linux-kernel - Re: [PATCH v3 0/6] clocksource: rework Atmel TCB timer driver

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Message-ID: <20180328153135.GG13942@piout.net>
Date:   Wed, 28 Mar 2018 17:31:35 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Alexander Dahl <ada@...rsis.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Boris Brezillon <boris.brezillon@...tlin.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 0/6] clocksource: rework Atmel TCB timer driver

On 28/03/2018 at 16:36:34 +0200, Daniel Lezcano wrote:
> On 28/03/2018 16:16, Alexandre Belloni wrote:
> > On 28/03/2018 at 15:03:11 +0200, Daniel Lezcano wrote:
> >> On 28/03/2018 12:29, Alexander Dahl wrote:
> >>> Hello Daniel,
> >>>
> >>> Am Dienstag, 27. März 2018, 13:30:22 CEST schrieb Daniel Lezcano:
> >>>> Can you can give a rough amount for the irq rate on the timer ?
> >>>
> >>> I used itop [1] now to get a rough estimate. First with kernel v4.14.29-rt25 
> >>> (fully preempt RT):
> >>>
> >>> INT                NAME          RATE             MAX
> >>>  19 [ vel     tc_clkevt]   397 Ints/s     (max:   432)
> >>>  26 [      vel     eth0]     4 Ints/s     (max:    38)
> >>>
> >>> Next test with kernel v4.15.13 gives (slightly slower, but non-RT):
> >>>
> >>> INT                NAME          RATE             MAX
> >>>  19 [ vel     tc_clkevt]   248 Ints/s     (max:   273)
> >>>  26 [      vel     eth0]     4 Ints/s     (max:    11)
> >>>
> >>> With kernel v4.16-rc7 plus this patch series and tcb as clocksource:
> >>>
> >>> INT                NAME          RATE             MAX
> >>>  17 [vel     timer@...a]  2164 Ints/s     (max:  2183)
> >>>  26 [      vel     eth0]     5 Ints/s     (max:    10)
> >>>
> >>> Is this the information you wanted? If not, could you point me on how to get 
> >>> the requested irq rate?
> >>
> >> It is perfect. Thanks!
> >>
> >> It confirms what I was worried about: the clocksource wraps up too
> >> quickly thus raising an interrupt every 400us. That is why I asked
> >> Alexande about a prescalar register.
> >>
> > 
> > The code should behave exactly the same between the previous and the new
> > driver. The interrupt is not coming from the clocksource but from the
> > clockevent and it is already on the slowest clock, the 32kHz one.
> 
> Do you have an explanation of why the rate is much higher ?
> 

The core is giving deltas of 31 clocks instead of much more than that, I
guess I messed up the initialization somewhere.


-- 
Alexandre Belloni, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com