linux-kernel - Re: [PATCH 1/5] clk: qcom: cmnpll: Account for reference clock divider

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Message-ID: <9f8eda70-e077-4925-bc80-f95e35464f87@oss.qualcomm.com>
Date: Thu, 4 Dec 2025 11:16:57 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Jie Luo <jie.luo@....qualcomm.com>,
        Bjorn Andersson
 <andersson@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>, Luo Jie <quic_luoj@...cinc.com>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        quic_kkumarcs@...cinc.com, quic_linchen@...cinc.com,
        quic_leiwei@...cinc.com, quic_pavir@...cinc.com,
        quic_suruchia@...cinc.com
Subject: Re: [PATCH 1/5] clk: qcom: cmnpll: Account for reference clock
 divider

On 12/4/25 8:44 AM, Jie Luo wrote:
> 
> 
> On 12/1/2025 9:42 PM, Konrad Dybcio wrote:
>> On 11/28/25 3:29 PM, Jie Luo wrote:
>>>
>>>
>>> On 11/28/2025 7:38 PM, Konrad Dybcio wrote:
>>>> On 11/28/25 9:40 AM, Luo Jie wrote:
>>>>> The clk_cmn_pll_recalc_rate() function must account for the reference clock
>>>>> divider programmed in CMN_PLL_REFCLK_CONFIG. Without this fix, platforms
>>>>> with a reference divider other than 1 calculate incorrect CMN PLL rates.
>>>>> For example, on IPQ5332 where the reference divider is 2, the computed rate
>>>>> becomes twice the actual output.
>>>>>
>>>>> Read CMN_PLL_REFCLK_DIV and divide the parent rate by this value before
>>>>> applying the 2 * FACTOR scaling. This yields the correct rate calculation:
>>>>> rate = (parent_rate / ref_div) * 2 * factor.
>>>>>
>>>>> Maintain backward compatibility with earlier platforms (e.g. IPQ9574,
>>>>> IPQ5424, IPQ5018) that use ref_div = 1.
>>>>
>>>> I'm not sure how to interpret this. Is the value fixed on these platforms
>>>> you mentioned, and always shows up as 0?
>>>>
>>>> Konrad
>>>
>>> On these platforms the hardware ref_div register comes up with a value
>>> of 1 by default. It is, however, still a programmable field and not
>>> strictly fixed to 1.
>>>
>>> The ref_div == 0 check in this patch is only meant as a safety net to
>>> avoid a divide‑by‑zero in the rate calculation.
>>
>> I think some sort of a warning/bugsplat would be good to have here,
>> however I see that clk-rcg2.c : clk_gfx3d_determine_rate() apparently
>> also silently fixes up a div0..
>>
>> Konrad
> 
> I agree it would be better to at least flag this as an invalid
> configuration. I can update the code to emit a warning (e.g. WARN_ON
> (!ref_div) while still clamping ref_div to a sane value to avoid
> crashing in production. That way we preserve the safety net but also
> get some visibility if this ever happens.

if (WARN_ON(div == 0))

sounds good!

Konrad