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

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Date:   Wed, 06 Sep 2023 14:14:16 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Devi Priya <quic_devipriy@...cinc.com>,
        Konrad Dybcio <konrad.dybcio@...aro.org>, 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

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.