netdev - Re: [EXT] Re: [PATCH v1] timer:clock:ptp: add support the dynamic posix clock alarm set for ptp

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Message-ID: <20190508170629.me5smui6n7n62x2l@localhost>
Date:   Wed, 8 May 2019 10:06:29 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Po Liu <po.liu@....com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "Y.b. Lu" <yangbo.lu@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Leo Li <leoyang.li@....com>, Roy Zang <roy.zang@....com>,
        Mingkai Hu <mingkai.hu@....com>,
        "deepa.kernel@...il.com" <deepa.kernel@...il.com>
Subject: Re: [EXT] Re: [PATCH v1] timer:clock:ptp: add support the dynamic
 posix clock alarm set for ptp

On Wed, May 08, 2019 at 07:36:54AM -0700, Richard Cochran wrote:
> No the alarm functionality has been removed.  It will not be coming
> back, unless there are really strong arguments to support it.

Here is some more background:

    commit 3a06c7ac24f9f24ec059cd77c2dbdf7fbfd0aaaf
    Author: Thomas Gleixner <tglx@...utronix.de>
    Date:   Tue May 30 23:15:38 2017 +0200

    posix-clocks: Remove interval timer facility and mmap/fasync callbacks
    
    The only user of this facility is ptp_clock, which does not implement any of
    those functions.
    
    Remove them to prevent accidental users. Especially the interval timer
    interfaces are now more or less impossible to implement because the
    necessary infrastructure has been confined to the core code. Aside of that
    it's really complex to make these callbacks implemented according to spec
    as the alarm timer implementation demonstrates. If at all then a nanosleep
    callback might be a reasonable extension. For now keep just what ptp_clock
    needs.
 
> Here is the result of a study of a prototype alarm method.  It shows
> why the hrtimer method is better.
> 
>    https://sourceforge.net/p/linuxptp/mailman/message/35535965/

That test was with a PCIe card.  With a SoC that has a PHC as a built
in peripheral, the hardware solution might outperform hrtimers.

So you might consider adding clock_nanosleep() for dynamic posix
clocks.  But your code will have to support multiple users at the same
time.

Thanks,
Richard