Re: [PATCH v3 1/4] hwmon: iio_hwmon: delay probing with late_initcall

[Jonathan Cameron <jic23@kernel.org>]

On 15 August 2016 18:07:30 BST, Guenter Roeck <linux@roeck-us.net> wrote:
>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 ?
Yes. Though whether it really adds much over doing that in drivers isn't clear.

Hmm. Needs more thought...
>
>Thanks,
>Guenter
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