linux-kernel - Re: [PATCH v2] x86/tsc: Convert ART in nanoseconds to TSC.

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Date:   Thu, 8 Mar 2018 15:25:48 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Rajvi Jingar <rajvi.jingar@...el.com>
cc:     mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        christopher.s.hall@...el.com
Subject: Re: [PATCH v2] x86/tsc: Convert ART in nanoseconds to TSC.

On Tue, 6 Mar 2018, Rajvi Jingar wrote:
> Device drivers use get_device_system_crosststamp() to produce precise
> system/device cross-timestamps. The PHC clock and ALSA interfaces,
> for example, make the cross-timestamps available to user applications.
> On Intel platforms, get_device_system_crosststamp() requires a TSC
> value derived from ART (Always Running Timer) to compute the monotonic
>  raw and realtime system timestamps.
> 
> Starting with Intel Goldmont platforms, the PCIe root complex supports
> the PTM time sync protocol. PTM requires all timestamps to be in units
> of nanoseconds. The Intel root complex hardware propagates system time –
> derived from ART - in units of nanoseconds performing the conversion
> as follows:
> 
> ART_NS = ART * 1e9 / <crystal frequency>
> 
> When user software requests a cross-timestamp, the system timestamps
> (generally read from device registers) must be converted to TSC by
> the driver software as follows:
> 
> TSC = ART_NS * TSC_KHZ / 1e6
> 
> This is valid when CPU feature flag X86_FEATURE_TSC_KNOWN_FREQ is set
> indicating the tsc_khz is derived from CPUID[15H]. Drivers should
> check that this flag is set before conversion to TSC is attempted.

Clear and coherent changelog. Well done!

> Changes from v1:
> 
> * use existing frequency hardcode for platforms where CPUID[15H].ECX == 0
> (v1 added redundant hardcode just for the ART.ns conversion)
> 
> * use tsc_khz for TSC conversion, also requires driver to check
> X86_FEATURE_TSC_KNOWN_FREQ (v1 used CPUID[15H].ECX value directly)

Maintainer lazyness request: Can you please put the changes paragraph below
the --- seperator so it is discarded when the patch is extracted from
mail. It's not part of the changelog which goes into git.

> +struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns)

Can you please add kernel doc format function documentation which explains
the calling conventions?

> +{
> +	u64 tmp, res, rem;
> +
> +	rem = do_div(art_ns, USEC_PER_SEC);
> +
> +	res = art_ns * tsc_khz;
> +	tmp = rem * tsc_khz;
> +
> +	do_div(tmp, USEC_PER_SEC);
> +	res += tmp;
> +
> +	return (struct system_counterval_t) {.cs = art_related_clocksource,
> +			.cycles = res};

Definitely way better than the previous one. Good job!

Thanks,

	tglx