linux-kernel - Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source

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Date:   Fri, 13 Jan 2017 15:50:48 +0100
From:   Richard Cochran <richardcochran@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, devel@...uxdriverproject.org,
        linux-kernel@...r.kernel.org,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        John Stultz <john.stultz@...aro.org>,
        Alex Ng <alexng@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH RFC] hv_utils: implement Hyper-V PTP source

On Fri, Jan 13, 2017 at 02:05:43PM +0100, Vitaly Kuznetsov wrote:
> Instead of doing in-kernel time adjustments offload the work to an
> NTP client by exposing TimeSync messages as a PTP device. Users my now
> decide what they want to use as a source.
> 
> I tested the solution with chrony, the config was:
> 
>  refclock PHC /dev/ptp0 poll 3 precision 1e-9
> 
> The result I'm seeing is accurate enough, the time delta between the guest
> and the host is almost always within [-10us, +10us], the in-kernel solution
> was giving us comparable results.

This approach is much nicer than the previous one.

> +static int hv_ptp_enable(struct ptp_clock_info *info,
> +			 struct ptp_clock_request *request, int on)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static int hv_ptp_gettime(struct ptp_clock_info *info, struct timespec64 *ts)
> +{
> +	u64 newtime;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&host_ts.lock, flags);
> +	newtime = host_ts.host_time + get_timeadj_latency(host_ts.ref_time);
> +	*ts = ns_to_timespec64((newtime - WLTIMEDELTA) * 100);
> +	spin_unlock_irqrestore(&host_ts.lock, flags);
> +
> +	return 0;
> +}
> +
> +struct ptp_clock_info ptp_hyperv_info = {
> +	.name		= "hyperv",
> +	.enable         = hv_ptp_enable,
> +	.gettime64      = hv_ptp_gettime,

The code in drivers/ptp/ptp_clock.c calls

	.adjfreq (or adjfine)
	.adjtime
	.settime64

unconditionally, so you need to implement these returning EOPNOTSUPP.
(See also Documentation/ptp/ptp.txt)

> +	.owner		= THIS_MODULE,
> +};

Thanks,
Richard