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

[Thomas Gleixner]

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