From: Scott Feldman <redacted>
Date: Thu, 24 Sep 2015 22:29:43 -0700

I'd rather keep 2-phase not optional, or at least make it some what of
a pain for drivers to opt-out of 2-phase.  Forcing the driver to see
both phases means the driver needs to put some code to skip phase 1
(and hopefully has some persistent comment explaining why its being
skipped).  Something like:

/* I'm skipping phase 1 prepare for this operation.  I have infinite hardware
 * resources and I'm not setting any persistent state in the driver or device
 * and I don't need any dynamic resources from the kernel, so its impossible
 * for me to fail phase 2 commit.  Nothing to prepare, sorry.
 */

I agree with Scott here.

If you can opt out of something, you can not think about it and thus
more likely get it wrong.

I can just see a driver not implementing prepare at all and then doing
stupid things in commit when they hit some resource limit or whatever,
rather than taking care of such issues in prepare.