linux-kernel - Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers

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Message-ID: <alpine.DEB.2.21.1907160952040.1767@nanos.tec.linutronix.de>
Date:   Tue, 16 Jul 2019 09:57:35 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Felipe Balbi <felipe.balbi@...ux.intel.com>
cc:     Richard Cochran <richardcochran@...il.com>, netdev@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Christopher S . Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers

Felipe,

On Tue, 16 Jul 2019, Felipe Balbi wrote:

-ENOCHANGELOG

As you said in the cover letter:

>  (3) The change in arch/x86/kernel/tsc.c needs to be reviewed at length
>      before going in.

So some information what those interfaces are used for and why they are
needed would be really helpful.

> +void get_tsc_ns(struct system_counterval_t *tsc_counterval, u64 *tsc_ns)
> +{
> +	u64 tmp, res, rem;
> +	u64 cycles;
> +
> +	tsc_counterval->cycles = clocksource_tsc.read(NULL);
> +	cycles = tsc_counterval->cycles;
> +	tsc_counterval->cs = art_related_clocksource;
> +
> +	rem = do_div(cycles, tsc_khz);
> +
> +	res = cycles * USEC_PER_SEC;
> +	tmp = rem * USEC_PER_SEC;
> +
> +	do_div(tmp, tsc_khz);
> +	res += tmp;
> +
> +	*tsc_ns = res;
> +}
> +EXPORT_SYMBOL(get_tsc_ns);
> +
> +u64 get_art_ns_now(void)
> +{
> +	struct system_counterval_t tsc_cycles;
> +	u64 tsc_ns;
> +
> +	get_tsc_ns(&tsc_cycles, &tsc_ns);
> +
> +	return tsc_ns;
> +}
> +EXPORT_SYMBOL(get_art_ns_now);

While the changes look innocuous I'm missing the big picture why this needs
to emulate ART instead of simply using TSC directly.

Thanks,

	tglx