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

Lars-Peter Clausen lars at metafoo.de
Mon Aug 1 14:54:46 UTC 2016


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 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.

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.

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.

The most likely solution is probably a mixture of all of them.

- Lars



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