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

[Jonathan Cameron <jic23@kernel.org>]

On 01/09/16 10:03, Quentin Schulz wrote:
> On 01/09/2016 09:15, Quentin Schulz wrote:
>> On 15/08/2016 23:35, Jonathan Cameron wrote:
>>>
>>>
>>> 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...
>>
>> Either we do the exact same "hack" as in the v2[1] in what you call
>> iio_channel_get_all_or_defer or we duplicate the code from
>> iio_channel_get_all in iio_channel_get_all_or_defer. Both do not seem
>> right to me but I really dislike the late_initcall method. With this
>> method we can only have one level of "channel dependency".
>>
>> This means if we ever create a new driver which depends on channels from
>> the driver using late_initcall, we will also have to use late_initcall
>> and we can't be sure the new driver will always be probed after the
>> driver he depends on.
>>
>> [1] https://lkml.org/lkml/2016/7/15/215
>>
>> Quentin
> 
> Should I revert back to the hack introduced in v2 then?
I think so.  Sorry I didn't see this until after you'd sent v4.

That hack had it's disadvantages but in many ways it was a least clean.

Jonathan
> 
>>>>
>>>> Thanks,
>>>> Guenter
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>>>
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