linux-kernel - Re: [PATCH V2] clk: qcom: clk-alpha-pll: Use determine_rate instead of round

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Message-ID: <6c3b1b45-4322-4582-9009-cda5749570c5@linaro.org>
Date:   Thu, 7 Sep 2023 11:07:45 +0200
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Stephen Boyd <sboyd@...nel.org>,
        Devi Priya <quic_devipriy@...cinc.com>, agross@...nel.org,
        andersson@...nel.org, linux-arm-msm@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        mturquette@...libre.com
Cc:     quic_saahtoma@...cinc.com
Subject: Re: [PATCH V2] clk: qcom: clk-alpha-pll: Use determine_rate instead
 of round_rate

On 6.09.2023 23:14, Stephen Boyd wrote:
> Quoting Konrad Dybcio (2023-09-06 00:33:38)
>> On 5.09.2023 22:40, Stephen Boyd wrote:
>>> Quoting Devi Priya (2023-09-01 00:00:41)
>>>> The round_rate() API returns a long value as the errors are reported using
>>>> negative error codes. This leads to long overflow when the clock rate
>>>> exceeds 2GHz.As the clock controller treats the clock rate above signed
>>>> long max as an error, use determine_rate in place of round_rate as the
>>>> determine_rate API does not possess such limitations.
>>>
>>> Does this fix something, or is it preparing for PLLs that run faster
>>> than 2GHz?
>> I did some grepping and we already have multiple of these.
>>
>> E.g. SM8250 CAMCC PLL2 (zonda) goes (or well, should go) up to 3.6 GHz.
>>
>> Today, only stromer PLL uses determine rate, but perhaps all of them
>> should.
>>
>> I would not at all be surprised if many otherwise inexplicable bugs
>> went away with that change.
> 
> Are any of those arm32 systems? It would only matter on arm32 systems
> because sizeof(long) is limited to 32-bits and we don't have negative
> frequencies.
Looking deeper, not sure if we have any armv7 chips falling under
this category, but there are definitely arm64 SoCs that can boot
arm32 kernels (e.g. 8953).

Konrad