On Wed, Aug 18, 2021 at 7:52 AM Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx> wrote: > > > > >>> 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. Additionally you mentioned that the consumer is unknown to the producer, so you are not able, for example, to use the newly exported device_driver_attach() to directly trigger the unblocked dependency.
