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

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Message-ID: <578CD99E.5000406@roeck-us.net>
Date:	Mon, 18 Jul 2016 06:29:02 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
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

On 07/18/2016 03:02 AM, Maxime Ripard wrote:
> 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?
>

Blindly converting -ENODEV to -EPROBEDEFER doesn't sound like a good idea
either.

>> 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.
>

Plus each time a new driver is loaded for whatever reason.

Also, are you sure that leaving a device in deferred probe state doesn't
have undesirable side effects ? For example, would driver_probe_done()
ever return true ?

Guenter