netdev - Re: [chrony-dev] Support for Multiple PPS Inputs on single PHC

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Message-ID: <Y+60JfLyQIXpSirG@hoboy.vegasvil.org>
Date:   Thu, 16 Feb 2023 14:54:29 -0800
From:   Richard Cochran <richardcochran@...il.com>
To:     Matt Corallo <ntp-lists@...tcorallo.com>
Cc:     chrony-dev@...ony.tuxfamily.org,
        Miroslav Lichvar <mlichvar@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [chrony-dev] Support for Multiple PPS Inputs on single PHC

On Thu, Feb 16, 2023 at 09:54:56AM -0800, Matt Corallo wrote:
> 
> 
> On 2/16/23 12:19 AM, Miroslav Lichvar wrote:
> > My first thought is that this should be addressed in the kernel, so
> > even different processes having open the PHC device can receive all
> > extts samples. If it turns out it's too difficult to do for the
> > character device (I'm not very familiar with that subsystem), maybe it
> > could be done at least in sysfs (/sys/class/ptp/ptp*/fifo or a new
> > file showing the last event like the PPS assert and clear).

The PPS thing has a race, and so I'd rather not copy that!
 
> As for duplicating the output across sockets, ptp_chardev.c's `ptp_read` is
> pretty trivial - just pop the next sample off the queue and return it.
> Tweaking that to copy the sample into every reader is probably above my
> paygrade (and has a whole host of leak risk I'd probably screw up).
> `extts_fifo_show` appears to be functionally identical.

Each extts in the fifo is delivered only once.  If there are multiple
readers, each reader will receive only some of the data.  This is
similar to how a pipe behaves.

HTH,
Richard