Re: [driver-core PATCH v6 6/9] driver core: Probe devices asynchronously instead of the driver

Probe devices asynchronously instead of the driver. This results in us seeing the same behavior if the device is registered before the driver or after. This way we can avoid serializing the initialization should the driver not be loaded until after the devices have already been added. The motivation behind this is that if we have a set of devices that take a significant amount of time to load we can greatly reduce the time to load by processing them in parallel instead of one at a time. In addition, each device can exist on a different node so placing a single thread on one CPU to initialize all of the devices for a given driver can result in poor performance on a system with multiple nodes.

I am using the driver_data member of the device struct to store the driver pointer while we wait on the deferred probe call. This should be safe to do as the value will either be set to NULL on a failed probe or driver load followed by unload, or the driver value itself will be set on a successful driver load. In addition I have used the async_probe flag to add additional protection as it will be cleared if someone overwrites the driver_data member as a part of loading the driver.