Date:   Thu, 1 Sep 2016 11:03:06 +0200
From:   Quentin Schulz <quentin.schulz@...e-electrons.com>
To:     Jonathan Cameron <jic23@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>
Cc:     Alexander Stein <alexander.stein@...tec-electronic.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 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@...ck-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?

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

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