RFC on cpufreq implementation

RFC on cpufreq implementation

[Mason]

Hello,

This is a follow-up to my previous thread.
"How many frequencies would cpufreq optimally like to manage?"
http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669

As I originally wrote, I'm running 3.14 on an ARM Cortex-A9
based SoC (namely Tango4 from Sigma Designs). I'd like to get
some feedback on the cpufreq driver I wrote for that platform.

I decided to expose only a small subset of frequencies (namely
{999,500,333,111} MHz) because, in my tests, the ondemand gov
chose mostly min and max, and the intermediate frequencies not
so much; so I figured "2 intermediate freqs" is good enough.
(I'm ready to hear otherwise.)

I tried to use as much generic framework as possible, but I've
read about the clk framework, and it looks to be an even greater
generalization. Are new platforms encouraged to use that, rather
than provide a cpufreq driver? Does it work when voltage scaling
comes in play? (This SoC doesn't have it, but the next will.)

I'm also wondering how cpufreq and cpuidle interact? Is one a
subset of the other? Are they orthogonal?

Regards.
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RFC on cpufreq implementation

[Krzysztof Kozlowski]

On czw, 2015-01-15 at 18:24 +0100, Mason wrote:
> Hello,
> 
> This is a follow-up to my previous thread.
> "How many frequencies would cpufreq optimally like to manage?"
> http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669
> 
> As I originally wrote, I'm running 3.14 on an ARM Cortex-A9
> based SoC (namely Tango4 from Sigma Designs). I'd like to get
> some feedback on the cpufreq driver I wrote for that platform.
> 
> I decided to expose only a small subset of frequencies (namely
> {999,500,333,111} MHz) because, in my tests, the ondemand gov
> chose mostly min and max, and the intermediate frequencies not
> so much; so I figured "2 intermediate freqs" is good enough.
> (I'm ready to hear otherwise.)
> 
> I tried to use as much generic framework as possible, but I've
> read about the clk framework, and it looks to be an even greater
> generalization. Are new platforms encouraged to use that, rather
> than provide a cpufreq driver? Does it work when voltage scaling
> comes in play? (This SoC doesn't have it, but the next will.)

The clock framework generalizes clocks, not cpufreq. Ideally you should
use clock framework in cpufreq driver. So instead manually setting
divider just do something like:

ret = clk_set_rate(cpu_clk, freq_exact);
if (ret) {
	dev_err(cpu_dev, "failed to set clock rate: %d\n", ret);
	return ret;
}

For voltage scaling you should use regulator framework.

Actually I think existing cpufreq-dt could serve your purpose. Why don't
you try it? Or look at it and use as an example.

> I'm also wondering how cpufreq and cpuidle interact? Is one a
> subset of the other? Are they orthogonal?

cpuidle and cpufreq are different subsystems. They don't interact, yet.
There are efforts to combine scheduler, cpufreq and cpuidle but this is
future. If your SoC has some deeper low power states than developing
cpuidle driver makes sense. If not - WFI will be used.

> 
> Regards.

RFC on cpufreq implementation

[Mason]

On 16/01/2015 10:08, Krzysztof Kozlowski wrote:

> On 2015-01-15 at 18:24 +0100, Mason wrote:
>  
>> This is a follow-up to my previous thread.
>> "How many frequencies would cpufreq optimally like to manage?"
>> http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669
>>
>> As I originally wrote, I'm running 3.14 on an ARM Cortex-A9
>> based SoC (namely Tango4 from Sigma Designs). I'd like to get
>> some feedback on the cpufreq driver I wrote for that platform.
>>
>> I decided to expose only a small subset of frequencies (namely
>> {999,500,333,111} MHz) because, in my tests, the ondemand gov
>> chose mostly min and max, and the intermediate frequencies not
>> so much; so I figured "2 intermediate freqs" is good enough.
>> (I'm ready to hear otherwise.)

I'll take a closer look at other drivers, but I'd like to hear
opinions on the subject.

