linux-kernel - Re: [PATCH v6 15/29] x86/hpet: Add helper function hpet_set_comparator

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Date:   Fri, 13 May 2022 14:19:44 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 15/29] x86/hpet: Add helper function
 hpet_set_comparator_periodic()

On Fri, May 06, 2022 at 11:41:13PM +0200, Thomas Gleixner wrote:
> On Thu, May 05 2022 at 16:59, Ricardo Neri wrote:
> > Programming an HPET channel as periodic requires setting the
> > HPET_TN_SETVAL bit in the channel configuration. Plus, the comparator
> > register must be written twice (once for the comparator value and once for
> > the periodic value). Since this programming might be needed in several
> > places (e.g., the HPET clocksource and the HPET-based hardlockup detector),
> > add a helper function for this purpose.
> >
> > A helper function hpet_set_comparator_oneshot() could also be implemented.
> > However, such function would only program the comparator register and the
> > function would be quite small. Hence, it is better to not bloat the code
> > with such an obvious function.
> 
> This word salad above does not provide a single reason why the periodic
> programming function is required and better suited for the NMI watchdog
> case

The goal of hpet_set_comparator_periodic() is to avoid code duplication. The
functions hpet_clkevt_set_state_periodic() and kick_timer() in the HPET NMI
watchdog need to program a periodic HPET channel when supported.


> and then goes on and blurbs about why a function which is not
> required is not implemented.

I can remove this.

> The argument about not bloating the code
> with an "obvious???" function which is quite small is slightly beyond my
> comprehension level.

That obvious function would look like this:

void hpet_set_comparator_one_shot(int channel, u32 delta)
{
	u32 count;

	count = hpet_readl(HPET_COUNTER);
	count += delta;
	hpet_writel(count, HPET_Tn_CMP(channel));
}

It involves one register read, one addition and one register write. IMO, this
code is sufficiently simple and small to allow duplication.

Programming a periodic HPET channel is not as straightforward, IMO. It involves
handling two different values (period and comparator) written in a specific
sequence, one configuration bit, and one delay. It also involves three register
writes and one register read.

Thanks and BR,
Ricardo