linux-kernel - Re: [PATCH v9 01/10] clk: qcom: clk-alpha-pll: Add support for dynamic update for slewing PLLs

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Message-ID: <744da3c1-04c6-4347-bafc-233471d07853@quicinc.com>
Date: Tue, 1 Jul 2025 11:49:13 +0530
From: Taniya Das <quic_tdas@...cinc.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Bjorn Andersson
	<andersson@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        "Stephen
 Boyd" <sboyd@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski
	<krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Konrad Dybcio
	<konradybcio@...nel.org>, Will Deacon <will@...nel.org>,
        Catalin Marinas
	<catalin.marinas@....com>
CC: Ajit Pandey <quic_ajipan@...cinc.com>,
        Imran Shaik
	<quic_imrashai@...cinc.com>,
        Jagadeesh Kona <quic_jkona@...cinc.com>,
        <linux-arm-msm@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v9 01/10] clk: qcom: clk-alpha-pll: Add support for
 dynamic update for slewing PLLs



On 6/27/2025 8:39 PM, Konrad Dybcio wrote:
> On 6/25/25 12:31 PM, Taniya Das wrote:
>>
>>
>> On 6/15/2025 12:20 AM, Konrad Dybcio wrote:
>>> On 6/12/25 11:55 AM, Taniya Das wrote:
>>>> The alpha PLLs which slew to a new frequency at runtime would require
>>>> the PLL to calibrate at the mid point of the VCO. Add the new PLL ops
>>>> which can support the slewing of the PLL to a new frequency.
>>>>
>>>> Reviewed-by: Imran Shaik <quic_imrashai@...cinc.com>
>>>> Signed-off-by: Taniya Das <quic_tdas@...cinc.com>
>>>> ---
> 
> [...]
> 
>>>> +	regmap_write(pll->clkr.regmap, PLL_L_VAL(pll), l);
>>>> +	regmap_write(pll->clkr.regmap, PLL_ALPHA_VAL(pll), lower_32_bits(a));
>>>> +	regmap_write(pll->clkr.regmap, PLL_ALPHA_VAL_U(pll), upper_32_bits(a));
>>>> +
>>>> +	/* Ensure that the write above goes before slewing the PLL */
>>>> +	mb();
>>>
>>> Here however, the write may not arrive at the clock controller before you
>>> proceed to slew_update()
>>>
>>
>> Yes, it is required here and will keep it.
> 
> You should change it to a readback instead
> 

Okay, I can introduce a readback, but as per the PLL code we have been
using mb() at most places. But I am okay to update it.

> Konrad