linux-kernel - Re: [PATCH] w1-gpio: handle of_get_gpio() returning -EPROBE

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Date:	Tue, 18 Feb 2014 09:24:36 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	zbr@...emap.net
Cc:	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] w1-gpio: handle of_get_gpio() returning -EPROBE_DEFER
 better

On Tue, Feb 18, 2014 at 05:26:20AM +0400, zbr@...emap.net wrote:
> Hi
> 
> 14.02.2014, 02:05, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de>:
> > of_get_gpio() might return -EPROBE_DEFER meaning that the driver
> > providing the gpio isn't ready yet. If that happens for the first gpio
> > the resulting kernel output without this patch is:
> >
> >         w1-gpio somename: Failed to parse DT
> >         platform somename: Driver w1-gpio requests probe deferral
> >
> > The first message is misleading and so is suppressed with this patch.
> >
> > Further if determining the gpio to switch the external pullup yields
> > -EPROBE_DEFER this error should be passed back to the caller instead of
> > just continuing without pullup.
> >
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> 
> I'm ok with this patch, but I virtually do not understand what it does :)
> Since I know nothing about device trees.
The thing is that the w1-gpio device might be probed before the gpio's
controller is available. In this case of_get_gpio returns -EPROBE_DEFER
and this makes the device core retry probing the w1-gpio device later
again. So this is not a dt-parsing problem but just dependency stuff.

The 2nd issue addressed didn't happen to me, but it might occur that the
first of_get_gpio for the data pin succeeds, but the getting the pullup
gpio fails with -EPROBE_DEFER (e.g. because it sits on a different
controller). In this case you still want to retry probing later instead
of ignoring the problem.
 
Does that make it clearer?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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