Re: [PATCH v15 6/6] clk: meson: a1: add Amlogic A1 Peripherals clock controller driver

[Jerome Brunet]

On Mon 22 May 2023 at 16:32, Dmitry Rokosov <ddrokosov@xxxxxxxxxxxxxx> wrote:

> Hello Martin,
>
> Thank you so much for the review, I really appreciate it!
> Please find my comments below.
>
> On Fri, May 19, 2023 at 11:03:54PM +0200, Martin Blumenstingl wrote:
>> Hi Dmitry,
>> 
>> On Wed, May 17, 2023 at 3:33 PM Dmitry Rokosov <ddrokosov@xxxxxxxxxxxxxx> wrote:
>> [...]
>> > +static struct clk_regmap sys_b_sel = {
>> > +       .data = &(struct clk_regmap_mux_data){
>> > +               .offset = SYS_CLK_CTRL0,
>> > +               .mask = 0x7,
>> > +               .shift = 26,
>> > +               .table = mux_table_sys,
>> > +       },
>> > +       .hw.init = &(struct clk_init_data){
>> > +               .name = "sys_b_sel",
>> > +               .ops = &clk_regmap_mux_ro_ops,
>> the sys_*_sel muxes and sys_*_gate are _ro...
>> 
>> > +               .parent_data = sys_parents,
>> > +               .num_parents = ARRAY_SIZE(sys_parents),
>> > +       },
>> > +};
>> > +
>> > +static struct clk_regmap sys_b_div = {
>> > +       .data = &(struct clk_regmap_div_data){
>> > +               .offset = SYS_CLK_CTRL0,
>> > +               .shift = 16,
>> > +               .width = 10,
>> > +       },
>> > +       .hw.init = &(struct clk_init_data){
>> > +               .name = "sys_b_div",
>> > +               .ops = &clk_regmap_divider_ops,
>> ...but the sys_*_div aren't
>> Is this on purpose? If it is: why can the divider be changed at
>> runtime but the mux can't?
>> 
>
> Ah, that's a good catch. Since the system clock is set up by the BootROM
> code, all sys_* dividers and gates should be read-only. I'll make sure
> to change that in the next version.
>
>> [...]
>> > +/*
>> > + * the index 2 is sys_pll_div16, it will be implemented in the CPU clock driver,
>> We need to add the "sys_pll_div16" input to the dt-bindings since they
>> should always describe the hardware (regardless of what the driver
>> implements currently).
>> I'm not sure how to manage this while we don't have the CPU clock
>> driver ready yet but I'm sure Rob or Krzysztof will be able to help us
>> here.
>> 
>
> I've shared my thoughts about it in the bindings thread. Please take a
> look.
>
>> > + * the index 4 is the clock measurement source, it's not supported yet
>> I suspect that this comes from the clock measurer IP block and if so
>> the dt-bindings should probably describe this input. But again, we'd
>> need to keep it optional for now since our clock measurer driver
>> doesn't even implement a clock controller.
>> 
>
> Indeed, this is a similar situation to what we have with the inputs and
> clocks of the CPU and Audio clock controllers. It seems like there is
> only one option here: we should mark it with a TODO tag...
>
>> [...]
>> > +static struct clk_regmap pwm_a_sel = {
>> > +       .data = &(struct clk_regmap_mux_data){
>> > +               .offset = PWM_CLK_AB_CTRL,
>> > +               .mask = 0x1,
>> > +               .shift = 9,
>> > +       },
>> > +       .hw.init = &(struct clk_init_data){
>> > +               .name = "pwm_a_sel",
>> > +               .ops = &clk_regmap_mux_ops,
>> > +               .parent_data = pwm_abcd_parents,
>> > +               .num_parents = ARRAY_SIZE(pwm_abcd_parents),
>> > +               /* For more information, please refer to rtc clock */
>> > +               .flags = CLK_SET_RATE_NO_REPARENT,
>> As mentioned in [0] we'll work with Heiner to see if we can improve
>> the decision making process of the PWM controller driver so that we
>> can just have .flags = 0 here.
>> This applies to all other occurrences of the same comment about the rtc clock.
>
> Sure, I'll make the change in v16. In my opinion, we should remove the
> CLK_SET_RATE_NO_REPARENT flag from all RTC related clock objects,
> including PWM, regardless of the outcome of the Heiner discussion. Based
> on our IRC talk, the decision has more pros than cons -
> https://libera.irclog.whitequark.org/linux-amlogic/2023-05-18

The clock scheme of PWM could indeed be handled like audio is but it
not strictly required.

In audio we have a limited number of PLLs (root sources). There is a lot
more consummers than there is root sources. If the root sources rate is
not carefully chosen to statisfy all needs, we could end in a situation
where we can't satisfy all consummers or we must glitch the source to do
so.

For the PWM, I think (but I'm not 100% sure) that the main clock controller
provides a source for each PWM. No risk of race there. That is why AML
decided to completly ignore the clock element in the PWM IP, because
they can do almost everything with what is in the main controller ... Still
ignoring those part is wrong

For the RTC, If you want/need to handle external RTCs, I don't think you
have much of a choice. If both the internal and external *report* the
same rate, CCF can't really know if one is best. It will just pick one,
no necessarily the one you want. I don't really see a way around manual
selection for this.