>> I tried to use as much generic framework as possible, but I've
>> read about the clk framework, and it looks to be an even greater
>> generalization. Are new platforms encouraged to use that, rather
>> than provide a cpufreq driver? Does it work when voltage scaling
>> comes in play? (This SoC doesn't have it, but the next will.)
>
> The clock framework generalizes clocks, not cpufreq. Ideally you should
> use clock framework in cpufreq driver. So instead manually setting
> divider just do something like:
>
> ret = clk_set_rate(cpu_clk, freq_exact);
> if (ret) {
> 	dev_err(cpu_dev, "failed to set clock rate: %d\n", ret);
> 	return ret;
> }

I will give clk a closer look.

> For voltage scaling you should use regulator framework.

OK. I'm also interested in frequency-throttling when temperatures
rise beyond specific thresholds. What subsystem ties sensors and
cpufreq together?

> Actually I think existing cpufreq-dt could serve your purpose. Why don't
> you try it? Or look at it and use as an example.

Will do. I've heard of device tree, but know nothing about it.

>> I'm also wondering how cpufreq and cpuidle interact? Is one a
>> subset of the other? Are they orthogonal?
>
> cpuidle and cpufreq are different subsystems. They don't interact, yet.
> There are efforts to combine scheduler, cpufreq and cpuidle but this is
> future. If your SoC has some deeper low power states than developing
> cpuidle driver makes sense. If not - WFI will be used.

AFAIU, there are no deeper power states on the Cortex-A9.

I didn't find where WFI is called :-(

In kernel/cpu/idle.c (file seems to have been removed in 3.15)
cpu_idle_loop() calls arch_cpu_idle()
http://lxr.free-electrons.com/source/kernel/cpu/idle.c?v=3.14#L98

In arch/kernel/process.c
http://lxr.free-electrons.com/source/arch/arm/kernel/process.c?v=3.14#L173
/*
  * Called from the core idle loop.
  */
void arch_cpu_idle(void)
{
	if (cpuidle_idle_call())
		default_idle();
}

default_idle calls cpu_do_idle (by default), a macro for cpu_v7_do_idle
which executes dsb+wfi, BUT...

ifndef CONFIG_CPU_IDLE then
static inline int cpuidle_idle_call(void) { return -ENODEV; }

Does that mean I MUST define CONFIG_CPU_IDLE if I want the idle
loop to call wfi (to save power), even if I don't have a cpuidle
driver?

Regards.

RFC on cpufreq implementation

[Krzysztof Kozlowski]

On pi?, 2015-01-16 at 12:10 +0100, Mason wrote:
> On 16/01/2015 10:08, Krzysztof Kozlowski wrote:
> 
> > On 2015-01-15 at 18:24 +0100, Mason wrote:
> >  
> >> This is a follow-up to my previous thread.
> >> "How many frequencies would cpufreq optimally like to manage?"
> >> http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669
> >>
> >> As I originally wrote, I'm running 3.14 on an ARM Cortex-A9
> >> based SoC (namely Tango4 from Sigma Designs). I'd like to get
> >> some feedback on the cpufreq driver I wrote for that platform.
> >>
> >> I decided to expose only a small subset of frequencies (namely
> >> {999,500,333,111} MHz) because, in my tests, the ondemand gov
> >> chose mostly min and max, and the intermediate frequencies not
> >> so much; so I figured "2 intermediate freqs" is good enough.
> >> (I'm ready to hear otherwise.)
> 
> I'll take a closer look at other drivers, but I'd like to hear
> opinions on the subject.
> 
> >> I tried to use as much generic framework as possible, but I've
> >> read about the clk framework, and it looks to be an even greater
> >> generalization. Are new platforms encouraged to use that, rather
> >> than provide a cpufreq driver? Does it work when voltage scaling
> >> comes in play? (This SoC doesn't have it, but the next will.)
> >
> > The clock framework generalizes clocks, not cpufreq. Ideally you should
> > use clock framework in cpufreq driver. So instead manually setting
> > divider just do something like:
> >
> > ret = clk_set_rate(cpu_clk, freq_exact);
> > if (ret) {
> > 	dev_err(cpu_dev, "failed to set clock rate: %d\n", ret);
> > 	return ret;
> > }
> 
> I will give clk a closer look.
> 
> > For voltage scaling you should use regulator framework.
> 
> OK. I'm also interested in frequency-throttling when temperatures
> rise beyond specific thresholds. What subsystem ties sensors and
> cpufreq together?

I believe thermal.

