100)
>                   配置
> 
> which according to google translate means
> 
> START_FREQ Bits  Default       Description
> ========== ===== ============= ================================================
> FRAC_N     23:16 0             Fractional part of the PLL multiplication factor
> 
>                   Depends on    Integer part of PLL multiplication factor
> M_PLL      15:8  NAND_D[3:0]   (Theoretically it can reach 255, [but] it is
>                   configuration  recommended not to exceed 100)
> 
> So just based on this description, I would expect that the formula to be
> something like
> 
> 	rate = parent * (255 * M_PLL + FRAC_N) / 255 / 4
> 
> However, the datasheet also gives the following formula:
> 
> 	rate = parent * (M_PLL + FRAC_N) / 4
> 
> which is what the Linux driver has implemented. I find this very unusual.
> First, the datasheet specifically says that these fields are the integer and
> fractional parts of the multiplier. Second, I think such a construct does not
> easily map to traditional PLL building blocks. Implementing this formula in
> hardware would likely require an adder, just to then set the threshold of a
> clock divider.
> 
> I think it is much more likely that the first formula is correct. The author of
> the datasheet may think of a multiplier of (say) 3.14 as
> 
> 	M_PLL = 3
> 	FRAC_N = 0.14
> 
> which together sum to the correct multiplier, even though the actual value
> stored in FRAC_N would be 36.
> 
> I suspect that this has slipped by unnoticed because when FRAC_N is 0, there is
> no difference in the formulae. The following patch is untested, but I suspect
> it will fix this issue. I would appreciate if anyone with access to the
> hardware could measure the output of the PLL (or one of its derived clocks) and
> determine the correct formula.
> 
> [1] https://lore.kernel.org/u-boot/20220418204519.19991-1-dhu@hodcarrier.org/T/#u
> 
> Fixes: b4626a7f4892 ("CLK: Add Loongson1C clock support")
> Signed-off-by: Sean Anderson <seanga2@...il.com>
> ---
> 
>   drivers/clk/loongson1/clk-loongson1c.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/clk/loongson1/clk-loongson1c.c b/drivers/clk/loongson1/clk-loongson1c.c
> index 703f87622cf5..2b98a116c1ea 100644
> --- a/drivers/clk/loongson1/clk-loongson1c.c
> +++ b/drivers/clk/loongson1/clk-loongson1c.c
> @@ -21,9 +21,9 @@ static unsigned long ls1x_pll_recalc_rate(struct clk_hw *hw,
>   	u32 pll, rate;
>   
>   	pll = __raw_readl(LS1X_CLK_PLL_FREQ);
> -	rate = ((pll >> 8) & 0xff) + ((pll >> 16) & 0xff);
> +	rate = (pll & 0xff00) + ((pll >> 16) & 0xff);
>   	rate *= OSC;
> -	rate >>= 2;
> +	rate >>= 10;
>   
>   	return rate;
>   }
> 

Since there have been no objections, can we apply this?

--Sean