linux-kernel - Re: [v2,1/4] hwmon: iio_hwmon: defer probe when no channel is found

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Message-ID: <20160718100232.GE4199@lukather>
Date:	Mon, 18 Jul 2016 12:02:32 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Quentin Schulz <quentin.schulz@...e-electrons.com>,
	jdelvare@...e.com, jic23@...nel.org, knaack.h@....de,
	lars@...afoo.de, pmeerw@...erw.net, wens@...e.org,
	lee.jones@...aro.org, linux-kernel@...r.kernel.org,
	linux-hwmon@...r.kernel.org, linux-iio@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	thomas.petazzoni@...e-electrons.com,
	antoine.tenart@...e-electrons.com
Subject: Re: [v2,1/4] hwmon: iio_hwmon: defer probe when no channel is found

Hi Guenter,

On Sat, Jul 16, 2016 at 10:00:13AM -0700, Guenter Roeck wrote:
> On Fri, Jul 15, 2016 at 11:59:11AM +0200, Quentin Schulz wrote:
> > iio_channel_get_all returns -ENODEV when it cannot find either phandles and
> > properties in the Device Tree or channels whose consumer_dev_name matches
> > iio_hwmon in iio_map_list. The iio_map_list is filled in by iio drivers
> > which might be probed after iio_hwmon.
> > 
> > It is better to defer the probe of iio_hwmon if such error is returned by
> > iio_channel_get_all in order to let a chance to iio drivers to expose
> > channels in iio_map_list.
> > 
> > Signed-off-by: Quentin Schulz <quentin.schulz@...e-electrons.com>
> > ---
> > 
> > No modifications for this patch since we did not settled for a solution.
> > What should we do?
> > 
> AFAICS the only thing we can do is to replace module_platform_driver() with
> its explicitly coded variant, and to use use late_initcall() instead of
> module_init().

I thought this kind of changes to the driver init time were
discouraged these days?

> Anything else would result in endless probe deferrals if there are
> no channels.

Well, technically, not endless. AFAIK, the kernel only retries when a
new driver is probed, which should hopefully settle down rather
quickly.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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