> 
> > Actually I think existing cpufreq-dt could serve your purpose. Why don't
> > you try it? Or look at it and use as an example.
> 
> Will do. I've heard of device tree, but know nothing about it.

Indeed it is driver meant to be used with Device Tree but beside DT code
it nicely shows how to scale the frequency CPU. If you don't use DT then
you can use only the set_target() as starting point.

On 3.14 the "cpufreq-dt" could be called "cpufreq-cpu0".

> 
> >> I'm also wondering how cpufreq and cpuidle interact? Is one a
> >> subset of the other? Are they orthogonal?
> >
> > cpuidle and cpufreq are different subsystems. They don't interact, yet.
> > There are efforts to combine scheduler, cpufreq and cpuidle but this is
> > future. If your SoC has some deeper low power states than developing
> > cpuidle driver makes sense. If not - WFI will be used.
> 
> AFAIU, there are no deeper power states on the Cortex-A9.
> 
> I didn't find where WFI is called :-(
> 
> In kernel/cpu/idle.c (file seems to have been removed in 3.15)
> cpu_idle_loop() calls arch_cpu_idle()
> http://lxr.free-electrons.com/source/kernel/cpu/idle.c?v=3.14#L98
> 
> In arch/kernel/process.c
> http://lxr.free-electrons.com/source/arch/arm/kernel/process.c?v=3.14#L173
> /*
>   * Called from the core idle loop.
>   */
> void arch_cpu_idle(void)
> {
> 	if (cpuidle_idle_call())
> 		default_idle();
> }
> 
> default_idle calls cpu_do_idle (by default), a macro for cpu_v7_do_idle
> which executes dsb+wfi, BUT...
> 
> ifndef CONFIG_CPU_IDLE then
> static inline int cpuidle_idle_call(void) { return -ENODEV; }
> 
> Does that mean I MUST define CONFIG_CPU_IDLE if I want the idle
> loop to call wfi (to save power), even if I don't have a cpuidle
> driver?

On current kernels you don't have to. But on 3.14... seems yes.

Best regards,
Krzysztof
> 
> Regards.

RFC on cpufreq implementation

[Javi Merino]

Hi Mason,

On Fri, Jan 16, 2015 at 11:10:23AM +0000, Mason wrote:
[...]
> OK. I'm also interested in frequency-throttling when temperatures
> rise beyond specific thresholds. What subsystem ties sensors and
> cpufreq together?

The thermal framework.  Have a look at
Documentation/thermal/sysfs-api.txt and
Documentation/thermal/cpu-cooling-api.txt

Cheers,
Javi

RFC on cpufreq implementation

[Mason]

On 16/01/2015 12:10, Mason wrote:

> I didn't find where WFI is called :-(
>
> In kernel/cpu/idle.c (file seems to have been removed in 3.15)
> cpu_idle_loop() calls arch_cpu_idle()
> http://lxr.free-electrons.com/source/kernel/cpu/idle.c?v=3.14#L98
>
> In arch/kernel/process.c
> http://lxr.free-electrons.com/source/arch/arm/kernel/process.c?v=3.14#L173
> /*
>   * Called from the core idle loop.
>   */
> void arch_cpu_idle(void)
> {
>      if (cpuidle_idle_call())
>          default_idle();
> }
>
> default_idle calls cpu_do_idle (by default), a macro for cpu_v7_do_idle
> which executes dsb+wfi, BUT...
>
> ifndef CONFIG_CPU_IDLE then
> static inline int cpuidle_idle_call(void) { return -ENODEV; }
>
> Does that mean I MUST define CONFIG_CPU_IDLE if I want the idle
> loop to call wfi (to save power), even if I don't have a cpuidle
> driver?

For the record, I read the code wrong.

cpuidle_idle_call() always returns -ENODEV, so the test
"if (cpuidle_idle_call())" is always true, and we always
call default_idle() and everything works as expected.

Sorry for the noise.

RFC on cpufreq implementation

[Viresh Kumar]

On 15 January 2015 at 22:54, Mason <mpeg.blue@free.fr> wrote:
> I decided to expose only a small subset of frequencies (namely
> {999,500,333,111} MHz) because, in my tests, the ondemand gov
> chose mostly min and max, and the intermediate frequencies not
> so much; so I figured "2 intermediate freqs" is good enough.
> (I'm ready to hear otherwise.)

