[RFC PATCH v3] drivercore: Add driver probe deferral mechanism

[akpm@linux-foundation.org (Andrew Morton)]

On Thu, 22 Sep 2011 12:51:23 -0600
Grant Likely <grant.likely@secretlab.ca> wrote:

> Allow drivers to report at probe time that they cannot get all the resources
> required by the device, and should be retried at a later time.
> 
> This should completely solve the problem of getting devices
> initialized in the right order.  Right now this is mostly handled by
> mucking about with initcall ordering which is a complete hack, and
> doesn't even remotely handle the case where device drivers are in
> modules.  This approach completely sidesteps the issues by allowing
> driver registration to occur in any order, and any driver can request
> to be retried after a few more other drivers get probed.

What happens is there is a circular dependency, or if a driver's
preconditions are never met?  AFAICT the code keeps running the probe
function for ever.

If so: bad.  The kernel should detect such situations, should
exhaustively report them and if possible, fix them up and struggle
onwards.

>
> ...
>
> +	 * This bit is tricky.  We want to process every device in the
> +	 * deferred list, but devices can be removed from the list at any
> +	 * time while inside this for-each loop.  There are two things that
> +	 * need to be protected against:
> +	 * - if the device is removed from the deferred_probe_list, then we
> +	 *   loose our place in the loop.  Since any device can be removed

s/loose/lose/

> +	 *   asynchronously, list_for_each_entry_safe() wouldn't make things
> +	 *   much better.  Simplest solution is to restart walking the list
> +	 *   whenever the current device gets removed.  Not the most efficient,
> +	 *   but is simple to implement and easy to audit for correctness.
> +	 * - if the device is unregistered, and freed, then there is a risk
> +	 *   of a null pointer dereference.  This code uses get/put_device()
> +	 *   to ensure the device cannot disappear from under our feet.
> +	 */
>
> ...
>
> +		/* Drop the mutex while probing each device; the probe path
> +		 * may manipulate the deferred list */

Please don't invent new coding styles.  Like this:

		/*
		 * Drop the mutex while probing each device; the probe path
		 * may manipulate the deferred list
		 */

(entire patch)

>
> ...
>