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

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Date:   Wed, 13 Sep 2023 11:27:25 +0800
From:   Jie Luo <quic_luoj@...cinc.com>
To:     Stephen Boyd <sboyd@...nel.org>, <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



On 9/13/2023 1:18 AM, Stephen Boyd wrote:
> 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.

Hi Stephen,
For clk_rcg2_ops, freq_tbl is used to find the closest rate to decided 
the parent clock, the configuration of clock source and clock divider 
are saved in the freq_tbl to configure the RCG hardware register, the 
mapping of parent clock and hardware register value is decided by the 
freq_tbl for the RCG clock.