linux-kernel - Re: [PATCH v4 7/7] clk: qcom: add the driver for the MSM8996 APCS clocks

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Date:   Wed, 25 Jan 2023 22:48:54 +0100
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Stephen Boyd <sboyd@...nel.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Taniya Das <quic_tdas@...cinc.com>
Cc:     linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/7] clk: qcom: add the driver for the MSM8996 APCS
 clocks



On 25.01.2023 22:38, Stephen Boyd wrote:
> Quoting Dmitry Baryshkov (2023-01-18 05:22:54)
>> diff --git a/drivers/clk/qcom/apcs-msm8996.c b/drivers/clk/qcom/apcs-msm8996.c
>> new file mode 100644
>> index 000000000000..7e46ea8ed444
>> --- /dev/null
>> +++ b/drivers/clk/qcom/apcs-msm8996.c
>> @@ -0,0 +1,76 @@
> [...]
>> +
>> +static int qcom_apcs_msm8996_clk_probe(struct platform_device *pdev)
>> +{
>> +       struct device *dev = &pdev->dev;
>> +       struct device *parent = dev->parent;
>> +       struct regmap *regmap;
>> +       struct clk_hw *hw;
>> +       unsigned int val;
>> +       int ret = -ENODEV;
>> +
>> +       regmap = dev_get_regmap(parent, NULL);
>> +       if (!regmap) {
>> +               dev_err(dev, "failed to get regmap: %d\n", ret);
>> +               return ret;
>> +       }
>> +
>> +       regmap_read(regmap, APCS_AUX_OFFSET, &val);
>> +       regmap_update_bits(regmap, APCS_AUX_OFFSET, APCS_AUX_DIV_MASK,
>> +                          FIELD_PREP(APCS_AUX_DIV_MASK, APCS_AUX_DIV_2));
>> +
>> +       /* Hardware mandated delay */
> 
> Delay for what? Setting the divider? What if the register value didn't
> change at all? Can you skip the delay in that case?
Waiting 5 us unconditionally in exchange for ensured CPU clock
source stability sounds like a rather fair deal.. Checking if
the register value changed would not save us much time..

Konrad
> 
>> +       udelay(5);
>> +
>> +       /*
>> +        * Register the clock as fixed rate instead of being a child of gpll0
>> +        * to let the driver register probe as early as possible.
> 
> The function doesn't block or return EPROBE_DEFER if the clk is orphaned
> when registered. Why is this necessary? Are you getting defered by the
> fw_devlink logic thinking it needs to defer probe of this driver until
> gpll0 provider probes? We should fix fw_devlink to not do that. Maybe if
> the node is a clk provider (#clock-cells exists) then we don't wait for
> clocks property to be provided, because the clk core already handles
> that itself.
> 
>> +        */
>> +       hw = devm_clk_hw_register_fixed_rate(dev, "sys_apcs_aux", NULL, 0, 300000000);