linux-kernel - Re: [PATCH v1 1/7] clk: qcom: clk-alpha-pll: Add support for Fabia PLL calibration

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Date:   Fri, 15 Nov 2019 13:41:20 +0530
From:   Taniya Das <tdas@...eaurora.org>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>
Cc:     David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andy Gross <agross@...nel.org>, devicetree@...r.kernel.org,
        robh@...nel.org, robh+dt@...nel.org
Subject: Re: [PATCH v1 1/7] clk: qcom: clk-alpha-pll: Add support for Fabia
 PLL calibration

Hi Stephen,

Thanks for your review.

On 11/6/2019 6:06 AM, Stephen Boyd wrote:
> Quoting Taniya Das (2019-10-31 05:21:07)
>> diff --git a/drivers/clk/qcom/clk-alpha-pll.c b/drivers/clk/qcom/clk-alpha-pll.c
>> index 055318f..8cb77ca 100644
>> --- a/drivers/clk/qcom/clk-alpha-pll.c
>> +++ b/drivers/clk/qcom/clk-alpha-pll.c
>> @@ -1141,15 +1160,11 @@ static int alpha_pll_fabia_set_rate(struct clk_hw *hw, unsigned long rate,
>>                                                  unsigned long prate)
>>   {
>>          struct clk_alpha_pll *pll = to_clk_alpha_pll(hw);
>> -       u32 val, l, alpha_width = pll_alpha_width(pll);
>> +       u32 l, alpha_width = pll_alpha_width(pll);
>>          u64 a;
>>          unsigned long rrate;
>>          int ret = 0;
>>
>> -       ret = regmap_read(pll->clkr.regmap, PLL_MODE(pll), &val);
>> -       if (ret)
>> -               return ret;
>> -
>>          rrate = alpha_pll_round_rate(rate, prate, &l, &a, alpha_width);
>>
>>          /*
> 
> How is this diff related? Looks like it should be split off into another
> patch to remove a useless register read.
> 

I will split this in different patch.

>> @@ -1167,7 +1182,66 @@ static int alpha_pll_fabia_set_rate(struct clk_hw *hw, unsigned long rate,
>>          return __clk_alpha_pll_update_latch(pll);
>>   }
>>
>> +static int alpha_pll_fabia_prepare(struct clk_hw *hw)
>> +{
> 
> Why are we doing this in prepare vs. doing it at PLL configuration time?
> Does it need to be recalibrated each time the PLL is enabled?
> 

In the case if PLL looses the configuration then we would encounter PLL 
locking issues. Thus want to go ahead with prepare. In the case it is 
calibrated it would return.

>> +       struct clk_alpha_pll *pll = to_clk_alpha_pll(hw);
>> +       const struct pll_vco *vco;
>> +       struct clk_hw *parent_hw;
>> +       unsigned long cal_freq, rrate;
>> +       u32 cal_l, regval, alpha_width = pll_alpha_width(pll);
>> +       u64 a;
>> +       int ret;
>> +
>> +       /* Check if calibration needs to be done i.e. PLL is in reset */
>> +       ret = regmap_read(pll->clkr.regmap, PLL_MODE(pll), &regval);
> 
> Please use 'val' instead of 'regval' as regval almost never appears in
> this file already.
> 

Sure, will use 'val'.

>> +       if (ret)
>> +               return ret;
>> +
>> +       /* Return early if calibration is not needed. */
>> +       if (regval & PLL_RESET_N)
>> +               return 0;
>> +
>> +       vco = alpha_pll_find_vco(pll, clk_hw_get_rate(hw));
>> +       if (!vco) {
>> +               pr_err("alpha pll: not in a valid vco range\n");
>> +               return -EINVAL;
>> +       }
>> +
>> +       cal_freq = DIV_ROUND_CLOSEST((pll->vco_table[0].min_freq +
>> +                               pll->vco_table[0].max_freq) * 54, 100);
> 
> Do we need to cast the first argument to a u64 to avoid overflow?
> 

No we do not need.

>> +
>> +       parent_hw = clk_hw_get_parent(hw);
>> +       if (!parent_hw)
>> +               return -EINVAL;
>> +
>> +       rrate = alpha_pll_round_rate(cal_freq, clk_hw_get_rate(parent_hw),
>> +                                       &cal_l, &a, alpha_width);
>> +       /*
>> +        * Due to a limited number of bits for fractional rate programming, the
>> +        * rounded up rate could be marginally higher than the requested rate.
>> +        */
>> +       if (rrate > (cal_freq + FABIA_PLL_RATE_MARGIN) || rrate < cal_freq) {
>> +               pr_err("Call set rate on the PLL with rounded rates!\n");
> 
> This message is weird. Drivers shouldn't need to call set rate with
> rounded rates. What is going on?
> 

:), my bad, copy paste from another function. I will remove this print.

>> +               return -EINVAL;
>> +       }
>> +
>> +       /* Setup PLL for calibration frequency */
>> +       regmap_write(pll->clkr.regmap, PLL_ALPHA_VAL(pll), cal_l);
>> +
>> +       /* Bringup the pll at calibration frequency */
> 
> capitalize PLL.
> 

Will take care of it.

>> +       ret = clk_alpha_pll_enable(hw);
>> +       if (ret) {
>> +               pr_err("alpha pll calibration failed\n");
>> +               return ret;
>> +       }
>> +
>> +       clk_alpha_pll_disable(hw);
>> +
>> +       return 0;
>> +}
>> +

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member
of Code Aurora Forum, hosted by The Linux Foundation.

--