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From: Clay McClure <clay@daemons.net>
To: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Sekhar Nori <nsekhar@ti.com>,
	Richard Cochran <richardcochran@gmail.com>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net: cpts: Condition WARN_ON on PTP_1588_CLOCK
Date: Mon, 20 Apr 2020 02:36:10 -0700	[thread overview]
Message-ID: <20200420093610.GA28162@arctic-shiba-lx> (raw)
In-Reply-To: <6fef3a00-6c18-b775-d1b4-dfd692261bd3@ti.com>

On Thu, Apr 16, 2020 at 02:11:45PM +0300, Grygorii Strashko wrote:

Grygorii,

> > CPTS_MOD merely implies PTP_1588_CLOCK; it is possible to build cpts
> > without PTP clock support. In that case, ptp_clock_register() returns
> > NULL and we should not WARN_ON(cpts->clock) when downing the interface.
> > The ptp_*() functions are stubbed without PTP_1588_CLOCK, so it's safe
> > to pass them a null pointer.
> 
> Could you explain the purpose of the exercise (Enabling CPTS with
> PTP_1588_CLOCK disabled), pls?

Hardware timestamping with a free-running PHC _almost_ works without
PTP_1588_CLOCK, but since PHC rollover is handled by the PTP kworker
in this driver the timestamps end up not being monotonic.

And of course the moment you want to syntonize/synchronize the PHC with
another clock (say, CLOCK_REALTIME), you'll need a PTP clock device. So
you're right, there's not much point in building CPTS_MOD without
PTP_1588_CLOCK.

Given that, I wonder why all the Ethernet drivers seem to just `imply`
PTP_1588_CLOCK, rather than `depends on` it?

In any case, I was surprised to get a warning during `ifdown` but not
during `ifup`. What do you think of this change, which prints an error
like this during `ifup` if PTP_1588_CLOCK is not enabled:

[    6.192707] 000: cpsw 4a100000.ethernet: error registering cpts device

--- 
diff --git a/drivers/net/ethernet/ti/cpts.c b/drivers/net/ethernet/ti/cpts.c
index 10ad706dda53..70b15039cd37 100644
--- a/drivers/net/ethernet/ti/cpts.c
+++ b/drivers/net/ethernet/ti/cpts.c
@@ -462,8 +462,8 @@ int cpts_register(struct cpts *cpts)
        timecounter_init(&cpts->tc, &cpts->cc, ktime_get_real_ns());
 
        cpts->clock = ptp_clock_register(&cpts->info, cpts->dev);
-       if (IS_ERR(cpts->clock)) {
-               err = PTR_ERR(cpts->clock);
+       if (IS_ERR_OR_NULL(cpts->clock)) {
+               err = cpts->clock ? PTR_ERR(cpts->clock) : -EOPNOTSUPP;
                cpts->clock = NULL;
                goto err_ptp;
        }

-- 
Clay

  reply	other threads:[~2020-04-20  9:36 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-16  8:56 Clay McClure
2020-04-16 11:11 ` Grygorii Strashko
2020-04-20  9:36   ` Clay McClure [this message]
2020-04-20 14:38     ` Arnd Bergmann
2020-04-20 17:00       ` Richard Cochran
2020-04-20 18:57         ` Arnd Bergmann
2020-04-20 21:18           ` Richard Cochran
2020-04-20 21:21             ` Arnd Bergmann
2020-04-20 21:34               ` Richard Cochran
2020-04-20 21:42                 ` Arnd Bergmann
2020-04-22 11:16                   ` Grygorii Strashko
2020-04-26  2:41                     ` Clay McClure

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