Following patch solved this issue in 3.17..

6393d6a1027e cpufreq: ondemand: Eliminate the deadband effect

RFC on cpufreq implementation

[Mason]

On 19/01/2015 08:52, Viresh Kumar wrote:
> On 15 January 2015 at 22:54, Mason wrote:
>> I decided to expose only a small subset of frequencies (namely
>> {999,500,333,111} MHz) because, in my tests, the ondemand gov
>> chose mostly min and max, and the intermediate frequencies not
>> so much; so I figured "2 intermediate freqs" is good enough.
>> (I'm ready to hear otherwise.)
> 
> Following patch solved this issue in 3.17..
> 
> 6393d6a1027e cpufreq: ondemand: Eliminate the deadband effect

Good to know, thanks! (I'm on 3.14)

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6393d6a1027ec1d69ec6246f6c7c2186f76c2abb

Do you have other comments/suggestions regarding my code?

Regards.

RFC on cpufreq implementation

[Viresh Kumar]

On 20 January 2015 at 03:33, Mason <mpeg.blue@free.fr> wrote:
> Good to know, thanks! (I'm on 3.14)
>
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6393d6a1027ec1d69ec6246f6c7c2186f76c2abb
>
> Do you have other comments/suggestions regarding my code?

No. It looked fine on the first look..

RFC on cpufreq implementation

[Amit Kucheria]

On Thu, Jan 15, 2015 at 10:54 PM, Mason <mpeg.blue@free.fr> wrote:
> Hello,
>
> This is a follow-up to my previous thread.
> "How many frequencies would cpufreq optimally like to manage?"
> http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669
>
> As I originally wrote, I'm running 3.14 on an ARM Cortex-A9
> based SoC (namely Tango4 from Sigma Designs). I'd like to get
> some feedback on the cpufreq driver I wrote for that platform.
>
> I decided to expose only a small subset of frequencies (namely
> {999,500,333,111} MHz) because, in my tests, the ondemand gov
> chose mostly min and max, and the intermediate frequencies not
> so much; so I figured "2 intermediate freqs" is good enough.
> (I'm ready to hear otherwise.)

How many states are really enough depends on the main workloads
running on your system. In a closed system (limited number of
applications) you can easily characterise your workloads and see what
operating points (OPP = voltage, frequency pair) the system spends
most of its time in (CPU_FREQ_STAT_DETAILS) and optimize out the
remaining OPPs.

In an open-ended system where you don't control what applications will
run on the system (e.g. android phone), it is probably a good idea to
expose more OPPs while keeping in mind that exposing 50 frequencies is
probably overkill (and silly) since you're spending more time reaching
the "optimum" OPP. Pick some high-impact ones e.g. ones that allow you
to lower your voltage.

> I tried to use as much generic framework as possible, but I've
> read about the clk framework, and it looks to be an even greater
> generalization. Are new platforms encouraged to use that, rather
> than provide a cpufreq driver? Does it work when voltage scaling
> comes in play? (This SoC doesn't have it, but the next will.)
>
> I'm also wondering how cpufreq and cpuidle interact? Is one a
> subset of the other? Are they orthogonal?

These queries have been answered by Krzysztof.

RFC on cpufreq implementation

[Mason]

On 19/01/2015 10:22, Amit Kucheria wrote:

> On Thu, Jan 15, 2015 at 10:54 PM, Mason wrote:
> 
>> This is a follow-up to my previous thread.
>> "How many frequencies would cpufreq optimally like to manage?"
>> http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669
>>
>> As I originally wrote, I'm running 3.14 on an ARM Cortex-A9
>> based SoC (namely Tango4 from Sigma Designs). I'd like to get
>> some feedback on the cpufreq driver I wrote for that platform.
>>
>> I decided to expose only a small subset of frequencies (namely
>> {999,500,333,111} MHz) because, in my tests, the ondemand gov
>> chose mostly min and max, and the intermediate frequencies not
>> so much; so I figured "2 intermediate freqs" is good enough.
>> (I'm ready to hear otherwise.)
> 
> How many states are really enough depends on the main workloads
> running on your system. In a closed system (limited number of
> applications) you can easily characterise your workloads and see what
> operating points (OPP = voltage, frequency pair) the system spends
> most of its time in (CPU_FREQ_STAT_DETAILS) and optimize out the
> remaining OPPs.

