linux-kernel - Re: [PATCH V6 3/3] clk: meson: s4: add support for Amlogic S4 SoC peripheral clock controller

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Date:   Mon, 30 Jan 2023 11:07:28 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Yu Tu <yu.tu@...ogic.com>, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Kevin Hilman <khilman@...libre.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     "kelvin . zhang" <Kelvin.Zhang@...ogic.com>,
        "qi . duan" <qi.duan@...ogic.com>
Subject: Re: [PATCH V6 3/3] clk: meson: s4: add support for Amlogic S4 SoC
 peripheral clock controller


On Mon 30 Jan 2023 at 17:59, Yu Tu <yu.tu@...ogic.com> wrote:

> On 2023/1/30 17:47, Jerome Brunet wrote:
>> [ EXTERNAL EMAIL ]
>> On Mon 30 Jan 2023 at 17:41, Yu Tu <yu.tu@...ogic.com> wrote:
>> 
>>> On 2023/1/30 17:06, Jerome Brunet wrote:
>>>> [ EXTERNAL EMAIL ]
>>>> On Sat 28 Jan 2023 at 18:17, Yu Tu <yu.tu@...ogic.com> wrote:
>>>>
>>>>> On 2023/1/20 17:47, Jerome Brunet wrote:
>>>>>> [ EXTERNAL EMAIL ]
>>>>>> On Fri 20 Jan 2023 at 11:33, Yu Tu <yu.tu@...ogic.com> wrote:
>>>>>>
>>>>>>> Hi
>>>>>>> On 2023/1/19 19:37, Jerome Brunet wrote:
>>>>>>>> [ EXTERNAL EMAIL ]
>>>>>>>> On Mon 16 Jan 2023 at 15:42, Yu Tu <yu.tu@...ogic.com> wrote:
>>>>>>>>
>>>>>>>>> Add the peripherals clock controller driver in the s4 SoC family.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Yu Tu <yu.tu@...ogic.com>
>>>>>>>> [...]
>>>>>>>>
>>>>>>>>> +
>>>>>>>>> +/* Video Clocks */
>>>>>>>>> +static struct clk_regmap s4_vid_pll_div = {
>>>>>>>>> +	.data = &(struct meson_vid_pll_div_data){
>>>>>>>>> +		.val = {
>>>>>>>>> +			.reg_off = CLKCTRL_VID_PLL_CLK_DIV,
>>>>>>>>> +			.shift   = 0,
>>>>>>>>> +			.width   = 15,
>>>>>>>>> +		},
>>>>>>>>> +		.sel = {
>>>>>>>>> +			.reg_off = CLKCTRL_VID_PLL_CLK_DIV,
>>>>>>>>> +			.shift   = 16,
>>>>>>>>> +			.width   = 2,
>>>>>>>>> +		},
>>>>>>>>> +	},
>>>>>>>>> +	.hw.init = &(struct clk_init_data) {
>>>>>>>>> +		.name = "vid_pll_div",
>>>>>>>>> +		/*
>>>>>>>>> +		 * The frequency division from the hdmi_pll clock to the vid_pll_div
>>>>>>>>> +		 * clock is the default value of this register. When designing the
>>>>>>>>> +		 * video module of the chip, a default value that can meet the
>>>>>>>>> +		 * requirements of the video module will be solidified according
>>>>>>>>> +		 * to the usage requirements of the chip, so as to facilitate chip
>>>>>>>>> +		 * simulation. So this is ro_ops.
>>>>>>>>> +		 * It is important to note that this clock is not used on this
>>>>>>>>> +		 * chip and is described only for the integrity of the clock tree.
>>>>>>>>> +		 */
>>>>>>>> If it is reset value and will be applicable to all the design, regarless
>>>>>>>> of the use-case, then yes RO ops is OK
>>>>>>>>
>>>>>>>> >From what I understand here, the value will depend on the use-case requirements.
>>>>>>>> This is a typical case where the DT prop "assigned-rate" should be used, not RO ops.
>>>>>>>
>>>>>>> Check the previous chip history, the actual scene is not used at all,
>>>>>>> basically is used in simulation. So the previous SOC was "ro_ops" without
>>>>>>> any problems.  This S4 SOC is not actually useful either.
>>>>>>>
>>>>>>> So when you were upstream, you had no problem making "ro_ops". I wonder if
>>>>>>> I could delete this useless clock, so you don't have to worry about it.
>>>>>> I don't know what to make of this. What is the point of adding a useless
>>>>>> clock ?
>>>>>
>>>>> As explained earlier this "vid_pll_div" is actually used in chip
>>>>> emulation. So next I'd like to know what you suggest to do with the clock?
>>>>>
>>>> If it does not exist in the actual SoC, please remove it
>>>>
>>>
>>> If I remove it, the "vid_pll_sel" clock will be missing a parent
>>> (vid_pll_div). I will use the table method and give the above reasons. Do
>>> you accept this method?
>> Either the clock exists or it does not.
>> If the HW actually exist, it is expected to be properly described.
>> If it does not, it obviously cannot be an input to another clock.
>> Please sort this out and make the necessary changes.
>> 
>
> The CLKCTRL_VID_PLL_CLK_DIV register is actually described, but it is not
> used in the actual board. According to your reply just now, description is
> required, but I want to know how to describe it to meet your requirements.
>
> Please give me some suggestions.

Implementing things is NOT about usage, it is about correctness.
Either there is actually a clock in the silicon you are producing at the
Amlogic factory, or there is not. 

If the clock is there in the actual HW should be properly
described/implemented, as it "might" be used as an input to other clocks
- even if you personnaly don't.

If clock does not exists (nothing behind the registers, or broken, etc
...)  then, yes you'll need to use parent tables and document this.