netdev - Re: [PATCH net-next] net: dsa: sja1105: poll for extts events from a timer

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Date:   Mon, 3 Aug 2020 11:26:31 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     davem@...emloft.net, f.fainelli@...il.com,
        vivien.didelot@...il.com, andrew@...n.ch, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: sja1105: poll for extts events from a
 timer

On Mon, Aug 03, 2020 at 08:51:58PM +0300, Vladimir Oltean wrote:
> The current poll interval is enough to ensure that rising and falling
> edge events are not lost for a 1 PPS signal with 50% duty cycle.

> Fix that by taking the following measures:
> - Schedule the poll from a timer. Because we are really scheduling the
>   timer periodically, the extts events delivered to user space are
>   periodic too, and don't suffer from the "shift-to-the-right" effect.
> - Increase the poll period to 6 times a second. This imposes a smaller
>   upper bound to the shift that can occur to the delivery time of extts
>   events, and makes user space (ts2phc) to always interpret correctly
>   which events should be skipped and which shouldn't.
> - Move the SPI readout itself to the main PTP kernel thread, instead of
>   the generic workqueue. This is because the timer runs in atomic
>   context, but is also better than before, because if needed, we can
>   chrt & taskset this kernel thread, to ensure it gets enough priority
>   under load.

Makes sense to me.

Acked-by: Richard Cochran <richardcochran@...il.com>