Re: [RFCv3 PATCH 11/18] v4l2-ctrls: add v4l2_fh pointer to the set control functions.

[Mauro Carvalho Chehab <mchehab@redhat.com>]

Em 28-06-2011 03:22, Hans Verkuil escreveu:
> On Monday, June 27, 2011 23:20:07 Mauro Carvalho Chehab wrote:
>> Em 07-06-2011 12:05, Hans Verkuil escreveu:
>>> From: Hans Verkuil <hans.verkuil@cisco.com>
>>>
>>> When an application changes a control you want to generate an event.
>>> However, you want to avoid sending such an event back to the application
>>> (file handle) that caused the change.
>>
>> Why? 
>>
>> I can see two usecases for an event-triggered control change:
>> 	1) when two applications are used, and one changed a value that could
>> affect the other;
>> 	2) as a way to implement async changes.
>>
>> However, it seems, from your comments, that you're covering only case (1).
>>
>> There are several reasons why we need to support case (2):
>>
>> Some controls may be associated to a servo mechanism (like zoom, optical
>> focus, etc), or may require some time to happen (like charging a flash device).
>> So, it makes sense to have events back to the application that caused the change.
>>
>> Kernel should not assume that the application that requested a change on a control
>> doesn't want to receive the notification back when the event actually happened.
>> This way, both cases will be covered.
>>
>> Yet, I failed to see where, in the code, such restriction were imposed.
> 
> Async changes are triggered by the driver, not an application. Any changes
> made by the driver will be sent to all applications.
> 
> That said, I think I should add a flag like V4L2_EVENT_SUB_FL_NO_FEEDBACK
> to explicitly let applications decide.

Agreed. it makes the code more generic.
> 
> That's easy enough.
> 
> Regards,
> 
> 	Hans