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

[Hans Verkuil]

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 
>>> ---
>>>
>>>  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(_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 

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

[Sakari Ailus]

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 
> > ---
> >
> >  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(_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 

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

[Hans Verkuil]

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 
> ---
>
>  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(_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
>>
>>
>> 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.

> - 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.

> - Post 1/4 separately, drop patch 2/4 -- 4/4 and create a new patch 
>   which removes all re-probe related code and post that as a new patch.  
>   I would feel a but uneasy about this without a consensus from all you 
>   guys since I don't understand all the ramifications in doing so.

I'm uneasy about this as well.

> - Post 1/4 separately, drop patch 2/4 -- 4/4 and try to rework the 
>   sub-notifier code to 

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

[Niklas Söderlund]

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 
> > > > ---
> > > > 
> > > >  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(_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
> 
> 
> 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.

- 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.

- Post 1/4 separately, drop patch 2/4 -- 4/4 and create a new patch 
  which removes all re-probe related code and post that as a new patch.  
  I would feel a but uneasy about this without a consensus from all you 
  guys since I don't understand all the ramifications in doing so.

- Post 1/4 separately, drop patch 2/4 -- 4/4 and try to rework the 
  sub-notifier code to work the intertwined v4l2 and re-probe portions 
  of the code.

> 
> > > 
> > > > +*
> > > > +* One might be tempted to use device_reprobe() to handle the 
> > > > re-
> > > > +* probing. Unfortunately this is 

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

[Niklas Söderlund]

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 
> > > ---
> > > 
> > >  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(_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


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?

> > 
> > > +  *
> > > +  * One might be tempted to use device_reprobe() to handle the re-
> > > +  * probing. Unfortunately this is not possible since some video
> > > +  * device drivers call v4l2_async_notifier_unregister() from
> > > +  * there remove function leading to a dead lock situation on
> > > +  * device_lock(dev->parent). This lock is held when video device
> > > +  * drivers remove function is called and device_reprobe() also
> > > +  * tries to take the same lock, so using it here could lead to a
> > > +  * dead lock situation.
> > > +  */
> > > +
> > >   for (i = 0; i < count; i++) {
> > >   
> > >   /* If we handled USB devices, we'd have to lock the parent too 
> */
> > >   device_release_driver(dev[i]);
> 
> -- 
> Regards,
> 
> Laurent Pinchart
> 

-- 
Regards,
Niklas Söderlund

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

[Laurent Pinchart]

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 
> > ---
> > 
> >  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(_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 :-)

> Or can someone come up with a valid reason why the re-probing code should
> stay? :-)
> 
> > +*
> > +* One might be tempted to use device_reprobe() to handle the re-
> > +* probing. Unfortunately this is not possible since some video
> > +* device drivers call v4l2_async_notifier_unregister() from
> > +* there remove function leading to a dead lock situation on
> > +* device_lock(dev->parent). This lock is held when video device
> > +* drivers remove function is called and device_reprobe() also
> > +* tries to take the same lock, so using it here could lead to a
> > +* dead lock situation.
> > +*/
> > +
> > for (i = 0; i < count; i++) {
> > 
> > /* If we handled USB devices, we'd have to lock the parent too 
*/
> > device_release_driver(dev[i]);

-- 
Regards,

Laurent Pinchart

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

[Sakari Ailus]

Hi Niklas,

Thanks for the patchset.

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 
> ---
>  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(_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.

Or can someone come up with a valid reason why the re-probing code should
stay? :-)

> +  *
> +  * One might be tempted to use device_reprobe() to handle the re-
> +  * probing. Unfortunately this is not possible since some video
> +  * device drivers call v4l2_async_notifier_unregister() from
> +  * there remove function leading to a dead lock situation on
> +  * device_lock(dev->parent). This lock is held when video device
> +  * drivers remove function is called and device_reprobe() also
> +  * tries to take the same lock, so using it here could lead to a
> +  * dead lock situation.
> +  */
> +
>   for (i = 0; i < count; i++) {
>   /* If we handled USB devices, we'd have to lock the parent too 
> */
>   device_release_driver(dev[i]);

-- 
Regards,

Sakari Ailus
e-mail: sakari.ai...@iki.fi