linux-kernel - Re: [PATCH v3 1/4] hwmon: iio_hwmon: delay probing with late

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Date:	Mon, 15 Aug 2016 10:07:30 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Alexander Stein <alexander.stein@...tec-electronic.com>,
	Quentin Schulz <quentin.schulz@...e-electrons.com>,
	linux-kernel@...r.kernel.org, jdelvare@...e.com, knaack.h@....de,
	lars@...afoo.de, pmeerw@...erw.net,
	maxime.ripard@...e-electrons.com, wens@...e.org,
	lee.jones@...aro.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: [PATCH v3 1/4] hwmon: iio_hwmon: delay probing with late_initcall

On Mon, Aug 15, 2016 at 04:40:21PM +0100, Jonathan Cameron wrote:
> On 26/07/16 17:04, Guenter Roeck wrote:
> > On Tue, Jul 26, 2016 at 12:00:33PM +0200, Alexander Stein wrote:
> >> On Tuesday 26 July 2016 11:33:59, Quentin Schulz wrote:
> >>> On 26/07/2016 11:05, Alexander Stein wrote:
> >>>> On Tuesday 26 July 2016 10:24:48, Quentin Schulz wrote:
> >>>>> On 26/07/2016 10:21, Alexander Stein wrote:
> >>>>>> On Tuesday 26 July 2016 09:43:44, 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.
> >>>>>>
> >>>>>> Would it work if iio_channel_get_all returning ENODEV is used for
> >>>>>> returning
> >>>>>> EPROBE_DEFER in iio_channel_get_all? Using late initcalls for
> >>>>>> driver/device
> >>>>>> dependencies seems not right for me at this place.
> >>>>>
> >>>>> Then what if the iio_channel_get_all is called outside of the probe of a
> >>>>> driver? We'll have to change the error code, things we are apparently
> >>>>> trying to avoid (see v2 patches' discussions).
> >>>>
> >>>> Maybe I didn't express my idea enough. I don't want to change the behavior
> >>>> of iio_channel_get_all at all. Just the result evaluation of
> >>>> iio_channel_get_all in iio_hwmon_probe. I have something link the patch
> >>>> below in mind.
> >>>>
> >>>> Best regards,
> >>>> Alexander
> >>>> ---
> >>>> diff --git a/drivers/hwmon/iio_hwmon.c b/drivers/hwmon/iio_hwmon.c
> >>>> index b550ba5..e32d150 100644
> >>>> --- a/drivers/hwmon/iio_hwmon.c
> >>>> +++ b/drivers/hwmon/iio_hwmon.c
> >>>> @@ -73,8 +73,12 @@ static int iio_hwmon_probe(struct platform_device
> >>>> *pdev)
> >>>>
> >>>>                 name = dev->of_node->name;
> >>>>         
> >>>>         channels = iio_channel_get_all(dev);
> >>>>
> >>>> -       if (IS_ERR(channels))
> >>>> -               return PTR_ERR(channels);
> >>>> +       if (IS_ERR(channels)) {
> >>>> +               if (PTR_ERR(channels) == -ENODEV)
> >>>> +                       return -EPROBE_DEFER;
> >>>> +               else
> >>>> +                       return PTR_ERR(channels);
> >>>> +       }
> >>>>
> >>>>         st = devm_kzalloc(dev, sizeof(*st), GFP_KERNEL);
> >>>>         if (st == NULL) {
> >>>
> >>> Indeed, I misunderstood what you told me.
> >>>
> >>> Actually, the patch you proposed is part of my v1
> >>> (https://lkml.org/lkml/2016/6/28/203) and v2
> >>> (https://lkml.org/lkml/2016/7/15/215).
> >>> Jonathan and Guenter didn't really like the idea of changing the -ENODEV
> >>> in -EPROBE_DEFER.
> >>
> >> Thanks for the links.
> >>
> >>> What I thought you were proposing was to change the -ENODEV return code
> >>> inside iio_channel_get_all. This cannot be an option since the function
> >>> might be called outside of a probe (it is not yet, but might be in the
> >>> future?).
> >>
> >> AFAICS this is a helper function not knowing about device probing itself. And 
> >> it should stay at that.
> >>
> >>> Of what I understood, two possibilities are then possible (proposed
> >>> either by Guenter or Jonathan): either rework the iio framework to
> >>> register iio map array earlier or to use late_initcall instead of init
> >>> for the driver consuming the iio channels.
> >>
> >> Interestingly using this problem would not arise due to module dependencies. 
> >> But using late_initcall would mean this needs to be done on any driver using 
> >> iio channels? I would rather keep those consumers simple.
> >>
> > Me too, but that would imply a solution in iio. The change you propose above
> > isn't exactly simple either, and would also be needed in each consumer driver.
> > 
> > Just for the record, I dislike the late_initcall solution as well, but I prefer
> > it over blindly converting ENODEV to EPROBE_DEFER.
> I'm falling on the other side on this one right now. Though I'd be tempted
> to renaming the function to something like iio_channel_get_all_or_defer
> to make it explicit that it can result in deferred probing.
> 
Would this new function return -EPROBE_DEFER instead of -ENODEV ?

Thanks,
Guenter