Re: [PATCH 4/4] v4l: async: add comment about re-probing to v4l2_async_notifier_unregister()

[Hans Verkuil <hverkuil@xs4all.nl>]

On 24/08/17 18:17, Sakari Ailus wrote:
> Hi Hans,
> 
> On Thu, Aug 24, 2017 at 09:59:41AM +0200, Hans Verkuil wrote:
>> On 08/23/17 21:03, Niklas Söderlund wrote:
>>> Hi,
>>>
>>> On 2017-08-18 15:42:37 +0200, Niklas Söderlund wrote:
>>>> Hi Sakari and Laurent,
>>>>
>>>> Thanks for your feedback.
>>>>
>>>> On 2017-08-18 14:20:08 +0300, Laurent Pinchart wrote:
>>>>> Hello,
>>>>>
>>>>> On Tuesday 15 Aug 2017 19:09:33 Sakari Ailus wrote:
>>>>>> On Mon, Jul 31, 2017 at 12:31:58AM +0200, Niklas Söderlund wrote:
>>>>>>> The re-probing of subdevices when unregistering a notifier is tricky to
>>>>>>> understand, and implemented somewhat as a hack. Add a comment trying to
>>>>>>> explain why the re-probing is needed in the first place and why existing
>>>>>>> helper functions can't be used in this situation.
>>>>>>>
>>>>>>> Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
>>>>>>> ---
>>>>>>>
>>>>>>>  drivers/media/v4l2-core/v4l2-async.c | 17 +++++++++++++++++
>>>>>>>  1 file changed, 17 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/media/v4l2-core/v4l2-async.c
>>>>>>> b/drivers/media/v4l2-core/v4l2-async.c index
>>>>>>> d91ff0a33fd3eaff..a3c5a1f6d4d2ab03 100644
>>>>>>> --- a/drivers/media/v4l2-core/v4l2-async.c
>>>>>>> +++ b/drivers/media/v4l2-core/v4l2-async.c
>>>>>>> @@ -234,6 +234,23 @@ void v4l2_async_notifier_unregister(struct
>>>>>>> v4l2_async_notifier *notifier)> 
>>>>>>>  	mutex_unlock(&list_lock);
>>>>>>>
>>>>>>> +	/*
>>>>>>> +	 * Try to re-probe the subdevices which where part of the notifier.
>>>>>>> +	 * This is done so subdevices which where part of the notifier will
>>>>>>> +	 * be re-probed to a pristine state and put back on the global
>>>>>>> +	 * list of subdevices so they can once more be found and associated
>>>>>>> +	 * with a new notifier.
>>>>>>
>>>>>> Instead of tweaking the code trying to handle unhandleable error conditions
>>>>>> in notifier unregistration and adding lengthy stories on why this is done
>>>>>> the way it is, could we simply get rid of the driver re-probing?
>>>>>>
>>>>>> I can't see why drivers shouldn't simply cope with the current interfaces
>>>>>> without re-probing to which I've never seen any reasoned cause. When a
>>>>>> sub-device driver is unbound, simply return the sub-device node to the list
>>>>>> of async sub-devices.
>>>>>
>>>>> I agree, this is a hack that we should get rid of. Reprobing has been there 
>>>>> from the very beginning, it's now 4 years and a half old, let's allow it to 
>>>>> retire :-)
>>>>
>>>> I would also be happy to see this code go away :-)
>>>>
>>>>>
>>>>>> Or can someone come up with a valid reason why the re-probing code should
>>>>>> stay? :-)
>>>>
>>>> Hans kindly dug out the original reason talking about why this code was 
>>>> added in the first place at
>>>>
>>>>     http://lkml.iu.edu/hypermail/linux/kernel/1210.2/00713.html
>>>>
>>>> I would also like record here what Laurent stated about this after 
>>>> reading the above on #v4l 
>>>>
>>>> 13:53  pinchartl : what could happen is this
>>>> 13:53  pinchartl : the master could export resources used by the subdev
>>>> 13:53  pinchartl : the omap3 isp driver, for instance, is a clock source
>>>> 13:54  pinchartl : and the clock can be used by sensors
>>>> 13:54  pinchartl : so if you remove the omap3 isp, the clock won't be 
>>>>    there anymore
>>>> 13:54  pinchartl : and that's bad for the subdev
> 
> Re-probing never helped anything with omap3isp driver as the clock is
> removed *after* unregistering async notifier. This means that the
> re-probing sub-device driver will get the same clock which is about to be
> removed and continues with that happily, only to find the clock gone in a
> brief moment.
> 
> This could be fixed in the omap3isp driver but it is telling that _no-one
> ever complained_.
> 
>>>>
>>>>
>>>> I don't claim I fully understand all the consequences of removing this 
>>>> reprobing now. But maybe it's safer to lave the current behavior in for 
>>>> now until the full problem is understood and move forward whit these 
>>>> patches since at least they document the behavior and removes another 
>>>> funky bit when trying to handle the situation where the memory 
>>>> allocation fails? What do you guys think?
>>>
>>> Any thoughts about how I can move forward with this? The reason I'm 
>>> asking is that this is a dependency for the sub-notifier patches which 
>>> in turn is dependency for the R-Car CSI-2 driver :-) If someone wants to 
>>> think more about this that is fine I just don't want it to be forgotten.  
>>> As I see it these are the options open to me, but as always I'm always 
>>> open to other solutions which I'm to narrow minded to see :-)
>>>
>>> - If after the latest discussions it feels the safest option is to keep 
>>>   the re-probe logic but separating the v4l2 housekeeping from re-probe 
>>>   logic move forward with this series as-is.
>>
>> I prefer this. We can always remove the reprobe code later once we have
>> a better understanding. I see no downside to this cleanup series and it
>> doesn't block any future development.
> 
> One thing we could do is to remove the memory allocation there. After that
> it couldn't fail anymore, leaving the device in an unknown state.
> 
>>
>>> - Post 1/4 separately and repost patch 2/4 -- 4/4 in a v2 to allow for 
>>>   more input on what is the right thing to do here.
>>
>> I'm OK with this as well, we missed the 4.14 merge window anyway.
> 
> Agreed.
> 

I want to add another option:

Keep it, finish the async work, then as a final patch remove the reprobe code.

If we discover that this actually breaks something then we can just revert that
final patch without having to rework the whole series.

Regards,

	Hans