Testing with CPU_FREQ_STAT_DETAILS enabled is on my TODO list.
Thanks for reminding me!

> In an open-ended system where you don't control what applications will
> run on the system (e.g. android phone), it is probably a good idea to
> expose more OPPs while keeping in mind that exposing 50 frequencies is
> probably overkill (and silly) since you're spending more time reaching
> the "optimum" OPP. Pick some high-impact ones e.g. ones that allow you
> to lower your voltage.

The current SoC does not support dynamic voltage scaling at all.
So I'm just tweaking the frequency. IIUC, if I divide freq by 4,
power should be divided by 4?

>> I tried to use as much generic framework as possible, but I've
>> read about the clk framework, and it looks to be an even greater
>> generalization. Are new platforms encouraged to use that, rather
>> than provide a cpufreq driver? Does it work when voltage scaling
>> comes in play? (This SoC doesn't have it, but the next will.)
>>
>> I'm also wondering how cpufreq and cpuidle interact? Is one a
>> subset of the other? Are they orthogonal?
> 
> These queries have been answered by Krzysztof.

The current SoC does not support any "deep" sleep; I was told that
the core just powers "itself" down after a WFI, nothing fancier.

Regards.

RFC on cpufreq implementation

[Mason]

On 15/01/2015 18:24, Mason wrote:

> This is a follow-up to my previous thread.
> "How many frequencies would cpufreq optimally like to manage?"
> http://thread.gmane.org/gmane.linux.ports.arm.kernel/373669

OK, so this is my TAKE 2 on the cpufreq driver, trying to remove
some dependencies on machine-specific definitions by getting the
virtual address at init via ioremap. (Is -EFAULT the right error
to return if ioremap fails?)

I'm not sure where machine-specific information is supposed to
be stored though? Such as register definitions, or the physical
addresses/offsets. What if they need to be shared among several
source files? I can't just duplicate them... (I've tentatively
called it temp.h for the time being.)

Regards.

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RFC on cpufreq implementation

[Viresh Kumar]

On 29 January 2015 at 22:13, Mason <mpeg.blue@free.fr> wrote:
> OK, so this is my TAKE 2 on the cpufreq driver, trying to remove
> some dependencies on machine-specific definitions by getting the
> virtual address at init via ioremap. (Is -EFAULT the right error
> to return if ioremap fails?)
>
> I'm not sure where machine-specific information is supposed to
> be stored though? Such as register definitions, or the physical
> addresses/offsets. What if they need to be shared among several
> source files? I can't just duplicate them... (I've tentatively
> called it temp.h for the time being.)

What do you want to do with this driver? If you want to get it reviewed,
please send it properly with git-send-email instead of attachments..

If its just an internal one, then sorry, the lists aren't for such reviews.

--
viresh

RFC on cpufreq implementation

[Mason]

On 30/01/2015 02:15, Viresh Kumar wrote:

> What do you want to do with this driver? If you want to get it reviewed,
> please send it properly with git-send-email instead of attachments..
> 
> If its just an internal one, then sorry, the lists aren't for such reviews.

The long-term goal is to mainline the whole port, but it's rather
overwhelming, and I haven't found a way to divide-and-conquer, yet.
I've been reading guides and documentation for weeks, but nothing
has made my brain click.

Everything seems to involve DeviceTree, and AFAIU, going down that
rabbit-hole means making lots of changes all over. (But I probably
misunderstood that part too.)

Right now, all I have is this cleaned up cpufreq driver. And I don't
even know where to put it!

I see some platforms have it in their machine-specific folder, others
are in drivers/cpufreq. (When to use mach vs plat?)

If it's supposed to go in drivers/cpufreq, I suppose there are naming
conventions to follow?

Also, if it's in drivers/cpufreq, we are not supposed to include
any machine-specific includes? And I'm back to my original question
where am I supposed to store machine-specific information, such as
register descriptions and MMIO addresses and offsets?

Two months ago, Arnd wrote:
> I meant the IO_ADDRESS stuff. Modern code uses ioremap() instead
> since the IO_ADDRESS was platform specific, and drivers can no longer
> use platform headers on CONFIG_ARCH_MULTIPLATFORM, which is used
> for all new code now.

