netdev - Re: [PATCH net] net: dsa: sja1105: the PTP_CLK extts input reacts on both edges

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Date:   Wed, 6 May 2020 12:58:28 -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] net: dsa: sja1105: the PTP_CLK extts input reacts on
 both edges

On Wed, May 06, 2020 at 08:48:13PM +0300, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> It looks like the sja1105 external timestamping input is not as generic
> as we thought. When fed a signal with 50% duty cycle, it will timestamp
> both the rising and the falling edge. When fed a short pulse signal,
> only the timestamp of the falling edge will be seen in the PTPSYNCTS
> register, because that of the rising edge had been overwritten. So the
> moral is: don't feed it short pulse inputs.
> 
> Luckily this is not a complete deal breaker, as we can still work with
> 1 Hz square waves. But the problem is that the extts polling period was
> not dimensioned enough for this input signal. If we leave the period at
> half a second, we risk losing timestamps due to jitter in the measuring
> process. So we need to increase it to 4 times per second.
> 
> Also, the very least we can do to inform the user is to deny any other
> flags combination than with PTP_RISING_EDGE and PTP_FALLING_EDGE both
> set.
> 
> Fixes: 747e5eb31d59 ("net: dsa: sja1105: configure the PTP_CLK pin as EXT_TS or PER_OUT")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

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