linux-kernel - Re: [PATCH v6 4/4] clk: qcom: add clock controller driver for qca8386/qca8084

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Date:   Tue, 12 Sep 2023 10:18:11 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Jie Luo <quic_luoj@...cinc.com>, agross@...nel.org,
        andersson@...nel.org, catalin.marinas@....com, conor+dt@...nel.org,
        konrad.dybcio@...aro.org, krzysztof.kozlowski+dt@...aro.org,
        mturquette@...libre.com, p.zabel@...gutronix.de,
        robh+dt@...nel.org, will@...nel.org
Cc:     linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_srichara@...cinc.com
Subject: Re: [PATCH v6 4/4] clk: qcom: add clock controller driver for qca8386/qca8084

Quoting Jie Luo (2023-09-12 05:07:02)
> 
> 
> On 9/12/2023 4:11 AM, Stephen Boyd wrote:
> > Quoting Jie Luo (2023-09-08 04:10:35)
> >>
> >> For example, when the uniphy works on PHY_INTERFACE_MODE_2500BASEX, then
> >> the parent uniphy clock rate is 312.5M, which is decided by hardware and
> >> can't be changed. when a branch clock requires a 25M clock, the parent
> >> uniphy clock maybe updated to 125M by clock framework if the flag
> >> CLK_SET_RATE_PARENT is set here, but the actual hardware clock rate of
> >> uniphy is still 315.5M since the uniphy still works in the interface
> >> mode PHY_INTERFACE_MODE_2500BASEX.
> >>
> > 
> > If the parent rate can't change because CLK_SET_RATE_PARENT is missing
> > and the hardware doesn't allow it, then perhaps instead of having a
> > frequency table we should have rcg clk ops for determine_rate that
> > simply looks at the parent rates and finds the rate closest to what is
> > desired. And for the set_rate clk_op we can have it be simple and just
> > program a fixed divider. The benefit is less frequency tables that don't
> > do anything and less hard-coding of the frequency. I thought we already
> > had those rcg clk_ops but I couldn't find them with a quick glance.
> 
> Thanks Stephen for the suggestion.
> looks you are saying the clk ops clk_dp_ops for the fix parent rate? 
> which seems not meet the clock requirement of this clock.

Yeah that is close, but the determine_rate clk_op needs to look at all
possible parents. With the dp clk_ops we assume that only one parent is
possible.

> 
> For the device qca8k, it is also possible to switch the interface modes 
> between PHY_INTERFACE_MODE_2500BASEX(312.5M) and 
> PHY_INTERFACE_MODE_SGMII(125M) during the running time, and there are 
> multiple parent clock source(P_UNIPHY0_RX or P_UNIPHY0_TX) for the RCG 
> clocks to select according to the current work mode. so the parent_map 
> and freq_tbl are necessary to this clock.

I still don't see why the freq_tbl is necessary.