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

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Date:   Thu, 16 Feb 2023 16:58:54 -0800
From:   Matt Corallo <ntp-lists@...tcorallo.com>
To:     Richard Cochran <richardcochran@...il.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 2/16/23 2:54 PM, Richard Cochran wrote:
> On Thu, Feb 16, 2023 at 09:54:56AM -0800, Matt Corallo wrote:
>> 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.

Right, sorry if the context wasn't clear, I only realized part of the message was removed in the 
first reply after sending. The question from Miroslav was, basically, "would kernel accept something 
to only get notified of extts pulses on a given channel, and, if so, how would we go about doing that".

The "we get pulses from all extts channels on the same socket" thing is a bit annoying to munge into 
chrony - it has the concept of "refclocks" which are a single clock, in this case a single pps pulse 
generator. If you have two of them on the same PTP clock but coming in on different pins/channels it 
doesn't have a way to express that outside of two refclocks. While we could take pulses from both 
refclocks on one socket and shove them into some queue and have the refclocks pick that data up its 
a bunch of complexity on the client side and not super clean in the current codebase.

Thanks,
Matt