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

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Message-ID: <525ef5cdefe987c3412249760324eb09.sboyd@kernel.org>
Date:   Wed, 25 Jan 2023 13:56:09 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        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

Quoting Konrad Dybcio (2023-01-25 13:48:54)
> 
> 
> 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..

So it is waiting for the CPU clk to be stable? The comment is not clear.