[RFC PATCH 1/2] driver core: export driver_deferred_probe_trigger()

[Pierre-Louis Bossart]

>>> The issue is that the driver core is using drivers completing probe as a
>>> proxy for resources becoming available.  That works most of the time
>>> because most probes are fully synchronous but it breaks down if a
>>> resource provider registers resources outside of probe, we might still
>>> be fine if system boot is still happening and something else probes but
>>> only through luck.
> 
>> The driver core is not using that as a proxy, that is up to the driver
>> itself or not.  All probe means is "yes, this driver binds to this
>> device, thank you!" for that specific bus/class type.  That's all, if
>> the driver needs to go off and do real work before it can properly
>> control the device, wonderful, have it go and do that async.
> 
> Right, which is what is happening here - but the deferred probe
> machinery in the core is reading more into the probe succeeding than it
> should.

I think Greg was referring to the use of the PROBE_PREFER_ASYNCHRONOUS
probe type. We tried just that and got a nice WARN_ON because we are
using request_module() to deal with HDaudio codecs. The details are in
[1] but the kernel code is unambiguous...

        /*
	 * We don't allow synchronous module loading from async.  Module
	 * init may invoke async_synchronize_full() which will end up
	 * waiting for this task which already is waiting for the module
	 * loading to complete, leading to a deadlock.
	 */
	WARN_ON_ONCE(wait && current_is_async());


The reason why we use a workqueue is because we are otherwise painted in
a corner by conflicting requirements.

a) we have to use request_module()
b) we cannot use the async probe because of the request_module()
c) we have to avoid blocking on boot

I understand the resistance to exporting this function, no one in our
team was really happy about it, but no one could find an alternate
solution. If there is something better, I am all ears.

Thanks
-Pierre

[1] https://github.com/thesofproject/linux/pull/3079