Regards.

RFC on cpufreq implementation

[Viresh Kumar]

On 31 January 2015 at 05:14, Mason <mpeg.blue@free.fr> wrote:
> I see some platforms have it in their machine-specific folder, others
> are in drivers/cpufreq. (When to use mach vs plat?)

drivers/cpufreq/

> If it's supposed to go in drivers/cpufreq, I suppose there are naming
> conventions to follow?

You can check existing drivers.

> Also, if it's in drivers/cpufreq, we are not supposed to include
> any machine-specific includes? And I'm back to my original question
> where am I supposed to store machine-specific information, such as
> register descriptions and MMIO addresses and offsets?

Define registers/etc in your driver only or somewhere in include/ .
Also, try to see if you can reuse cpufreq-dt.c, your driver wouldn't make
it to the kernel otherwise.

RFC on cpufreq implementation

[Mason]

Viresh Kumar wrote:

> Mason wrote:
>
>> If it's supposed to go in drivers/cpufreq, I suppose there are naming
>> conventions to follow?
>
> You can check existing drivers.

All of them?

~/linux-3.19-rc7$ wc drivers/cpufreq/*
  31542  95730 813482 total

Are there perhaps 1 or 2 "golden standard" drivers that are well-written
and up-to-date with respect to current conventions?

>> Also, if it's in drivers/cpufreq, we are not supposed to include
>> any machine-specific includes? And I'm back to my original question
>> where am I supposed to store machine-specific information, such as
>> register descriptions and MMIO addresses and offsets?
>
> Define registers/etc in your driver only or somewhere in include/ .

Like Samsung did with include/dt-bindings/clock/exynos*.h ?

> Also, try to see if you can reuse cpufreq-dt.c, your driver wouldn't make
> it to the kernel otherwise.

Are you saying that use of DeviceTree is mandatory for new ARM ports?

In other words, ports using "board files" will not be accepted into
mainline, nor will drivers not using DT?

If that is correct, then my proposed cpufreq driver has exactly 0%
chance of being mainlined as-is, right?

I took a look at cpufreq-dt.c but I think it doesn't quite fit my use-case.
In my driver, I define "clock dividers" (typically 1,2,3,5,9) and these are
used to divide some baseline frequency that the cpufreq code doesn't need to
know. The baseline frequency is set by the boot loader, and Linux is not
supposed to change that, only apply the dividers if necessary.

What do you think of this use-case?

Regards.

RFC on cpufreq implementation

[Måns Rullgård]

Mason <mpeg.blue@free.fr> writes:

> Are you saying that use of DeviceTree is mandatory for new ARM ports?

Yes.

-- 
M?ns Rullg?rd
mans at mansr.com

RFC on cpufreq implementation

[Viresh Kumar]

On 4 February 2015 at 05:37, Mason <mpeg.blue@free.fr> wrote:
> All of them?
>
> ~/linux-3.19-rc7$ wc drivers/cpufreq/*
>  31542  95730 813482 total
>
> Are there perhaps 1 or 2 "golden standard" drivers that are well-written
> and up-to-date with respect to current conventions?

cpufreq-dt.c

> Like Samsung did with include/dt-bindings/clock/exynos*.h ?

Yeah.

> Are you saying that use of DeviceTree is mandatory for new ARM ports?

Yes.

> In other words, ports using "board files" will not be accepted into
> mainline, nor will drivers not using DT?

It wouldn't be good for you as well, that's all I can say.

> If that is correct, then my proposed cpufreq driver has exactly 0%
> chance of being mainlined as-is, right?

:)

> I took a look at cpufreq-dt.c but I think it doesn't quite fit my use-case.
> In my driver, I define "clock dividers" (typically 1,2,3,5,9) and these are
> used to divide some baseline frequency that the cpufreq code doesn't need to
> know. The baseline frequency is set by the boot loader, and Linux is not
> supposed to change that, only apply the dividers if necessary.
>
> What do you think of this use-case?

You need to write a backend clock driver for this. Most of the other platforms
are doing this. cpufreq-dt driver does clk_get(), clk_get_rate(), clk_set_rate()
and your backend driver should handle these to change the frequencies.