linux-kernel - Re: [PATCH net-next 0/3] ptp: add pulse signal loopback support for debugging

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Message-ID: <20250903151138.malmfj53hew7caze@skbuf>
Date: Wed, 3 Sep 2025 18:11:38 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Richard Cochran <richardcochran@...il.com>
Cc: Wei Fang <wei.fang@....com>, andrew+netdev@...n.ch, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	xiaoning.wang@....com, Frank.Li@....com, yangbo.lu@....com,
	christophe.leroy@...roup.eu, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org, imx@...ts.linux.dev
Subject: Re: [PATCH net-next 0/3] ptp: add pulse signal loopback support for
 debugging

On Wed, Sep 03, 2025 at 06:41:30AM -0700, Richard Cochran wrote:
> On Wed, Sep 03, 2025 at 04:37:46PM +0800, Wei Fang wrote:
> > Some PTP devices support looping back the periodic pulse signal for
> > debugging,
> 
> What kinds of debugs can be resolved by this loopback feature?

The commit message of patch 2/3 shows that you can record extts events
for the periodic output signal emitted by the device, without this
signal ever being required to be routed in any particular way on the PCB,
through the SoC's pins.

So you can make sure that the pulse intervals and their phase alignment
are correct from the perspective of the emitting PHC's time base.

Or you can use it as a built-in extts event generator when you have no
external equipment which does that, as I did a few years ago to validate
the extts functionality on ptp_qoriq.

> It seems pointless to me...

Well, it's pointless in the same sense that a mirror is pointless.