linux-kernel - Re: [PATCH] net: cpts: Condition WARN_ON on PTP_1588

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Date:   Mon, 20 Apr 2020 23:21:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Clay McClure <clay@...mons.net>,
        Grygorii Strashko <grygorii.strashko@...com>,
        "David S. Miller" <davem@...emloft.net>,
        Sekhar Nori <nsekhar@...com>,
        Networking <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: cpts: Condition WARN_ON on PTP_1588_CLOCK

On Mon, Apr 20, 2020 at 11:18 PM Richard Cochran
<richardcochran@...il.com> wrote:
> On Mon, Apr 20, 2020 at 08:57:05PM +0200, Arnd Bergmann wrote:
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 172) #if IS_REACHABLE(CONFIG_PTP_1588_CLOCK)
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 173)
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 174) /**
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 175)  * ptp_clock_register() - register a PTP hardware clock driver
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 176)  *
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 177)  * @info:   Structure describing the new clock.
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 178)  * @parent: Pointer to the parent device of the new clock.
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 179)  *
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 180)  * Returns a valid pointer on success or PTR_ERR on failure.  If PHC
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 181)  * support is missing at the configuration level, this function
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 182)  * returns NULL, and drivers are expected to gracefully handle that
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 183)  * case separately.
> > > d1cbfd771ce82 (Nicolas Pitre       2016-11-11 184)  */
> >
> > The key here is "gracefully". The second patch from Clay just turns NULL into
> >  -EOPNOTSUPP and treats the compile-time condition into a runtime error.
>
> You are talking about the cpts driver, no?
>
> I'm worried about ptp_clock_register(), because it does return NULL if
> IS_REACHABLE(CONFIG_PTP_1588_CLOCK), and this is the "correct"
> behavior ever since November 2016.
>
> If somebody wants to change that stub to return EOPNOTSUPP, then fine,
> but please have them audit the callers and submit a patch series.

It's not great, but we have other interfaces like this that can return NULL for
success when the subsystem is disabled. The problem is when there is
a mismatch between the caller treating NULL as failure when it is meant to
be "successful lack of object returned".

       Arnd