[Ksummit-discuss] [TECH TOPIC] Addressing complex dependencies and semantics (v2)

[Andrzej Hajda]

On 08/01/2016 03:55 PM, Mauro Carvalho Chehab wrote:
> Em Mon, 1 Aug 2016 15:33:22 +0200
> Lars-Peter Clausen <lars at metafoo.de> escreveu:
>
>> On 08/01/2016 03:21 PM, Hans Verkuil wrote:
>>>
>>> On 08/01/2016 03:09 PM, Laurent Pinchart wrote:  
>>>> On Friday 29 Jul 2016 12:13:03 Mark Brown wrote:  
>>>>> On Fri, Jul 29, 2016 at 09:45:55AM +0200, Hans Verkuil wrote:  
>>>>>> My main problem is not so much with deferred probe (esp. for cyclic
>>>>>> dependencies it is a simple method of solving this, and simple is good).
>>>>>> My main problem is that you can't tell the system that driver A needs to
>>>>>> be probed after drivers B, C and D are probed first.
>>>>>>
>>>>>> That would allow us to get rid of v4l2-async.c which is a horrible hack.
>>>>>>
>>>>>> That code allows a bridge driver to wait until all dependent drivers are
>>>>>> probed. This really should be core functionality.
>>>>>>
>>>>>> Do other subsystems do something similar like
>>>>>> drivers/media/v4l2-core/v4l2-async.c? Does anyone know?  
>>>>> ASoC does, it has an explicit card driver to join things together and
>>>>> that just defers probe until everything it needs is present.  This was
>>>>> originally open coded in ASoC but once deferred probe was implemented we
>>>>> converted to that.  
>>>> Asynchronous bindings of components, as done in ASoC, DRM and V4L2, is a 
>>>> problem largely solved (or rather hacked around), but I'm curious to know how 
>>>> ASoC handles device unbinding (due to device removal or manual unbinding 
>>>> through sysfs). With asynchronous binding we can more or less easily wait for 
>>>> all components to be present before creating circular dependencies, but 
>>>> breaking them to implement unbinding is an unsolved problem at least in V4L2.
>>>>  
>>> We need to prevent subdevice drivers from being unbound. It's easy enough to
>>> do that (set suppress_bind_attrs to true), we just never did that. It's been
>>> on my TODO list for ages to make a patch adding that flag...
>>>
>>> You can only unbind bridge drivers. Unbinding subdevs is pointless in general
>>> and should be prohibited. Perhaps in the future with dynamically reconfigurable
>>> video pipelines (FPGA) you want that, but then you need to do a lot of
>>> additional work. For everything we have today we should just set
>>> suppress_bind_attrs to true.  
>> suppress_bind_attrs is the lazy solution and as you pointed out does not
>> work too well for all cases.
> Agreed. 
>
> What we really need is a kind of "usage count" behavior to suppress
> unbinds, e. g. a device driver can be unbound only if any other driver
> using resources on it gets unbind first.
>
> That will solve most of unbind issues at the media subsystem.

When I was investigating issues with unbind sysfs attribute I have found
claim by Greg KH that unbind should be rather unavoidable, like in case
of hw removal - kernel is not able to prevent users from removing usb
device, even if it is in use.

Assuming the claim is still valid, the only solution I see are callbacks
notifying resource consumers about removal of the resources.

Regards
Andrzej

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