Re: [PATCH v1 1/2] clocksource/drivers/exynos_mct_v2: introduce Exynos MCT version 2 driver for next Exynos SoC

[Youngmin Nam]

On Tue, Oct 26, 2021 at 01:00:51PM +0200, Krzysztof Kozlowski wrote:
> On 26/10/2021 12:45, Youngmin Nam wrote:
> > On Tue, Oct 26, 2021 at 09:10:28AM +0200, Krzysztof Kozlowski wrote:
> >> On 26/10/2021 03:47, Youngmin Nam wrote:
> >>>> If everyone added a new driver to avoid integrating with existing code,
> >>>> we would have huge kernel with thousands of duplicated solutions. The
> >>>> kernel also would be unmaintained.
> >>>>
> >>>> Such arguments were brought before several times - "I don't want to
> >>>> integrating with existing code", "My use case is different", "I would
> >>>> need to test the other cases", "It's complicated for me".
> >>>>
> >>>> Instead of pushing a new vendor driver you should integrate it with
> >>>> existing code.
> >>>>
> >>> Let me ask you one question.
> >>> If we maintain as one driver, how can people who don't have the new MCT test the new driver?
> >>
> >> I assume you talk about a case when someone else later changes something
> >> in the driver. Such person doesn't necessarily have to test it. The same
> >> as in all other cases (Exynos MCT is not special here): just ask for
> >> testing on platform one doesn't have.
> >>
> >> Even if you submit this as separate driver, there is the exact same
> >> problem. People will change the MCTv2 driver without access to hardware.
> >>
> > Yes, I can test the new MCT driver if someone ask for testing after modifying the new driver.
> > But in this case, we don't need to test the previous MCT driver. We have only to test the new MCT driver.
> 
> Like with everything in Linux kernel. We merge instead of duplicate.
> It's not an argument.
> 
> >> None of these differ for Exynos MCT from other drivers, e.g. mentioned
> >> Samsung PMIC drivers, recently modified (by Will and Sam) the SoC clock
> >> drivers or the ChipID drivers (changed by Chanho).
> > From HW point of view, the previous MCT is almost 10-year-old IP without any major change and
> > it will not be used on next new Exynos SoC.
> > MCTv2 is the totally newly designed IP and it will replace the Exynos system timer.
> > Device driver would be dependent with H/W. We are going to apply a lot of changes for this new MCT.
> > For maintenance, I think we should separate the new MCT driver for maintenance.
> > 
> 
> There are several similarities which actually suggest that you
> exaggerate the differences.
> 
> The number of interrupts is the same (4+8 in older one, 12 in new one...).

I didn't "exaggerate" at all.
The numer of interrups is the same. But their usage is completely different.
The type of each timer is different.
And previous MCT can only support upto 8 cores.

* MCTv1 (Let me call previous MCT as MCTv1)
 - 4 global timer + 8 local timer
 - Global timer and local timer are totally different.
 - 4 global timer have only one 64bit FRC that serves as the "up-counter" with 4 "comparators"
 - 8 local timer have 8 of 32bit FRC that serves as the "down-counter" without any "comparators".(just expire timer)
 - local timer can be used as per-cpu event timer, so it can only support upto 8 cores.

* MCTv2
 - There are no global timer and local timer anymore.
 - 1 of 64bit FRC that serves as "up-counter" (just counter without "comparators")
 - 12 comaprators (These are not "counter") can be used as per-cpu event timer so that it can support upto 12 cores.
 - RTC source can be used as backup source.

> You assign the MCT priority also as higher than Architected Timer
> (+Cc Will and Mark - is it ok for you?)
>     evt->rating = 500;      /* use value higher than ARM arch timer *
> 
Yes, this is absolutely correct on event timer.
We cannot use arm arch timer which is operating based on PPI as per-cpu event timer because of poewr mode.
We have to use SPI for per-cpu timer interrupt. (This is the same in all Exynos platform)

> All these point that block is not different. Again, let me repeat, we
> support old Samsung PMICs with new Samsung PMICs in one driver. Even
> though the "old one" won't be changed, as you mentioned here. The same
> Samsung SoC clock drivers are used for old Exynos and for new ones...
> Similarly to pinctrl drivers. The same ChipId.
> 
> Everywhere we follow the same concept of unification instead of
> duplication. Maybe Exynos MCT timer is an exception but you did not
> provide any arguments supporting this. Why Exynos MCTv2 should be
> treated differently than Exynos850 clocks, chipid, pinctrl and other blocks?
> 

If MCTv2 has only changes in register layout, I can consider merging work.
But this is not that case.

You gave a example with PMIC, SoC clock, Pinctrl, ChipId.
These H/W IP have only changes in register layout which came from difference of each SoC.

Were these H/W IP version changed?
Were these H/W IP control method changed ?
No. It only has minor chagnes not major changes.

* PMIC - controls the PMIC reigster with I2C interface regarding their SoC usecase.
       - there is no changes on H/W control method itself.

* SoC Clock - changes only in register layout regarding SoC
            - Clock control method still the same.

* Pinctrl - changes only in gpio pin register layout (pin number, pin type, pin map..) regarding SoC.
	  - Is there any changes on control method ?

* Chipid - This is very simple H/W IP. It only supports unique chip id value with read-only register.
         - It really only have changes in register layout.

MCTv2 is different.
Not only register layout but also it's control method has to be changed regarding H/W difference.

> Daniel,
> Any preferences from you? Integrating MCT into existing driver (thus
> growing it) or having a new one?
> 
> Best regards,
> Krzysztof
>