RE: [PATCH 0/6] Add RZ/V2M Compare-Match Timer (TIM) support

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Hi Thomas Gleixner,

Thanks for the feedback.

> Subject: RE: [PATCH 0/6] Add RZ/V2M Compare-Match Timer (TIM) support
> 
> Biju!
> 
> On Wed, Dec 07 2022 at 07:52, Biju Das wrote:
> >> On Tue, Dec 06 2022 at 09:40, Geert Uytterhoeven wrote:
> >> > When multiple clocksources are registered, the clocksource
> >> > subsystems picks the best one anyway, right?
> >>
> >> As it does for the clock event devices. If there is an architected
> >> timer then that should be always preferred.
> >>
> >> No idea why there is a need for the extra hardware and the drivers
> >> which are both never utilized.
> >
> > I got feedback from BSP team for the actual usage of this timer.
> >
> > Basically, this HW timer is used for measuring the processing time of
> > DRP-AI accurately compared to the CPU timer normally we use.
> 
> How is a slow to access timer with a lower clock frequency more accurate?

But our tick frequency for arm64 defconfig is CONFIG_HZ_250=y. So we get timer interrupt
at every 4 msec. 

How do we get timer event interrupt, eg: for 1 microsec?

> 
> > The example use cases,
> > Timer in FREERUN mode, Check the timer value after the restart(1usec)"
> > Timer in FREERUN mode, Check the timer value after the
> restart(10000000usec)"
> >
> > What is the model to be used for this kind of HW usage? Counter or
> Timer?
> >
> > I can think of one possible HW usage by using Counter model.
> > Not sure how timer model can be used for this kind of HW usage??
> >
> > Eg: we can set ceiling values 1usec and 10000000usec using counter
> framework
> >   And that will trigger interrupt events corresponding to the ceiling
> values
> >   to user space and user space app can accurately measure the DRP-AI
> processing time.
> >
> > Also counter model exposes count values to user space from the counter
> HW.
> 
> Counter subsystem != clocksource/event subsystem.
> 
> We are debating a clocksource/clockevent driver and not a counter driver,
> right?

Yes, Rob pointed out we should not misuse the compatibles as I have both
Timer and counter bindings for a given HW timer.

Timer, It can be used as broadcast and highres timer for RT.

Counter, It can be used as measuring the processing time of DRP-AI.

What is the best way to use this hardware to take care of all this use cases? 

So far all the Renesas timers used timer model. But for RZ/V2M the HW usage is different.
The customer BSP mainly uses this timer for measuring the processing time of DRP-AI, 
so the expectation is we should at least have support for this use case.

Cheers,
Biju





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