netdev - Re: [PATCH] ptp: Set lookup cookie when creating a PTP PPS source.

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Date:   Fri, 2 Jul 2021 03:39:36 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Richard Cochran <richardcochran@...il.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Cc:     netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] ptp: Set lookup cookie when creating a PTP PPS source.

On Mon, Jun 28, 2021 at 04:38:35PM -0700, Richard Cochran wrote:
> On Mon, Jun 28, 2021 at 11:25:33AM -0700, Jonathan Lemon wrote:
> > When creating a PTP device, the configuration block allows
> > creation of an associated PPS device.  However, there isn't
> > any way to associate the two devices after creation.
> >
> > Set the PPS cookie, so pps_lookup_dev(ptp) performs correctly.
>
> Setting lookup_cookie is harmless, AFAICT, but I wonder about the use
> case.  The doc for pps_lookup_dev() says,

Harmless you say?

Let's look at the code in a larger context:

 struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info,
 				     struct device *parent)
 {
 	struct ptp_clock *ptp;
 
 	...
 	ptp = kzalloc(...);
 	...
 	ptp->info = info;
 	...
 
 	if (ptp->info->do_aux_work) {
 		...
+		ptp->pps_source->lookup_cookie = ptp;
 	}
 
 	/* Register a new PPS source. */
 	if (info->pps) {
 		struct pps_source_info pps;
 		...
 		ptp->pps_source = pps_register_source(&pps, PTP_PPS_DEFAULTS);
 		...
 	}

Notice anything out of the ordinary?
Like perhaps the fact that ptp->pps_source is an arbitrary NULL pointer
at the time the assignment to ->lookup_cookie is being made, because it
is being created later?

How on earth is this patch supposed to work?