Thierry Reding thierry.reding@gmail.com writes:
From: Thierry Reding treding@nvidia.com
The Tegra HDA controller driver committed in v3.16 causes deadlocks when loaded as a module. The reason is that the driver core will lock the HDA controller device upon calling its probe callback and the probe callback then goes on to create child devices for detected codecs and loads their modules via a request_module() call. This is problematic because the new driver will immediately be bound to the device, which will in turn cause the parent of the codec device (the HDA controller device) to be locked again, causing a deadlock.
This problem seems to have been present since the modularization of the HD-audio driver in commit 1289e9e8b42f ("ALSA: hda - Modularize HD-audio driver"). On Intel platforms this has been worked around by splitting up the probe sequence into a synchronous and an asynchronous part where the request_module() calls are asynchronous and hence avoid the deadlock.
An alternative proposal is provided in this series of patches. Rather than relying on explicit request_module() calls to load kernel modules for HDA codec drivers, this implements a uevent callback for the HDA bus to advertises the MODALIAS information to the userspace helper.
Effectively this results in the same modules being loaded, but it uses the more canonical infrastructure to perform this. Deferring the module loading to userspace removes the need for the explicit request_module() calls and works around the recursive locking issue because both drivers will be bound from separate contexts.
Tested-by: Kevin Hilman khilman@linaro.org
FWIW, I tested this on top of next-20150923, and it solves the boot problem I'm seeing on tegra124-jetson-tk1 with module loading.
Kevin