netdev - Re: [PATCH net-next 4/4] net: dsa: sja1105: configure the PTP_CLK pin as EXT_TS or PER

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Message-ID: <20200323232648.GC9140@localhost>
Date:   Mon, 23 Mar 2020 16:26:48 -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, christian.herber@....com,
        yangbo.lu@....com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next 4/4] net: dsa: sja1105: configure the PTP_CLK
 pin as EXT_TS or PER_OUT

On Tue, Mar 24, 2020 at 12:59:24AM +0200, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> The SJA1105 switch family has a PTP_CLK pin which emits a signal with
> fixed 50% duty cycle, but variable frequency and programmable start time.
> 
> On the second generation (P/Q/R/S) switches, this pin supports even more
> functionality. The use case described by the hardware documents talks
> about synchronization via oneshot pulses: given 2 sja1105 switches,
> arbitrarily designated as a master and a slave, the master emits a
> single pulse on PTP_CLK, while the slave is configured to timestamp this
> pulse received on its PTP_CLK pin (which must obviously be configured as
> input). The difference between the timestamps then exactly becomes the
> slave offset to the master.
> 
> The only trouble with the above is that the hardware is very much tied
> into this use case only, and not very generic beyond that:
>  - When emitting a oneshot pulse, instead of being told when to emit it,
>    the switch just does it "now" and tells you later what time it was,
>    via the PTPSYNCTS register. [ Incidentally, this is the same register
>    that the slave uses to collect the ext_ts timestamp from, too. ]
>  - On the sync slave, there is no interrupt mechanism on reception of a
>    new extts, and no FIFO to buffer them, because in the foreseen use
>    case, software is in control of both the master and the slave pins,
>    so it "knows" when there's something to collect.
> 
> These 2 problems mean that:
>  - We don't support (at least yet) the quirky oneshot mode exposed by
>    the hardware, just normal periodic output.
>  - We abuse the hardware a little bit when we expose generic extts.
>    Because there's no interrupt mechanism, we need to poll at double the
>    frequency we expect to receive a pulse. Currently that means a
>    non-configurable "twice a second".
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

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