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

[Lars-Peter Clausen <lars@metafoo.de>]

On 08/01/2016 05:34 PM, Andrzej Hajda wrote:
> On 08/01/2016 04:54 PM, Lars-Peter Clausen wrote:
>> On 08/01/2016 04:44 PM, Andrzej Hajda wrote:
>>> On 08/01/2016 03:55 PM, Mauro Carvalho Chehab wrote:
>>>> Em Mon, 1 Aug 2016 15:33:22 +0200
>>>> Lars-Peter Clausen <lars@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.
>> There are multiple options.
>>
>> One option, which I think is currently the most used option in the kernel,
>> is to unregister the resource when the provider is removed, but keep the
>> resource object alive as long as there are users. Any further operation on
>> such object will fail with an error. This works to the point where things
>> don't crash, but it wont function in any meaningful way. There is no way to
>> automatically recover if the resource reappears.
> 
> For me it is not a real solution, it is just dirty workaround to just avoid
> invalid pointers. It 'works' only because unbinding is rarely used.
> For example, how the device is supposed to work if its regulator or clock
> disappeared?
> 
>>
>> Other options are as you pointed out notifier callbacks that allows the
>> resource use to be aware that a resource has disappeared and it might adjust
>> and continue to function with limited functionality.
>>
>> Another option is to teach the device core about critical resource
>> dependencies so that a consumer is automatically unbound by the core if any
>> of its resource dependencies are unregistered. The device can also
>> automatically be re-bound once the critical resources re-appear.
> 
> That would be OK only for critical resources.
> 
>>
>> The most likely solution is probably a mixture of all of them.
> 
> If we implement callbacks, we do not need other two 'options'.

Having to manually register callbacks for every resource in every driver
will result in a massive amount of boilerplate code. I'd rather avoid that.
In addition to that doing resource tracking at the framework level will help
with probe ordering.